51 drug, accompanied by negative affective systems. They generally provide users with states— is also developing. Together, these lower overall nicotine levels than they insights on abuse and addiction are leading receive with tobacco, as well as totally to new therapies. eliminating exposure to smoke and its deadly contents. The first non-nicotine prescription Nicotine, the addicting substance in drug, bupropion, an antidepressant marketed tobacco, acts through the well-known as Zyban, has been approved for use as a cholinergic nicotinic receptor. This drug can pharmacological treatment for nicotine act as both a stimulant and a sedative. addiction. Behavioral treatments are Immediately after exposure to nicotine, there important for helping an individual learn is a “kick” caused in part by the drug’s coping skills for both short and long-term stimulation of the adrenal glands and prevention of relapse. resulting discharge of epinephrine. The rush of adrenaline stimulates the body and Psycho stimulants A popular, chemically causes a sudden release of glucose as well altered form of cocaine, crack, is smoked by as an increase in blood pressure, respiration, large number of addicts. It enters the brain in and heart rate. Nicotine also suppresses seconds, producing a rush of euphoria and insulin output from the pancreas, which feelings of power and self-confidence. The means that smokers are always slightly key biochemical factor that underlies the hyperglycemic. In addition, nicotine releases reinforcing effects of psycho stimulants is the dopamine in the brain regions that control brain chemical dopamine. We feel pleasure pleasure and motivation. This mechanism is when dopamine-containing neurons release thought to underlie the pleasurable dopamine into specific brain areas that sensations experienced by many smokers. include a special portion of the nucleus accumbens. Cocaine and amphetamines Much better understanding of addiction, produce intense feelings of euphoria by coupled with the identification of nicotine as increasing the amount of dopamine that is an addictive drug, has been instrumental in available to send messages within the brain the development of treatments. Nicotine reward system. gum, the transdermal patch, nasal spray, and inhalers all appear to be equally Cocaine users often go on binges (=heavy effective in treating more than 1 million drinking or eating), consuming a large amount people addicted to nicotine. These of the drug in just a few days. A crash occurs techniques are used to relieve withdrawal after this period of intense drug-taking and symptoms and produce less severe includes symptoms of emotional and physiological alterations than tobacco-based physical exhaustion and depression. These
52 BRAIN DRUG REWARD SYSTEMS. Scientists are not certain about all the structures involved in the human brain reward system. However, studies of rat and monkey brains, and brain imaging studies in humans, have provided many clues. These illustrations show what areas are most likely part of the reward systems in the human brain. A central group of structures is common to the actions of all drugs. These structures include a collection of dopamine containing neurons found in the ventral tegmental area. These neurons are connected to the nucleus accumbens and other areas, such as the prefrontal cortex. Cocaine exerts its effects mainly through this system. Opiates act in this system and many other brain regions, including the amygdala, that normally use opioid peptides. Opioids are naturally occurring brain chemicals that induce the same actions as drugs, such as heroin and morphine. Alcohol activates the core reward system and additional structures throughout the brain because it acts where GABA and glutamate are used as neurotransmitters. GABA and glutamate are widely distributed in the brain, including the cortex, hippocampus, amygdala, and nucleus accumbens. physical ailments, including severe muscle pain, stomach cramps, diarrhea, and unpleasant mood. symptoms may result from an actual crash in Opiates, like psycho stimulants, increase dopamine function and the activity of another the amount of dopamine released in the brain chemical, serotonin, as well as an brain reward system and mimic the effects of increased response of the brain systems that endogenous opioids such as opioid peptides. react to stress. Vaccines to produce Heroin injected into a vein reaches the brain antibodies to cocaine in the bloodstream are in 15 to 20 seconds and binds to opiate in clinical trials. receptors found in many brain regions, including the reward system. Activation of Opiates Humans have used opiate drugs, the receptors in the reward circuits causes a brief rush of intense euphoria, followed by a such as morphine, for thousands of years. couple of hours of a relaxed, contented state. Monkeys and rats readily self-administer Opiates create effects like those elicited heroin or morphine and, like humans, will by the naturally occurring opioid peptides. They relieve pain, depress breathing, cause become tolerant and physically dependent nausea and vomiting, and stop diarrhea - important medical uses. In large doses, with unlimited access. Withdrawal symptoms heroin can make breathing shallow or stop Altogether the cause of death in thousands range from mild flu-like discomfort to major of people who have died of heroin overdose.
53 HOW CRACK COCAINE AFFECTS THE BRAIN. Crack cocaine takes the same route as nicotine by entering the bloodstream through the lungs. Within seconds, it is carried by the blood to the brain. The basis for increased pleasure occurs at the gap where the impulses that represent neural messages are passed from one neuron to another. This gap is called a synapse. Dopamine-containing neurons normally relay their signals by releasing dopamine into many synapses. Dopamine crosses the synapse and fits into receptors on the surface of the receiving cell. This triggers an electrical signal that is relayed through the receiver. Then, to end the signal, dopamine molecules break away from the receptors and are pumped back into the nerve terminals that released them. Cocaine molecules block the pump or “transporter,” causing more dopamine to accumulate in the synapse. Pleasure circuits are stimulated again and again, producing euphoria.
54 A standard treatment for opiate addiction cause of mental retardation. Chronic liver involves methadone, a long-acting oral diseases, including cirrhosis—the main opiate that helps keep craving, withdrawal, and relapse under control. Methadone helps chronic health problem associated with opiate addicts rehabilitate themselves by alcohol addiction—are responsible for more preventing withdrawal symptoms that are powerful motivators of drug use. A synthetic than 25,000 deaths each year. opiate, known as LAAM, can exert its effects on heroin for up to 72 hours with minimal Genetic and environmental factors side effects when taken orally. Its long contribute to alcoholism, but no single factor duration of action permits dosing just three or combination of factors enables doctors to times per week, eliminating the need for predict who will become an alcoholic. daily dosing. LAAM will be increasingly available in clinics that already dispense Ethanol, the active ingredient in alcoholic methadone. Naloxone and naltrexone are medications that also block the effects of beverages, reduces anxiety, tension, and morphine, heroin, and other opiates. As antagonists, they are especially useful as inhibitions. In low doses it may act as a antidotes. Another medication to treat heroin addiction, buprenorphine, causes weaker stimulant, whereas at higher doses, it acts as opiate effects and is less likely to cause overdose problems. Buprenorphine is a depressant. In both cases, it significantly expected to become an important treatment. alters mood and behavior. It can also cause Alcohol Although legal, alcohol is highly addictive. Alcohol abuse and alcohol heat loss and dehydration. addiction—sometimes referred to as alcoholism or alcohol dependence—are the The drug, which is easily absorbed into nation’s major drug problem, with some the bloodstream and the brain, affects people being more susceptible than others. several neurotransmitter systems. For Nearly14millionpeopleabusealcoholorarealco example, alcohol’s interaction with the holic.Fetalalcohol syndrome, affecting about GABA receptor can calm anxiety, impair 0.5 to 3 of every 1,000 babies born in the muscle control, and delay reaction time. At United States, is the leading preventable higher doses, alcohol also decreases the function of NMDA receptors that recognize the neurotransmitter glutamate. This interaction can cloud thinking and eventually lead to coma. Researchers are developing treatments that interfere with molecules, such as the opioid peptides, that trigger alcohol’s positive reinforcing effects. One such drug, naltrexone, recently has been approved for treating alcoholism. Marijuana This drug can distort perception and alter the sense of time, space, and self. In certain situations,
55 marijuana can produce intense anxiety. particularly with alcohol, can lead to In radioactive tracing studies, scientists unexpected adverse reactions and even death after high doses. Physical exhaustion found that tetrahydrocannabinol (THC), the also can enhance some toxicities and active ingredient in marijuana, binds to problems. specific receptors, many of which coordinate movement. This may explain why people MDMA, called “Adam,” “ecstasy,” or who drive after they smoke marijuana are “XTC” on the street, is a synthetic, impaired. The hippocampus, a structure psychoactive drug with hallucinogenic and involved with memory storage and learning, amphetamine-like properties. Users also contains many THC receptors. This may encounter problems similar to those found explain why heavy users or those intoxicated with the use of amphetamines and cocaine. on marijuana have poor short-term memory Recent research also may link ecstasy use and problems processing complex to long-term damage to those parts of the information. Scientists recently discovered brain critical to thought, memory, and that these receptors normally bind to a pleasure. natural internal chemical called anandamide, and they are now working to see how this Rohypnol, GHB (gamma hydroxyl- chemical affects brain function. butyrate), and ketamine are predominantly central nervous system depressants. Club Drugs Ecstasy, herbal ecstasy, Because they are often colorless, tasteless, rohypnol (“roofies”), GHB, and ketamine are and odorless, they can be easily added to among the drugs used by some teens and beverages and ingested unknowingly. young adults as part of rave and trance These drugs have emerged as the so- called events, which are generally night-long date-rape drugs. When mixed with alcohol, dances, often held in warehouses. The drugs rohypnol can incapacitate victims and are rumored to increase stamina and to prevent them from resisting sexual assault. produce intoxicating highs that are said to Also, rohypnol may be lethal when mixed deepen the rave or trance experience with alcohol and other depressants. .Recent hard science, however, is uncovering the serious damage that can Since about 1990, GHB has been abused occur in several parts of the brain from use in the United States for euphoric, sedative, of some of these drugs. and anabolic (body building) effects. It, too, has been associated with sexual assault. Many users tend to experiment with a Ketamine is another central nervous system variety of club drugs in combination. This depressant abused as a date-rape drug. practice creates a larger problem, because Ketamine, or “Special K,” is a fast-acting combinations of any of these drugs, general anesthetic. It has sedative, hypnotic,
56 analgesic, and hallucinogenic properties. It is stemming from a learning disability. ADHD marketed in the United States and a number also is thought to have a strong genetic of foreign countries for use as a general influence. anesthetic in both human and veterinary medical practice. In addition to behavioral therapy, ADHD is commonly treated with medication—largely Attention deficit hyperactivity disorder stimulants. Ritalin, one brand name under which the stimulant methylphenidate is sold, Attention deficit hyperactivity disorder is one of the most widely prescribed drugs for treating ADHD. Its use is controversial. (ADHD) was first described more than 100 Debate about treatment with Ritalin centers on the benefits of a more focused child, on years ago. Today it is the focus of hundreds the one hand, and doubts about the long- term risks of exposing children to of studies. Characterized by excessively psychotropic drugs, on the other. inattentive, hyperactive, or impulsive Alzheimer’s disease behaviors, ADHD affects 3 percent to 5 One of the most frightening and devastating of all neurological disorders is the dementia percent of children. Studies show that that occurs in the elderly. The most common cause of this illness is Alzheimer’s disease 30percent to70percent of these children will (AD). continue to experience ADHD symptoms as The earliest symptoms are forgetfulness and memory loss; adults. disorientation to time or place; and difficulty with concentration, calculation, language, By definition, symptoms of ADHD appear and judgment. Some patients have severe before age 7, last for six months or longer, behavioral disturbances and may even and impair normal functioning in at least two become psychotic. The illness is areas of a child’s life—at school, among progressive. In the final stages, the affected friends, or at home, for example. Adults must individual is in capable of self-care. show impairment at home and at work. Unfortunately, no effective treatments exist, Normal children sometimes show similar and patients usually die from pneumonia or behavior, and other conditions, disorders, or some other complication. environmental triggers— such as novelty— may also be present with ADHD children; In the earliest stages, the clinical therefore, diagnosis requires a diagnosis of possible or probable AD can be comprehensive evaluation, using parent and teacher rating scales, a clinical interview, and testing. Currently, ADHD is diagnosed solely on the basis of behavioral symptoms. Some studies show a correlation between ADHD and changes in brain structure, suggesting the possibility of using neuroimaging techniques in the future to help identify targets for treatment or to help distinguish ADHD symptoms from those
57 made with greater than 80 percent accuracy. clinical symptoms. As the course of the disease progresses, the accuracy of diagnosis at Alzheimer’s Rare individuals with AD have a research centers exceeds 90 percent. The dominantly inherited form of the disease. diagnosis depends on medical history, These patients often have early-onset physical and neurological examinations, illness. Recently, scientists have identified psychological testing, laboratory tests, and mutations in AD-linked genes on three brain imaging studies. At present, final chromosomes. The gene encoding the confirmation of the diagnosis requires amyloid precursor protein is on chromosome examination of brain tissue, usually obtained 21. In other families with early onset AD, at autopsy. mutations have been identified in the presenilin 1 and 2 genes, which are on The causes and mechanisms of the brain chromosomes 14 and 1, respectively. abnormalities are not yet fully understood, Apolipoprotein E (apoE), a chromosome 19 but great progress has been made through gene, which influences susceptibility in late genetics, biochemistry, cell biology, and life, exists in three forms, with apoE4 experimental treatments. Microscopic clearly associated with enhanced risk. examination of AD brain tissue shows Treatments are available mostly only for abnormal accumulations of a small fibrillar some symptoms of AD, such as agitation, peptide, termed a beta amyloid, in the anxiety, unpredictable behavior, sleep spaces around synapses (neuritic plaques) disturbances, and depression. Three drugs and by abnormal accumulations of a treat cognitive symptoms in patients with modified form of the protein tau in the cell mild to moderate Alzheimer’s. These agents bodies of neurons (neurofibrillary tangles improve memory deficits temporarily. (=twisted together into a jumbled mass)) are mostly in Several other approaches, such as brain regions important for memory and antioxidants, are being tested. intellectual functions. An exciting area of research is the use of In cases of AD, reductions occur in levels approaches in which genes are introduced in of markers for several neurotransmitters, mice. These transgenic mice carrying mutant including acetylcholine, somatostatin, genes linked to inherited AD develop monoamine, and glutamate, that allow cells behavioral abnormalities and some of the to communicate with one another. Damage cellular changes that occur in humans. It is to these neural systems, which are critical for anticipated that these mice models will prove attention, memory, learning, and higher very useful for studying the mechanisms of cognitive abilities, is believed to cause the AD and testing novel therapies. Moreover, researchers have begun to knock out genes playing critical roles in the
58 production of amyloid. These enzymes, syndrome, severely sore paws from termed beta and gamma secretase, which compulsive licking, respond to the cleave the amyloid peptide from the serotonergic antidepressant clomipramine, precursor, are clearly targets for which was the first effective treatment development of drugs to block amyloid. developed for OCD in people. Anxiety disorders Serotonergic antidepressants especially the tricyclics, such as clomipramine, and the The most widespread mental illnesses, selective serotonin reuptake inhibitors anxiety disorders annually affect an (SSRIs), such as sertraline (Zoloft) and estimated 12.6 percent of the adult paroxetine (Paxil)—are effective in treating population. They include phobias such as OCD. A specialized type of behavioral fear of heights, agoraphobia, and social intervention, exposure and response anxiety disorder; generalized anxiety prevention is also effective in many patients. disorder; post-traumatic stress disorder; panic disorder; and obsessive-compulsive Panic disorder, which affects large disorder (OCD). Some can keep people number of people usually, starts “out of the completely housebound or, as in the case of blue.” Patients experience an overwhelming panic disorder, contribute to suicide. Many of sense of impending doom, accompanied by these disorders occur with depression, and sweating, weakness, dizziness, and individuals so afflicted are at high risk of shortness of breath. With repeated attacks, suicide. patients may develop anxiety in anticipation of another attack and avoid public settings In OCD, people become trapped, often for where attacks might occur. If these patients many years, in repetitive thoughts and are untreated, their lives may constrict until behaviors, which they recognize as they develop agoraphobia, becoming groundless but cannot stop, such as virtually house bound. repeatedly washing hands, or checking that doors are locked or stoves turned off. Phobia is an intense, irrational fear of a particular object or situation. Individuals can Social learning and genetics likely play a develop phobias of almost anything, such as role in developing the disorder. Positron dogs, dating, or driving over bridges. emission tomography (PET) scans reveal Exposure to the feared object or situation abnormalities in both cortical and deep areas can trigger an extreme fear reaction that may of the brain, suggesting central Nervous include a pounding heart, shortness of system changes in OCD patients. breath, and sweating. Scientists recently discovered that certain Experiencing or witnessing a crime or breeds of large dogs that develop acral lick
59 being a victim of sexual abuse can lead to a a highly structured environment and form of stress that can last a lifetime. specialized education and language Termed post-traumatic stress disorder, the intervention programs. The earlier the condition afflicts large number of people interventions begin, the better the outcome. aged 18 to 54 each year. Currently, autism is diagnosed on the The recent discovery of brain receptors basis of behavioral symptoms. New research for the benzodiazepine antianxiety drugs has shows that brain imaging data is 95 percent sparked research to identify the brain’s own accurate in identifying how the brains of antianxiety chemical messengers. This individuals with autism differ from those of finding may lead to ways to regulate this typically developing young children. brain system and correct its possible defects in panic anxiety disorders. PET Research has also revealed that genetic scans reveal that during such attacks, the tip factors contribute to the development of of the brain’s temporal lobe is unusually autism. Success in identifying the so-called active compared with controls. When normal “vulnerability” genes for autism may allow people expect to receive a shock to the scientists to develop an improved diagnostic finger, the same general area is activated. technique that combines the detection of behavioral indicators with biological The SSRIs, the serotonin-norepinephrine abnormalities to better identify infants and reuptake inhibitors (SSRNs), cognitive toddlers at risk for autism. behavior therapy, or a combination of these are now the first-choice treatments of most A clear understanding of the biological anxiety disorders. Tricyclic antidepressants, abnormalities that alter brain development monoamine oxidase inhibitors (MAOIs), and in autism could guide the formulation of new high-potency benzodiazepines are also therapies that target the disorder on a effective for many of these disorders. molecular level. These research efforts will mean that health-care practitioners will be Autism better armed with the necessary tools for early diagnosis and more effective Characterized by communication interventions. difficulties, delayed development of language, impaired social skills, Brain tumors communication difficulties, and narrow, obsessive interests or repetitive behaviors, Although brain tumors are not always autism is extremely isolating. There is no malignant—a condition that spreads and cure, but children with autism respond well to becomes potentially lethal—these growths are always serious because they can cause pressure in the brain and compression of nearby structures, interfering with normal
60 brain activity. restricted; immunotherapy, which uses the Primary brain tumors arise within the body’s own immune system against the tumor; gene therapy, in which bioengineered brain, whereas secondary brain tumors genes are delivered to the cancer cells to kill spread from other parts of the body through them; and several approaches for a targeted the blood stream. For tumors starting in the delivery of antibodies, toxins, or growth brain, about 60 percent of which are inhibiting molecules that attach specifically to malignant, the cause is unknown. Tumors the cancer cells and kill them. that begin as cancer elsewhere and then spread to the brain are always malignant. Down syndrome Symptoms vary according to location and Down syndrome, the most frequently size. The compression of brain tissue or occurring chromosomal abnormality, nerve tracts, as well as expansion of the appears in one of every 800 to 1,000babies. tumor, can cause symptoms such as It occurs when an extra copy of chromosome seizures, headaches, muscle weakness, loss 21—or part of its long arm—is present in the of vision or other sensory problems, and egg or, less commonly, in the sperm, at the speech difficulties. An expanding tumor can time of conception. It is not known why this increase pressure within the skull, causing error occurs, and it is not linked to any headache, vomiting, visual disturbances, and environmental or behavioral factors, either impaired mental functioning. Brain tumors before or during pregnancy, but the risk is are diagnosed with MRI (magnetic markedly increased with the age of the resonance imaging) and CT (computed mother. At age 35, the risk is about one in tomography) scanning. 365 births; at age 40,it is one in 110. It is important to note, however, that the average Surgery is a common treatment if the age of women who give birth to children with tumor is accessible and vital structures will Down syndrome is 28, because younger not be disturbed. Radiation is used to stop a women give birth more often. Prenatal tumor’s growth or cause it to shrink. screening tests, such as the Triple Screen Chemotherapy destroys tumor cells that may and Alpha fetaproteinPlus, can accurately remain after surgery and radiation. Steroid detect Down syndrome in about 60 percent drugs relieve swelling and other symptoms. of fetuses. Available treatments are primarily Down syndrome is associated with palliative and at best prolong life by a few weeks. A number of promising experimental therapies, however, are currently being explored. These include antiangiogenic therapy, in which the tumor’s blood supply is
61 approximately 50 physical and understood, they hope to decode the biochemical processes that occur in Down developmental characteristics. An individual syndrome and learn to treat or cure this disorder. with Down syndrome is likely to possess, to Learning disorders various degrees, some of these Learning disabilities include difficulties in the acquisition and use of listening, speaking, characteristics: mild to moderate mental reading, writing, reasoning, or mathematical abilities. These challenges often occur in retardation; low muscle tone; an upward people with normal or high intelligence. slant to the eyes; a flat facial profile; an Dyslexia, or specific reading disability, is the most common and most carefully studied enlarged tongue and an increased risk of of the learning disabilities. It affects 80 percent of all of those identified as learning congenital heart defects, respiratory disabled. Dyslexia is characterized by an unexpected difficulty in reading in children problems, and digestive tract obstruction. All and adults who otherwise possess the intelligence, motivation, and schooling people with Down syndrome show the considered necessary for accurate and fluent neuropathological changes of Alzheimer’s reading. Studies indicate that dyslexia is a persistent, chronic condition. It does not disease by age 40, and most show cognitive represent a transient “developmental lag.” decline by age 60. Babies with Down syndrome develop much like typical children, but at a somewhat slower rate. They will learn to sit, walk, talk, and toilet train, just like their peers. Early intervention programs can begin shortly after birth and can help foster an infant’s development. Thanks to medical advances and a greater understanding of the potential of those with this condition, people with Down syndrome have been able to have longer and fuller lives. Individuals with Down syndrome are being educated in their neighborhood schools, participating in community activities, and finding rewarding employment and relationships. Although there is no cure for or means of preventing Down syndrome, scientists are moving closer to understanding the role that the genes on chromosome 21 play in a person’s development. Once this mystery is
62 Reading Therapy from those who are poor readers. Scientists believe that people with dyslexia A range of investigations indicates that and other reading problems often have difficulty breaking words into their various there are differences in brain regions constituent sounds. Because they cannot between dyslexic and non impaired readers, distinguish these sounds from one another, especially the temporo - parieto - occipital dyslexics often do not associate the sounds and frontal opercular regions. Recent data with the corresponding letters that would using functional brain imaging indicate that enable them to read words. In this photo, a dyslexic readers demonstrate a functional child with reading problems participates in a disruption in an extensive system in the therapy program designed to stimulate and posterior portion of the brain. The disruption correct the part of her brain that processes occurs within the neural systems linking simple sounds and recognizes them as visual representations of the letters to the speech. phonologic structures they represent. The specific cause of the disruption of neural . There is now a strong consensus that the systems in dyslexia is thought to result from developmental missteps relating to neuronal central difficulty in most forms of dyslexia migration to the cerebral cortex. It is clear that dyslexia runs in families, and active reflects a deficit within the language research aims to identify what appear to be system—and more specifically, in a several dyslexia-susceptibility genes. component of the language system called Interventions to help children with dyslexia focus on teaching the child that words can phonology. This is illustrated in difficulty be segmented into smaller units of sound and that these sounds are linked with transforming the letters on the page to the specific letter patterns. In addition, children with dyslexia require practice in reading sounds of language. A current debate exists stories, both to allow them to apply their newly acquired decoding skills to reading as to whether this difficulty reflects a general words in context and to experience reading for meaning. sound processing deficit or a problem Neurological AIDS specific to sounds of language, phonemes, In 2003, about 4.8 million people became infected with human immunodeficiency virus and conscious awareness of these sounds. (HIV), the largest number since the onset of the AIDS epidemic; 38 million are now living As children approach adolescence, one manifestation of dyslexia may be a very slow reading rate. Children may learn to read words accurately, but they will not be fluent or automatic, reflecting the lingering effects of a phonologic deficit. Because they can read words accurately—although very slowly—dyslexic adolescents and young adults may mistakenly be assumed to have “outgrown” their dyslexia. The ability to read aloud accurately, rapidly, and with good expression, as well as facility with spelling, may be most useful clinically in distinguishing students who are average
63 with HIV. The epidemic is still the most their mental processes. At the same time, patients may develop leg weakness and a intense in sub-Saharan Africa but is gaining loss of balance. Imaging techniques, such as computed tomography (CT) and magnetic speed in Asia and Eastern Europe. The resonance imaging (MRI), show that the brains in these patients have undergone impact of AIDS in the United States has some shrinkage. The examination of brain cells under a microscope suggests that been muted because of life-prolonging abnormalities are present principally in subcortical areas. Neurons in the cortex drugs,but in developing countries only also may be altered or lost, however. 400,000 of 6 million people are receiving Recent studies indicate that highly active combination antiretroviral treatment— such treatment. Women now represent cocktails of three or more drugs active against HIV—is effective in reducing the nearly half of all worldwide cases. incidence of AIDS dementia. Such treatment also can effectively reverse the cognitive Although the principal target of HIV is the abnormalities attributed to brain HIV infection. immune system, the nervous system also Peripheral neuropathy is also a major may be profoundly affected. Some 20 neurological problem seen commonly in HIV patients. It is believed that the virus triggers percent to 40 percent of patients with full a distal sensory neuropathy through neurotoxic mechanisms. This has often blown AIDS also develop clinically been unmasked or exacerbated by certain of the anti retroviral drugs that have significant dementia that includes movement mitochondrial toxicity and tend to make the neuropathies more frequent and serious. In impairment, with a smaller percentage still current cohorts of advanced patients, more than half have neuropathy, making it a suffering from an overt dementia. Those major area for preventive and symptomatic therapeutic trials. affected have mental problems ranging from Despite remarkable progress in mild difficulty with concentration or developing therapy, some patients develop these problems and fail to respond to coordination to progressive, fatal dementia. Despite advances in treating other aspects of the disease, AIDS dementia remains incompletely understood. Most current hypotheses center on an indirect effect of HIV infection related to secreted viral products or cell-coded signal molecules called cytokines. Nonetheless, HIV infection appears to be the prime mover in this disorder because antiviral treatment may prevent or reverse this condition in some patients. Experts believe that serious neurologic symptoms are uncommon early in HIV/AIDS infection. Later, however, patients develop difficulty with concentration and memory and experience general slowing of
64 treatment, thus requiring additional Patients so injured are not allowed to lie flat on their backs in bed. Rather, they are approaches to prevention and treatment of positioned in a modified sitting position, which raises the head to reduce pressure these symptoms. In addition, because of effects within the skull. immunodeficiency in HIV patients, encountering otherwise rare opportunistic infections and malignancies is still relatively Treatments for increases in intracranial common. pressure include the removal of Neurological trauma cerebrospinal fluid, moderate No magic bullet has yet been found, but hyperventilation to decrease blood volume, doctors have discovered several methods to stave off severe neurological damage and the administration of drugs to reduce caused by head and spinal cord injuries and to improve neurological function. These cellular metabolism or to remove water from treatments include better imaging techniques, methods to understand and the injured tissue. Treatments for the injury improve the brain’s ability to regenerate and repair itself, and improved rehabilitation induced reduction of cerebral blood flow techniques. include the administration of drugs that Greater access to and use of computed tomography (CT) and magnetic resonance increase mean arterial blood pressure. In imaging (MRI) offer physicians the opportunity to diagnose the extent of trauma combination with the reduction in intracranial and to avoid secondary injury related to edema, or swelling, and a reduction in blood pressure, these results in an increase in flow to the brain (ischemia). In general, patients who arrive in the emergency room blood flow, allowing more blood to reach vital and are diagnosed with a severe head injury have a pressure monitoring device inserted areas. in to their brain, usually within the lateral ventricle. As swelling progresses, the CT or In addition to helping the physician avoid MRI images of the brain show the surface of the brain being pressed against the inside of cerebral edema and reductions in cerebral the skull. This pressure inside the skull increases and can become life-threatening. blood flow following traumatic brain injury, imaging can reveal mass lesions produced by the initial injury. These mass lesions can consist of bleeding on the surface or within the brain as well as the formation of contusions. Once blood leaves its respective vessels and comes into direct contact with brain tissue, it can add focal pressure, thereby reducing cerebral blood flow, or it can by itself be toxic to brain cells. As a consequence, once detected, it is usually surgically removed. Contusions* can also be
65 troubling, because they can increase promote nerve regeneration after brain and spinal cord injury. Although these new pressure as well as contribute to the therapies have not yet reached the clinic, several approaches are on the path to development of post-traumatic epilepsy. clinical trials. Depending on the location and type, they are Schizophrenia also candidates for surgical removal. Marked by disturbances in thinking, emotional reactions, and social behavior, Researchers have found that people who schizophrenia usually results in chronic suffer spinal cord injuries become less illness and personality change. Delusions, severely impaired if they receive high hallucinations, and thought disorder are intravenous doses of a commonly used common. steroid drug, methylprednisolone, within eight hours of the injury. Building on this Schizophrenia is thought to reflect knowledge, researchers hope to decipher the precise order of chemical reactions that changes in the brain, possibly caused by lead to damage and to develop new therapies to block these reactions. disease or injury at the time of birth, and a Scientists have known that, after a spinal genetic disposition that may be exacerbated cord injury, animals can regain the ability to bear their weight and walk at various speeds by environmental stress. Recently, several on a treadmill belt. More recently, scientists have recognized that the level of this genes have been identified that appear to recovery depends to a large degree on whether these tasks are practiced—that is, increase the risk of developing trained for—after injury. It appears that humans with spinal cord injury also respond schizophrenia. Brain systems using the to training interventions. chemicals dopamine, glutamate, and GABA Recently, scientists have discovered that new nerve cells can be born in the adult appear to be particularly involved in the brain, but these new cells do not seem capable of helping the injured brain pathogenesis of the disorder. Brain scans regenerate. Studies are underway to determine how to “jumpstart” the pathway and postmortem studies show abnormalities that stimulates neurogenesis, the birth of new nerve cells. Researchers are also trying in some people with schizophrenia, such as to decipher how certain environmental cues can be used or overcome to attract these enlarged ventricles (fluid-filled spaces) and new cells—or transplanted stem or progenitor cells—to areas of brain injury to reduced size of certain brain regions. facilitate regeneration and repair. Functional neuroimaging scans such as These and other recent discoveries are positron emission tomography (PET) and pointing the way toward new therapies to functional magnetic resonance imaging (fMRI) taken during intellectual tasks show abnormal functioning in specific brain areas of people with this illness.
66 The disorder usually begins between the approximately 30 percent of patients who are ages of 15 and 25. Some patients fully not helped by conventional medications. The recover following treatment, but most drug can, however, induce a potentially fatal continue to have moderate or severe blood disorder, agranulocytosis, in about 1 symptoms, particularly in response to stress. percent of patients. To prevent this disorder, About 15 percent of patients return to normal patients must take regular weekly to life after a single episode, 60 percent will biweekly blood tests, a precaution that have intermittent episodes throughout their makes using the drug very costly. lives, and another 25 percent will not recover their ability to live as independent Several new antipsychotics— adults. Deficits in cognition, particularly risperidone, olanzapine, quetiapine, involving attention and memory, are ziprasadone, and aripiprazole—offer some of frequent, lifelong manifestations in most the benefits of clozapine without risk of patients, even in those who show good angranulocytosis, but their long term side recovery from acute symptoms. effect profiles are not fully known. After a long search for an effective Stroke antipsychotic medication, scientists synthesized the drug chlorpromazine during Until recently, if you or a loved one had a the late 1940s. Bythe1950s, it was found to stroke, your doctor would tell your family be useful in treating psychotic states and there was no treatment. In all likelihood, the later became a mainstay of drug treatment. patient would live out the remaining months Since that time, many agents similar to or years with severe neurological chlorpromazine have been developed. When impairment. given as long-acting injections, these drugs reduce some symptoms and aid patients’ This dismal scenario is now brightening. readiness for adjustment back into their For one, use of the clot dissolving communities. Chronic use of the drugs, bioengineered drug, tissue plasminogen however, may cause abnormal muscle activator (tPA), is now a standard treatment movements and tremors in some patients. in many hospitals. This approach rapidly Safer treatments are being sought. opens blocked vessels to restore circulation before oxygen loss causes permanent Thus far, most drugs are successful in damage. Given within three hours of a treating hallucinations and thought disorder. stroke, it often can help in limiting the Clozapine acts somewhat differently from ensuing brain damage. Also, attitudes about other antipsychotic drugs. It treats the the nation’s third leading cause of death are changing rapidly. Much of this has come from new and better understandings of the mechanisms that lead to the death of
67 neurons following stroke and devising ways Controlling risk factors such as obesity, to protect these neurons. A stroke occurs when a blood vessel bringing oxygen and blood pressure, diabetes, and high nutrients to the brain bursts or is clogged by a blood clot or some other particle. This cholesterol can help prevent stroke. Other deprives the brain of blood, causing the death of neurons within minutes. Depending specific treatments involving surgery can on its location, a stroke can cause many permanent disorders, such as paralysis on clear clogs in the arteries of the neck region one side of the body and loss of speech. and help prevent a cut off of blood supply. Stroke often occurs in individuals over 65 Treatments that target the heart’s blood years of age, yet a third are younger. Stroke tends to occur more in males and blacks and flow can prevent stroke. Surgery can help in those with diabetes, high blood pressure, repair damaged heart valves. Anticoagulant heart disease, obesity, high cholesterol, and drugs can reduce the chance of clots a family history of stroke. forming, traveling to the brain and causing a stroke. In addition to tPA, increased use of preventive measures is battling the disorder. Other experimental therapies under investigation may lead to even bigger payoffs for patients in the future. Some strategies target mechanisms inside the neuron. In this way, the vicious cycle of local damage followed by a widening
68 STROKE. A stroke occurs when a blood vessel bringing oxygen and nutrients to the brain bursts or is clogged by a blood clot (1). This lack of blood leads to a cascade of petrochemical abnormalities that can cause cell death within minutes. Free radicals are released, causing damage to endothelial cells (2) and the mitochondria (3) of neurons. Normally the body readily disarms free radicals (4), but in stroke, endothelial cell damage allows many more than can be controlled to move into brain tissue. Depending on its location, a stroke can have different symptoms such as paralysis on one side of the body or a loss of speech
69 fringe of biochemical-induced neuronal death can be slowed. A number of classes of drugs have been shown to be effective in animal studies. Another promising possibility is the use of neural stem cells. Some animal studies have shown that an injection of stem cells aids recovery even if administered several days after the injury. Administration of growth factors may further enhance the benefits of stem cell transplantation.
70 Chapter- 10 These and other methods have resulted in many advances in knowledge about the New diagnostic workings of the nervous system and are still methods useful today. New methods, safely applicable to humans, promise to give even more Many of the recent advances in precise information. understanding the brain are due to the development of techniques that allow Imaging techniques scientists to directly monitor neurons throughout the body. Positron emission tomography (PET) PET Electrophysiological recordings trace brain is one of the most important techniques for electrical activity in response to a specific measuring blood flow or energy consumption external stimulus. In this method, electrodes in the brain. This method of measuring brain placed in specific parts of the brain— function is based on the detection of depending on which sensory system is being radioactivity emitted when positrons, tested—make recordings that are then positively charged particles, undergo processed by a computer. The computer radioactive decay in the brain. Small makes an analysis based on the time lapse amounts of radiation are introduced into the between stimulus and response. It then blood, which is then taken up into different extracts this information from background brain areas in proportion to how hard the activity. neurons are working. Computers build three dimensional images of the brain based on Following the discovery that material is the amount of radiation emitted in these transported within neurons, methods have different areas. been developed to visualize activity and precisely track fiber connections within the PET studies have helped scientists nervous system. This can be done by understand more about how drugs affect the injecting a radioactive amino acid into the brain and what happens during learning, brain of an experimental animal; the animal is when using language, and in certain brain killed a few hours later, and then the disorders—such as stroke, depression, and presence of radioactive cells is visualized on Parkinson’s disease. Within the next few film. In another technique, the enzyme years, PET could enable scientists to identify horseradish peroxidase is injected and taken the biochemical nature of neurological and up by nerve fibers that can be later identified mental disorders and determine how well under a microscope. therapy is working in patients. PET has revealed marked changes in the depressed brain. Knowing the location of these changes helps researchers understand the causes of
71 depression and monitor the effectiveness of the background field. As they swing back, specific treatments. they create a signal that can be picked up Another technique, single photon emission computed tomography (SPECT), is similar to and converted into an image. Tissue that PET, but its pictures are not as detailed. contains a lot of water and fat produces a SPECT is much less expensive than PET because the tracers it uses have a longer bright image; tissue that contains little or no half-life and do not require an accelerator nearby to produce them. water, such as bone, appears black. Magnetic resonance imaging (MRI) MRI images can be constructed in any Providing a high quality, three-dimensional image of organs and structures inside the plane, and the technique is particularly body without X-rays or other radiation, MRIs are unsurpassed in anatomical detail and valuable in studying the brain and spinal may reveal minute changes that occur over time. cord. It reveals the precise extent of tumors MRIs tell scientists when structural rapidly and vividly. And MRI provides early abnormalities first appear in the course of a disease, how they affect subsequent evidence of potential damage from stroke, development, and precisely how their progression correlates with mental and allowing physicians to administer proper emotional aspects of a disorder. treatments early. During the 15-minute MRI procedure, a patient lies inside a massive, hollow, Magnetic resonance spectroscopy (MRS), cylindrical magnet and is exposed to a powerful, steady magnetic field. Different a technique related to MRI, uses the same atoms in the brain resonate to different frequencies of magnetic fields. In MRI, a machinery but measures the concentration of background magnetic field lines up all the atoms in the brain. A second magnetic field, specific chemicals—such as oriented differently from the background field, is turned on and off many times a second; at neurotransmitters—in different parts of the certain pulse rates, particular atoms resonate and line up with this second field. When the brain instead of blood flow. MRS also holds second field is turned off, the atoms that were lined up with it swing back to align with great promise: by measuring the molecular and metabolic changes that occur in the brain, this technique has already provided new information on brain development and aging, Alzheimer’s disease, schizophrenia, autism, and stroke. Because it is noninvasive, MRS is ideal for studying the natural course of a disease or its response to treatment. Functional magnetic resonance imaging (fMRI) Among the most popular neuroimaging techniques today is fMRI. This technique compares brain activity under resting and activated conditions. It combines the high-spatial-resolution, noninvasive imaging of brain anatomy offered by standard MRI with a strategy for detecting increases
72 in blood oxygen levels when brain activity activity in a particular task, whereas MEG brings fresh blood to a particular area of the tells researchers and physicians when brain. This technique allows for more detailed certain areas become active. Together, this maps of brain areas underlying human information leads to a much more precise mental activities in health and disease. To understanding of how the brain works in date, fMRI has been applied to the study of health and disease. various functions of the brain, ranging from primary sensory responses to cognitive Optical imaging techniques Optical activities. imaging relies on shining weak lasers through the skull to visualize brain activity. Magnetoencephalography (MEG) MEG These techniques are inexpensive and is a recently developed technique that relatively portable. They are also silent and reveals the source of weak magnetic fields safe: Because only extremely weak lasers emitted by neurons. An array of cylinder- are used, they can be used even to study shaped sensors monitors the magnetic field young infants. In a technique called near pattern near the patient’s head to determine infrared spectroscopy (NIRS), technicians the position and strength of activity in various shine lasers through the skull at near infrared regions of the brain. In contrast with other frequencies, which renders the skull imaging techniques, MEG can characterize transparent. Blood with oxygen in it absorbs rapidly changing patterns of neural activity— different frequencies of light than blood in down to millisecond resolution—and can which the oxygen has been consumed. By provide a quantitative mea- sure of the observing how much light is reflected back strength of this activity in individual subjects. from the brain at each frequency, Moreover, by presenting stimuli at various researchers can track blood flow. Diffuse rates, scientists can deter mine how long optical tomography is then used to create neural activation is sustained in the diverse maps of brain activity. A related technique, brain areas that respond. the event-related optical signal, records how light is scattered in response to cellular One of the most exciting developments in changes that arise when neurons fire and imaging is the combined use of information potentially can assess very fast—well under from fMRI and MEG. The former provides a second— changes in neural activity. detailed information about the areas of brain
73 Chapter- 11 Control of behavior by electrical stimulation brain THE MIND – BRAIN RELATIONSHIP In 1870 Fritsch and Hitzing reported that mild nondestructive The biological explanation to human electrical stimulation of the portions of behavior is a recent school of thinking dog’s cerebral cortex could cause and scientific enquiry. Still it is hard to muscle movements. Movements were believe that our thoughts and actions always on the side of the body opposite are the result of physical processes in to the stimulation. Depending on the the brain. There are many philosophic exact point stimulated, the dog would views to explain the mind body relation move its neck, back, abdomen, tail, leg or mind brain relation. These views have or some other parts of the body. their on merits and demerits. Some of Repeated stimulation on the same point the important philosophical views of consistently elicited the same response. mind brain relations are as follows: Electrical activity occurs naturally Dualism : Holds that mind and brain are in the brain at al times. Nerves carrying fundamentally deferent and each can exist messages from the sense organs cause independently of each other. this electrical activity. Their impulses combine with the electrical activity Monism : Holds that universe consists of already present in certain areas of the only one kind of substance variously viewed brain to produce activity in other areas, as either physical, mental or some which then activates still other areas. combination of the two. Eventually, the areas of the brain that control movements generate activities. These two versions of mind brain relationship brings only vague ideas and Electrical stimulation of the brain they are unscientific too. The mind brain can evoke not only simple muscle relationship is exceedingly difficult to movements but also more complex investigate scientifically. The scientific sequences of behaviour, particularly if evidences come from a variety of electrical the animal is awake during the stimulation experiments conducted on the stimulation and free to move about. brain. If stimulating brain areas can elicit a Scientists implanted electrodes wide variety of experiences or behavioral permanently into the lower parts of the changes then brain activity must be brain of the chicken and passed week responsible for mental events. electric current to the brain. Stimulation of various areas elicited such behaviour as feeding, drinking, cackling, grooming, turning the body to one side, sitting down, sleeping, escape flight and aggressive attack. Similarly it was also found that electrical stimulation of certain brain areas could induce an animal to fall asleep suddenly. The behaviour elicited depends on exact location of electrode and intensity of
74 stimulation. The elicited behaviour also varied depending on environmental stimuli. Eg. Stimulation at a particular point of brain in rats caused eating if only food was present and drinking if only water was present. Electrical stimulation of the human brain Though we cannot conduct experiments on human brain, medical purposes allow to investigate the electrical stimulation of human brain. The point of origin of epilepsy has been identified in this way. People suffering from severe pain and depression were found relief by receiving electrical stimulation on the so-called pleasure areas of the brain. R G Heath in 1964 experimented a patient by stimulating the septal areas of the brain and found that there was a sudden change in the mood from depression to anger. Because electrical stimulation in the brain can elicit not only sensations and movements but also emotional changes it appears that brain activity is responsible for what will be called mind.
75 Chapter- 12 In this method electrode are implanted in the brain and briefly stimulated certain METHODS OF INVESTIGATING areas. The resultant behaviour is HOW THE BRAIN CONTROLS studied and their relation is established between the stimulated area. Eg BEHAVIOUR stimulating the lateral hypothalamus increases eating From the ‘phrenologist’ view of behaviour scientists have advanced a 5 Labeling the brain activity. long way in research of brain areas and a) Auto radiography:-. its relationship with behaviour. A Auto radiography is method of number of techniques have been determining where a chemical is located developed in pursuit of this goal. in the brain by processing the signal 1) Stereotaxic instruments produced radioactively by that chemical Investigators some times wish to study Eg. 2 Deoxy- D-glucose – (radioactive) the effect of stimulating or damaging or is injected into blood and the neuron recording the activity of deeper part of take up them for metabolism. The the brain of animals. The Stereotaxic metabolism of it can be auto radio instrument is provided with scale and graphed. electrodes. Researchers refer to a b) Immunohistochemstry. previously formed map called Stereotaxic atlas to help this process of It involves immunological, histological study. and chemical techniques Eg. Acetylcholine receptor from monkey 2) Lesion and ablation is injected to rabbit. Antibodies are A lesion is the destruction or functional collected and labeled. It is then treated disruption of an area of the brain. An with slice of monkeys brain to find out ablation is the removal of a part of the the distribution pattern of Acetylcholine brain. Both of which produce behavioral receptor distribution pattern in the brain . deficit indicating the role of that part of the brain in developing the lost Studies on the structure of living character. A small window on the skull is human brains. cut to remove the brain or part either by Non invasive technique to study live cutting or suction. human brain 3) Histological techniques i) Computerized Axial After the lesion test is conducted the Topography(CAT scan) brain is removed and embedded using X- rays are passed through the brain at chemicals. And sliced on microtoms. every one degree of the brain till 1800 These slices are stained to study the completed. Computer reconstructs the lesioned part images. Dyes and ionic compounds are used for better contrast. This technique 4 Stimulation and recording from the is used to identify Alzheimer’s disease, brain Schizophrenia etc ii)Magnetic Resonance Imaging (MRI)
76 High degree resonance images of the brain are produced without exposing the brain to any radiation. MRI reveals structural deficit, like enlarged ventricles. It is slow (15 minutes) and so only stationary structures are visualized. Eco planar MRI is a fast technique to record blood flow Measurement of human brain activity At any moment certain areas of the brain are more active. To record this many noninvasive techniques are available i) Electro Encephalo Graph ( EEG) Records electrical activities of brain . Identifies epilepsy, tumor, or other medical problem and sleep, awake, dreaming or excited state. ii) Positron Emission Tomography ( PET scan ) Provides a high-resolution image of brain activity. PETS is based on the positron emission due to decay of radio active elements (Radioactive glucose is injected before PET Scan) PETS helps to identify tumors, epilepsy, and other disorders that alter metabolic rate of a given brain area. It was used to study the time when various brain structures become active infants.
77 Chapter- 13 smaller somewhat spherical cerebellum (lower right), and the THE BRAIN AND ITS brainstem (center). Prominent in the CAPACITIES brainstem are the Brain is a portion of the central medulla oblongata (the egg-shaped nervous system contained within the enlargement at center) and the skull. The brain is the control center thalamus (between the medulla and for movement, sleep, hunger, thirst, the cerebrum). The cerebrum is and virtually every other vital activity responsible for intelligence and necessary to survival. All human reasoning. The cerebellum helps to emotions—including love, hate, fear, maintain balance and posture. The anger, elation, and sadness—are medulla is involved in maintaining controlled by the brain. It also involuntary functions such as receives and interprets the countless respiration, and the thalamus acts as signals that are sent to it from other a relay center for electrical impulses parts of the body and from the traveling to and from the cerebral external environment. The brain cortex. makes us conscious, emotional, and intelligent. . The human brain has three major Left and Right Brain Functions Although the structural components: the large cerebrum is symmetrical in structure, with dome-shaped cerebrum (top), the two lobes emerging from the brain stem and matching motor and sensory areas in each, certain intellectual functions are restricted to one hemisphere. A person’s dominant hemisphere is usually occupied with language and logical operations, while the other hemisphere controls emotion and artistic and spatial skills. In nearly all right-
78 handed and many left-handed people, the region of the motor area controls the left hemisphere is dominant. movement of the right leg. The middle region controls right side of the trunk, head, right arm and hand. Lower region, which is the larger part, controls the muscles in the right side of the face. And the right motor area controls the left side. Behaviour and emotion area Language area It is the most complicated are of the brain The language area of the brain works known as the frontal cortex, responsible for together with the speech area. The first is intellect, learning and personality. It also responsible for the understanding of spoken produces reasoning, conscience and and written language. Messages are sent judgment, the qualities that make human from here to the speech area, then to the more advanced than animals. Development voice box, tongue and lip. This process of this area is affected directly by our translates thoughts of what we want to say environment and upbringing. into spoken words. The motor area Hearing area The motor area in our brain controls the The hearing area of our brain is also known movement of different parts of the body. as the auditory cortex receives nerve The amount(size) of motor area devoted to impulses from our ears. It interprets the a particular movement depends on the skill volume, rhythm, and pitch of each sound and precision of the movement. The upper and compares each new sound with the earlier hearing experiences. This is how a person recognize words, music, and other sounds.
79 Information processing area the main link between the brain and the rest of our body. This area on the sides of our brain includes several small parts. Together, they receive The taste area and organize information send from our seeing and hearing sensors. Eg. The brain The taste area is located to the sides of the recognizes the feel of an object and the brain, at the base of the sensory area. meaning of written and spoken words. Messages about the taste of the food travel here from the taste buds on the tongue. The The seeing area interpretation of the taste is also affected by the sense of smell. Sensors in the nose The seeing area of the brain, also called the send messages to the brain, which then visual cortex receives electrical impulses analyses the different kinds of smell caused the by light entering our eyes. The seeing area compares the incoming signals The speech area with earlier visual experiences, so that one can recognize patterns of light as familiar The speech area is responsible for the objects. This area also helps to focus the production of words. It is usually situated in eyes. the left half of the cerebral cortex, and sends impulses to the muscles that alter the The balance area shapes and positions of your vocal cords, tongue and lips enabling us to produce This is the second largest area of the brain. sounds during speech Skeletal muscle movements are coordinated here, including the voluntary and involuntary movements. This area also responds to the impulses send from the muscles, which maintain muscle tone. The spinal cord The spinal cord is about 45 cm long and as thick as a finger. It is protected by a column of bones called vertebrae that form the spine or back bone. The spinal cord and the brain together make up the central nervous system and control most of our body activities. The 31 pairs of spinal nerves, which arise from the spinal cord, form the peripheral nervous system. These provide
80 Chapter- 14 neurotransmitter in the body, and neurons that leave the central nervous system (for NEUROTRANSMITTERS example, those running to skeletal muscle) use acetylcholine as their neurotransmitter; Neurotransmitter, chemical made by neurons that run to the heart, blood vessels, neurons, or nerve cells. Neurons send out and other organs may use acetylcholine or neurotransmitters as chemical signals to norepinephrine. Dopamine is involved in the activate or inhibit the function of neighboring movement of muscles, and it controls the cells. Within the central nervous system, secretion of the pituitary hormone prolactin, which consists of the brain and the spinal which triggers milk production in nursing cord, neurotransmitters pass from neuron to mothers. neuron. In the peripheral nervous system, which is made up of the nerves that run The second neurotransmitter family is from the central nervous system to the rest composed of amino acids, organic of the body, the chemical signals pass compounds containing both an amino group between a neuron and an adjacent muscle (NH2) and a carboxylic acid group (COOH). or gland cell. Amino acids that serve as neurotransmitters include glycine, glutamic and aspartic acids, TYPES OF and gamma-amino butyric acid (GABA). Glutamic acid and GABA are the most NEUROTRANSMITTERS abundant neurotransmitters within the central nervous system, and especially in Nine chemical compounds—belonging to the cerebral cortex, which is largely three chemical families—are widely responsible for such higher brain functions as thought and interpreting sensations. recognized as neurotransmitters. In The third neurotransmitter family is addition, certain other body chemicals, composed of peptides, which are compounds that contain at least 2, and including adenosine, histamine, sometimes as many as 100 amino acids. Peptide neurotransmitters are poorly enkephalins, endorphins, and epinephrine, understood, but scientists know that the peptide neurotransmitter called substance P have neurotransmitter like properties. influences the sensation of pain. Experts believe that there are many more In general, each neuron uses only a single compound as its neurotransmitter. However, neurotransmitters as yet undiscovered. some neurons outside the central nervous The first of the three families is composed of amines, a group of compounds containing molecules of carbon, hydrogen, and nitrogen. Among the amine neurotransmitters are acetylcholine, norepinephrine, dopamine, and serotonin. Acetylcholine is the most widely used
81 system are able to release both an amine Neurotransmitters are released into a and a peptide neurotransmitter. microscopic gap, called a synapse, that separates the transmitting neuron from the Neurotransmitters are manufactured from cell receiving the chemical signal. The cell precursor compounds like amino acids, that generates the signal is called the glucose, and the dietary amine called presynaptic cell, while the receiving cell is choline. Neurons modify the structure of termed the postsynaptic cell. these precursor compounds in a series of reactions with enzymes. Neurotransmitters After their release into the synapse, that come from amino acids include neurotransmitters combine chemically with serotonin, which is derived from tryptophan; highly specific protein molecules, termed dopamine and norepinephrine, which are receptors, that are embedded in the surface derived from tyrosine; and glycine, which is membranes of the postsynaptic cell. When derived from threonine. Among the this combination occurs, the voltage, or neurotransmitters made from glucose are electrical force, of the postsynaptic cell is glutamate, aspartate, and GABA. Choline either increased (excited) or decreased serves as the precursor for acetylcholine. (inhibited). When a neuron is in its resting state, its HOW NEUROTRANSMITTERS voltage is about -70 millivolts. An excitatory WORK neurotransmitter alters the membrane of the postsynaptic neuron, making it possible for In the nervous system, a message-carrying ions (electrically charged molecules) to impulse travels from one end of a nerve cell move back and to the other by means of an electrical impulse. When it reaches the terminal end forth across the neuron’s membranes. This of a nerve cell, the impulse triggers tiny flow of ions makes the neuron’s voltage rise sacs called presynaptic vessicles to release toward zero. If enough excitatory receptors their contents, chemical messengers called have been activated, the postsynaptic neurotransmitters. The neurotransmitters neuron responds by firing, generating a float across the synapse, or gap between nerve impulse that causes its own adjacent nerve cells. When they reach the neighboring nerve cell, the neurotransmitters fit into specialized receptor sites much as a key fits into a lock, causing that nerve cell to “fire,” or generate an electric message-carrying impulse. As the message continues through the nervous system, the presynaptic cell absorbs the excess neurotransmitters, and repackages them in presynaptic vessicles in a process called neurostransmitter reuptake.
82 neurotransmitter to be released into the next Once neurotransmitters have been secreted synapse. An inhibitory neurotransmitter into synapses and have passed on their causes different ions to pass back and forth chemical signals, the presynaptic neuron across the postsynaptic neuron’s clears the synapse of neurotransmitter membrane, lowering the nerve cell’s voltage molecules. For example, acetylcholine is to -80 or -90 millivolts. The drop in voltage broken down by the enzyme makes it less likely that the postsynaptic cell acetylcholinesterase into choline and will fire. acetate. Neurotransmitters like dopamine, serotonin, and GABA are removed by a If the postsynaptic cell is a muscle cell physical process called reuptake. In rather than a neuron, an excitatory reuptake, a protein in the presynaptic neurotransmitter will cause the muscle to membrane acts as a sort of sponge, contract. If the postsynaptic cell is a gland causing the neurotransmitters to reenter the cell, an excitatory neurotransmitter will presynaptic neuron, where they can be cause the cell to secrete its contents. broken down by enzymes or repackaged for reuse. While most neurotransmitters interact with ROLES OF NEUROTRANSMITTERS IN their receptors to create new electrical DISEASE nerve impulses that energize or inhibit the adjoining cell, some neurotransmitter Neurotransmitters are known to be involved interactions do not generate or suppress in a number of disorders, including nerve impulses. Instead, they interact with a Alzheimer’s disease. Victims of Alzheimer’s second type of receptor that changes the disease suffer from loss of intellectual internal chemistry of the postsynaptic cell by capacity, disintegration of personality, either causing or blocking the formation of mental confusion, hallucinations, and chemicals called second messenger aggressive—even violent—behavior. These molecules. These second messengers symptoms are the result of progressive regulate the postsynaptic cell’s biochemical degeneration in many types of neurons in processes and enable it to conduct the the brain. Forgetfulness, one of the earliest maintenance necessary to continue symptoms of Alzheimer’s disease, is partly synthesizing neurotransmitters and caused by the destruction of neurons that conducting nerve impulses. Examples of normally release the neurotransmitter second messengers, which are formed and acetylcholine. Medications that increase entirely contained within the postsynaptic brain levels of acetylcholine have helped cell, include cyclic adenosine restore short-term memory and reduce monophosphate, diacylglycerol, and inositol mood swings in some Alzheimer’s patients. phosphates.
83 Neurotransmitters also play a role in reuptake of Serotonin. Serotonin then Parkinson disease, which slowly attacks the remains in the synapse for a longer time, nervous system, causing symptoms that and its ability to act as a signal is prolonged, worsen over time. Fatigue, mental which contributes to the relief of depression confusion, a masklike facial expression, and the control of obsessive-compulsive stooping posture, shuffling gait, and behaviors. problems with eating and speaking are among the difficulties suffered by Parkinson victims. These symptoms have been partly linked to the deterioration and eventual death of neurons that run from the base of the brain to the basal ganglia, a collection of nerve cells that manufacture the neurotransmitter dopamine. The reasons why such neurons die are yet to be understood, but the related symptoms can be alleviated. L-dopa, or levodopa, widely used to treat Parkinson disease, acts as a supplementary precursor for dopamine. It causes the surviving neurons in the basal ganglia to increase their production of dopamine, thereby compensating to some extent for the disabled neurons. Many other effective drugs have been shown to act by influencing neurotransmitter behavior. Some drugs work by interfering with the interactions between neurotransmitters and intestinal receptors. For example, belladonna decreases intestinal cramps in such disorders as irritable bowel syndrome by blocking acetylcholine from combining with receptors. This process reduces nerve signals to the bowel wall, which prevents painful spasms. Other drugs block the reuptake process. One well-known example is the drug fluoxetine (Prozac), which blocks the
84 Chapter- 15 The medulla is broad above, where it joins the pons; and narrows down below, where Gross Anatomy of the Brainstem it becomes continuous with the spinal cord. Its length is about 3 cm and its width is The brainstem consists (from above about 2 cm at its upper end. The junction of downwards) of the midbrain, the pons and the medulla and cord is usually described as the medulla. The midbrain is continuous, lying at the level of the upper border of the above, with the cerebral hemispheres. The atlas vertebra. The medulla is continuous, below, with the spinal cord. Posteriorly, the pons and transition is, in fact, not abrupt but occurs medulla are separated from the cerebellum over a certain distance. The medulla is by the fourth ventricle. The ventricle is divided into a lower closed part, which continuous, below, with the central canal, surrounds the central canal; and an upper which traverses the lower part of the open part, which is related to the lower part medulla, and becomes continuous with the of the fourth ventricle. The surface of the central canal of the spinal cord. Cranially, medulla is marked by a series of fissures or the fourth ventricle is continuous with the sulci that divide it into a number of regions. aqueduct, which passes through the The anterior median fissure and the midbrain. The midbrain, pons and medulla posterior median sulcus are upward are connected to the cerebellum by the continuations of the corresponding features superior, middle and inferior cerebellar seen on the spinal cord. On each side the peduncles, respectively. anterolateral sulcus lies in line with the ventral roots of spinal nerves. The rootlets A number of cranial nerves are attached of the hypoglossal nerve emerge from this to the brainstem. The third and fourth sulcus. The posterolateral nerves emerge from the surface of the midbrain; and the fifth from the pons. The sulcus lies in line with the dorsal nerve roots sixth, seventh and eighth nerves emerge at of spinal nerves, and gives attachment to the junction of the pons and medulla. The rootlets of the glossopharyngeal, vagus and ninth, tenth, eleventh and twelfth cranial accessory nerves. The !egion between the nerves emerge from the surface of the anterior median sulcus and the medulla. The surface of the brainstem is anterolateral sulcus is occupied (on either intimately related to the meninges and to side of the midline) by an elevation called arteries and veins. the pyramid. The elevation is caused by a large bundle of fibres that descend from the The Medulla: gross anatomy cerebral cortex to the spinal cord. Some of these fibres cross from one side to the other in the lower part of the medulla, obliterating the anterior median fissure. These crossing fibres constitute the decussation of the pyramids. Some other fibres emerge from the anterior median fissure, above the decussation, and wind laterally over the surface of the medulla. These are the anterior external arcuate fibres. In the upper part of the medulla, the region between the anterolateral and
85 posterolateral sulci shows a prominent, The Pons: gross anatomy elongated, oval swelling named the olive. This swelling is about half an inch long. It is The pons shows a convex anterior surface, produced by a large mass of grey matter marked by prominent transversely running called the inferior olivary nucleus. The fibres. Laterally, these fibres collect to form posterior part of the medulla, between the a bundle, the middle cerebellar peduncle. posterior median sulcus and the The trigeminal nerve emerges from the posterolateral sulcus, contains tracts that anterior surface, and the point of its enter it from the posterior funiculus of the emergence is taken as a landmark to define spinal cord. These are the fasciculus gracilis the plane of junction between the pons and lying medially, next to the midline, and the the middle cerebellar peduncle. The fasciculus cuneatus lying laterally. These anterior surface of the pons is marked, in fasciculi end in rounded elevations called the midline, by a shallow groove, the sulcus the gracile and cuneatetubercles. These basilaris, which lodges the basilar artery. tubercles are produced by masses of grey The line of junction between the pons and matter called the nucleus gracilis and the the nucleus cuneatus respectively. Just above these tubercles the posterior aspect of the medulla is marked by a groove through medulla is occupied by a triangular fossa which a number of cranial nerves emerge. which forms the lower part of the floor of The abducent nerve emerges from just the fourth ventricle. This fossa is bounded above the pyramid and on either side by the inferior cerebellar peduncle. The lower part of the medulla, runs upwards in close relation to the immediately lateral to the fasciculus anterior surface of the pons. The facial and cuneatus, is marked by another longitudinal vestibulocochlear nerves emerge from the elevation called the tuberculum cinereum. interval between the olive and the pons. This elevation is produced by an underlying The posterior aspect of the pons forms the collection of grey matter called the spinal upper part of the floor of the fourth nucleus of the trigeminal nerve. The grey ventricle. On either side of the lower part of matter of this nucleus is covered by a layer the pons there is a region called the of nerve fibres that form the spinal tract of cerebella-pontine angle. This region lies the trigeminal nerve. near the lateral aperture of the fourth ventricle. The facial, vestibulocochlear and glossopharyngeal nerves, the nervus intermedius, and sometimes the labyrinthine arteries lie in this region. The Midbrain: gross anatomy When the midbrain is viewed from the
86 anterior aspect, we see two large bundles of a membrane, the superior medullary of fibres, one on each side of the middle velum, which stretches between the two line. These are the crura of the midbrain. superior cerebellar peduncles, and helps to The crura are separated by a deep fissure. form the roof of the fourth ventricle. The Near the pons the fissure is narrow, but trochlear nerve emerges from the velum, broadens as the crura diverge to enter the and then winds round the side of the corresponding cerebral hemispheres. The midbrain to reach its ventral aspect. parts of the crura just below the cerebrum form the posterior boundary of a space In the description of the surface features called the interpeduncular fossa. The of the brainstem, given above, reference oculomotor nerve emerges from the medial has been made to the floor of the fourth aspect of the crus (singular of crura) of the ventricle. It is important that this same side. description be read at this stage, so as to obtain a 'complete idea of the posterior The posterior aspect of the midbrain is aspect of the pons and medulla. marked by four rounded swellings. These are the colliculi, one superior and one inferior on each side. Each colliculus is related laterally to a ridge called the brachium. The superior brachium (also called the superior quadrigeminal brachium, or brachium of superior colliculus) connects the superior colliculus to the lateral geniculate body. Similarly, the inferior brachium (also called the inferior quadrigeminal brachium or brachium of inferior colliculus) connects the inferior colliculus to the medial geniculate body. Just below the colliculi, there is the uppermost part
87 Chapter- 16 For convenience of description each cerebral hemisphere is divided into four Exterior of the Cerebral Hemispheres major subdivisions or lobes. To consider the boundaries of these lobes reference Poles, Surfaces, and Borders has to be made to some sulci and other The cerebrum consists of two cerebral features to be seen on each hemisphere. hemispheres that are partially connected with each other. When (a) On the superolateral surface of the viewed from the lateral aspect each hemisphere there are two prominent cerebral hemisphere has the sulci. One of these is the posterior ramus appearance shown in. Three somewhat of the lateral sulcus which begins near pointed ends or poles can be the temporal pole and runs backwards recognised. These are the frontal pole and slightly upwards. Its posteriormost anteriorly, the occipital pole part curves sharply upwards. The second posteriorly, and the temporal pole that sulcus that is used to delimit the lobes is lies between the frontal and occipital the ,central sulcus. It begins on the poles, and points forwards and superomedial margin a little behind the somewhat downwards. midpoint between the frontal and A coronal section through the cerebral occipital poles, and runs downwards and hemispheres shows that each forwards to end a little above the hemisphere has three borders: posterior ramus of the lateral sulcus. superomedial, inferolateral and inferomedial. These borders divide the (b) On the medial surface of the surface of the hemisphere into three hemisphere, near the occipital pole, large surfaces: superolateral, medial there is a sulcus called the and inferior. The inferior surface is parietooccipital sulcus . The upper end. further subdivided into an anterior of this sulcus reaches the superomedial 'orbital part and a posterior tentorial border and a small part of it can be seen part on the superolateral surface . Corresponding to these subdivisions, the inferomedial border is divided into (c) A little anterior to the occipital pole an anterior part called the medial the inferolateral border shows a slight orbital border and a posterior part indentation called the preoccipital called the medial occipital border. The notch (or preoccipital incisure). orbital part of the inferolateral border is called the superciliary border(as it lies To complete the subdivision of the just above the level of the eyebrows). hemisphere into lobes we now have to The surfaces of the cerebral draw two imaginary lines. The first hemisphere are not smooth. They show imaginary line connects the upper end a series of grooves or sulci which are of the parieto-occipital sulcus to the separated by intervening areas that are preoccipital notch. The second called gyri. imaginary line is a backward Lobes continuation of the posterior ramus of the lateral sulcus (excluding the posterior upturned part) to meet the first line. We are now in a position to define the limits of the various lobes as follows. (1) The frontal lobe lies anterior to the
88 central sulcus, and above the Further Subdivisions of the posterior ramus' of the lateral sulcus Superolateral Surface (2) The parietal lobe lies behind the central sulcus. It bounded below The subdivisions of the superolateral by the posterior ramus of the lateral sulcus and by the surface are described below second imaginary line; and behind by Frontal lobe the upper part of the first imaginary line The frontal lobe is further subdivided as follows. The precentral sulcus runs (3) The occipital lobe is the area lying downwards and forwards parallel to and behind the first imaginary line. a little anterior to the central sulcus. The (4) The temporal lobe lies below the area between it and the central sulcus is posterior ramus of the lateral sulcus the precentral gyrus. In the region and the second imaginary the anterior to the precentral gyrus there superolateral surface. On reaching this are two sulci that run in an surface it divides into three rami anteroposterior direction. These are the (branches). These rami are anterior (or superior and inferior frontal sulci. They anterior horizontal), ascending (or divide this region into superior, middle anterior ascending) and posterior. The and inferior frontal gyri.The ne anterior anterior and ascending rami are short and ascending rami of the lateral sulcus and run into the frontal lobe in the extend into the inferior frontal gyrus directions indicated by their names. The dividing it into three parts. The part posterior ramus has already been below the anterior ramus is the pars considered. Unlike most other sulci, the orbitalis; that between the anterior and lateral sulcus is very deep. Its walls cover ascending rami is the pars triangularis; a fairly large area of the surface of the and the part posterior to the ascending hemisphere called the insula ramus is the pars opercularis. Temporal lobe
89 The temporal lobe has two sulci that run Occipital lobe parallel to the posterior ramus of the The occipital lobe shows three rather lateral sulcus. They are termed the short sulci. One of these, the lateral superior and inferior temporal sulci. occipital sulcus lies horizontally and They divide the superolateral surface of divides the lobe into superior and this lobe into superior, middle and inferior occipital gyri. The lunate sulcus inferior temporal gyri. runs downwards and slightly forwards just in front of the occipital pole. The The parietal lobe vertical strip just in front of it is the gyrus descend ens. The transverse The parietal lobe shows the following occipital sulcus is located in the subdivisions. The postcentral sulcus runs uppermost part of the occipital lobe. downwards and forwards parallel to and The upper end of the parietooccipital a little behind the central sulcus. The sulcus (which just reaches the area between these two sulci is the superolateral surface from the medial postcentral gyrus. The rest of the surface) is surrounded by the arcus parietal lobe is divided into a superior parieto-occipita/is. As its name parietal lobule and an inferior parietal suggests, it belongs partly to the parietal lobule by the intraparietal sulcus. The lobe and partly to the occipital lobe. upturned posterior end of the posterior ramus of the lateral sulcus extends into Insula the inferior parietal lobule. The In the depth of the stem and posterior posterior ends of the superior and ramus of the lateral sulcus there is a inferior temporal sulci also turn upwards part of the cerebral hemisphere called to enter this lobule. The upturned ends the insula (insula = hidden). It is of these three sulci divide the inferior surrounded by a circular sulcus. During parietal lobule into three parts. The part development of the cerebral that arches over the upturned posterior hemisphere of this area grows less than end of the posterior ramus of the lateral surrounding areas which, therefore, sulcus is called the supramarginal gyrus. come to overlap it and occlude it from The part that arches over the superior surface view. These surrounding areas temporal sulcus is called the angular are called opercula (= lids). The frontal gyrus. The part that arches over the operculum lies between the anterior and posterior end of the inferior temporal ascending rami of the lateral sulcus. The sulcus is called the arcus frontoparietal operculum lies above the temporooccipitalis. posterior ramus of the lateral sulcus. The temporal operculum lies below this sulcus. The temporal operculum has a superior surface hidden in the depth of the lateral sulcus . On this surface are located two gyri called the anterior and posterior transverse temporal gyri. The surface of the insula itself is divided into a number of gyri.
90 Medial Surface of Cerebral Hemisphere In the anterior wall of the third ventricle there are the anterior When the two cerebral hemispheres are commissure and the lamina separated from each other by a cut in terminalis. The anterior commissure is attached to the genu of the corpus the midline the appearances seen are callosum through a thin lamina of fibres that constitutes the rostrum of the shown in Figure. The structures seen are corpus callosum. Below, the anterior commissure is continuous with the as follows. lamina terminalis which is a thin lamina The corpus callosum is a prominent of nervous tissue. The lower end of the arched structure consisting of lamina terminalis is attached to the commissural fibres passing from one optic chiasma. Just in front of the lamina hemisphere to the other . It consists of a terminalis there are the paraterminal central part called the trunk, a posterior gyrus and the parolfactory gyrus (Fig. end or splenium, and an anterior end or 16.5). Posteriorly, the third ventricle is genu. A little below the corpus callosum related to the pineal body {or pineal gland} and inferiorly to the hypophysis we see the third ventricle of the brain. A cerebri. Above the corpus callosum (and number of structures can be identified also in front of and behind it) we see the sulci and gyri of the medial surface of in relation to this ventricle. The the hemisphere . The most prominent of interventricular foramen through which the sulci is the cingulate sulcus which the third ventricle communicates with follows a curved course parallel to the the lateral ventricle can be seen in the upper convex margin of the corpus callosum. Anteriorly, it ends below the upper and anterior part. rostrum of the corpus callosum. Posteroinferiorly, the ventricle is Posteriorly, it turns upwards to reach the superomedial border a little behind continuous with the cerebral aqueduct. the upper end of the central sulcus. The The lateral wall of the ventricle is area between the cingulate sulcus and formed in greater part by a large mass of the corpus callosum is called the gyrus grey matter called the thalamus. The right and left thalami are usually interconnected (across the midline) by a strip of grey matter called the interthalamic connexus. The anteroinferior part of the lateral wall of the third ventricle is formed by a collection of grey matter that constitutes the hypothalamus. Above the thalamus there is a bundle of fibres called the fornix. Posteriorly, the fornix is attached to the under surface of the corpus callosum, but anteriorly it disappears from view just in front of the interventricular foramen. Extending between the fornix and the corpus callosum there is a thin lamina called the septum pellucidum {or septum lucidum}, which separates the right and left lateral ventricles from each other. Removal of the septum pellucidum brings the interior of the lateral ventricle into view.
91 cinguli. It is separated from the corpus with the parieto-occipital sulcus are callosum by the callosal sulcus. separated by a deeply situated anterior cuneolingual gyrus. The part of the medial surface of the hemisphere between the cingulate Inferior Surface of Cerebrum sulcus and the superomedial border When the cerebrum is separated from consists of two parts. The smaller the hindbrain by cutting across the posterior part which is wOl,lnd around midbrain, and is viewed from below, the the end of the central sulcus is called the appearances seen are shown in Fig. 8.7. paracentral lobule. The large anterior Posteriorto the midbrain we see the part is called the medial frontal gyrus under surface of the splenium of the corpus callosum. Anterior to the These two parts are separated by a midbrain there is a depressed area short sulcus continuous with the called the interpeduncular fossa. The cingulate sulcus. fossa is bounded in front by the optic chiasma and on the sides by the right The part of the medial surface behind and left optic tracts. The optic tracts the paracentral lobule and the gyrus wind round the sides of the midbrain to cinguli shows two major sulci that cut terminate on its posterolateral aspect. off a triangular area called the cuneus. In this region two swellings, the medial The triangle is bounded anteriorly and and lateral above by the parieto-occipital sulcus; geniculate bodies, can be seen. Certain inferiorly by the calcarine sulcus; and structures are seen within the posteriorly by the superomedial border interpeduncular fossa. These are of the hemisphere. The calcarine sulcus closely related to the floor of the third extends forwards beyond its junction ventricle (see also Fig. 8.6). Anterior with the parieto-occipital sulcus and and medial to the crura of the midbrain ends a little below the splenium of the there are two rounded swellings called corpus callosum. The small area the mamillary bodies. Anterior to these separating the splenium from the bodies there is a median elevation calcarine sulcus is called the isthmus. called the tubercinereum, to which the Between the parieto-occipital sulcus and infundibulum of the hypophysis cerebri the paracentral lobule there is a is attached. The triangular interval quadrilateral area called the precuneus. between the mamillary bodies and the Anteroinferiorly the precuneus is midbrain is pierced by numerous small separated from the posterior part of the blood vessels and is called the posterior gyrus cinguli by the suprasplenial (or perforated substance. A similar area subparietal) sulcus. lying on each side of the optic chiasma is called the anterior perforated The precuneus and the posterior part substance. The anterior perforated of the paracentral lobule form the substance is bounded anterolaterally by medial surface of the parietal lobe. the lateral olfactory stria and posterolaterally by the uncus. The Although the parieto-occipital and anterior perforated substance is calcarine sulci appear to be continuous connected to the insula by a band of with each other on surface view, they grey matter called the limen insulae are separated by the cuneate gyrus (or which lies in the depth of the stem of cuneolingual gyrus) which lies in the the lateral sulcus. depth of the area where the two sulci meet. The parts of the calcarine sulcus anterior and posterior to the junction
92 In addition to these structures we see occipitotemporal gyrus. This gyrus is the sulci and gyri on the orbital and continuous (around the inferolateral tentorial parts of the inferior surface of margin of the cerebral hemisphere) with the each cerebral hemisphere. These the inferior temporal gyrus parts are separated from each other by the stem of the lateral sulcus. Sulci and gyri on orbital surface Close to the medial border of the orbital surface there is an anteroposterior sulcus: it is called the olfactory sulcus because the olfactory bulb and tract lie superficial to it. The area medial to this sulcus is called the gyrus rectus. The rest of the orbital surface is divided by an H-shaped orbital sulcus into anterior, posterior, medial and lateral orbital gyri.Sulci and gyri on tentorial surface The tentorial surface is marked by two major sulci that run in an anteroposterior direction. These are the collateral sulcus medially, and the occipitotemporal sulcus laterally. The posterior part of the collateral sulcus runs parallel to the calcarine sulcus: the area between them is the lingual gyrus. Anteriorly, the lingual gyrus becomes continuous with the parahippocampal gyrus which is related medially to the midbrain and to the interpeduncular fossa. The anterior end of the parahippocampal gyrus is cut off from the curved temporal pole of the hemisphere by a curved rhinal sulcus. This part of the parahippocampal gyrus forms a hook-like structure called the uncus, details of which are considered later. Posteriorly, the parahippocampal gyrus becomes continuous with the gyrus cinguli through the isthmus . The area between the collateral sulcus and the rhinal sulcus medially, and the occipitotemporal sulcus laterally, is the medial occipitotemporal gyrus. The area lateral to the occipitotemporal sulcus is called the lateral
93 Chapter- 17 The ontogenetic explanation describes how a structure or behaviour BIOLOGICAL EXPLANATION develops. It begins with the genes and TO BEHAVIOUR traces how those genes combine with the influence of the environment to Just as the psychologists try produce the final outcome. to explain behavior the biological psychologists try to explain An evolutionary explanation behaviour in biological terms. For Eg relates a structure or behaviour to the , consider how various types of evolutionary history of a species. Eg. psychologists try to explain human Our capacities for behaviour are language: Cognitive psychologists evolutionarily modified from the study the relationship between what capacities that the other mammals is said and the meaning behind it. exhibit. Developmental psychologists study how children’s language capacities A functional explanation developed as the children grow describes why a structure or behaviour older. Social psychologists explore evolved as it did. A functional the relationship between language explanation demonstrates how a and culture and how social pressure particular gene increased in a influences speech. Biological population (through reproductive psychologists try to determine what succession) goes on in the brain that make speech possible. Similarly the Consider an example of bird song, explanations the biological which offers an excellent illustration of psychologists seek does not confine how physiological, ontogenetic, to brain activity alone. It can be evolutionary and functional explanations classified into four categories. apply to single behaviour. 1) Physiological explanation The question is why do birds sing ? 2) Ontogenetic explanation (Explanations attached) * 3) Evolutionary explanation In the case of human beings too for any 4) Functional explanation such behaviour there will be biological explanations.For example chemical A physiological explanation changes inside brain increase or relates an activity to how the brain and decrease the aggressiveness, change the other organs function even at the the mood and influence the sexual cellular and chemical level. We can behaviour. The full explanation to such understand why a particular type of behaviours is complicated, but never behavior occurs by the detailed study of the less biological. The past the brain chemical machinery. experiences exert their effects by means of our brain. The perception of an individual’s current situation is a
94 brain activity. So is his competing promoting this behavior. It is too easy to motivations. And ultimately human say that birds sing because they have behaviour is a an instinct to sing, that mother squirrels take care of their babies because they * BIRDS SONG have a maternal instinct, or that cats attack mice because they have a Let us consider how these four types hunting instinct. In each of these cases, biological explanation apply to a specific people may believe that they have example of behavior. That example is explained the behavior, but they have Birds song. But apart from its interest only named it. For this reason, many as a behavior, birdsong offers an investigators of animal behavior avoid excellent illustration of how the term instinct altogether. physiological, ontogenetic, evolutionary, and functional explanations apply to a A Physiological single behavior. Explanation of Birdsong The question is, Why do birds sing? But we can make the question more Birdsong depends on two areas of the specific. Not all birds sing. Even among brain (known as the caudal nucleus of songbirds, adult males generally do the hyperstriatum ventrale and the most of the singing. Depending on the robust nucleus of the archistriatum). species, females and immature males These areas are well developed in may sing less or not at all. The adult songbirds, such as sparrows and males sing vigorously in spring and finches; they are small or absent in birds early summer (the mating season); in with only simple vocalizations, such as most species, they become silent or chickens and pigeons. In the songbirds, sing only an occasional fragment of a the relevant brain areas are larger in song during the fall and winter. So the males than they are in females question becomes, Why do particular kinds of birds sing at the particular times The size of these brain areas they do? depends on testosterone, a hormone generally occurring in higher levels in First, let us note what is not an males than in females. At the start of the explanation: We cannot explain breeding season, testosterone levels birdsong by saying that it is an instinct. rise in males, causing the brain areas The term instinct is at best a label for a responsible for birdsong to increase in category of behaviors that depend more size. When those areas grow large on species membership than on enough, the bird begins to sing. If the individual experience. But labeling a testes are surgically removed from a behavior \"instinctive\" does not tell us male songbird, his testosterone level how the behavior developed or why the species evolved the set of genes
95 drops, the brain areas responsible for ify how the brain areas control the song decrease in size, and he stops muscles to produce song. The answer singing. Similarly, if an investigator to that question is known to some extent damages the relevant brain areas, the (Arnold, 1982), though not completely. bird stops singing. As is usually the case in science, answering one question leads to other, What do you suppose happens if an more detailed questions. experimenter injects large amounts of testosterone into a female? The relevant In songbirds, certain areas of the brain are larger in areas of her brain grow, and she begins males (left) than in females (right). Generally, only to sing-even in species in which females the males sing. (Photos courtesy of Arthur P. Arnold.) seldom if ever sing! This evidence indicates that both the size of the brain areas and the ability to sing depend on testosterone, not on being a genetic male (Nottebohm, 1980a). (It also indicates that at least a few brain areas can change their size in adult vertebrates. ) Further, what do you suppose happens if an investigator damages the song control brain areas in a female bird? The damage cannot decrease her singing, because she did not sing even with an intact brain. The effect on her behavior is that she no longer recognizes the song of her species. For example, a female canary adopts a sexually receptive position only if she hears a male singing a normal canary song. After damage to her song-control areas, she adopts that receptive position even after hearing other kinds of bird songs, such as sparrow songs (Brenowitz, 1991). In short, the physiological explanation states that testosterone causes the growth of certain brain areas necessary for the production and recognition of birdsong. Note that this explanation is somewhat incomplete: It does not spec-
96 An sparrows, it lasts from about age 20 Ontogenetic days to about age 60 days. Exposure to Explanation of that recording before age 20 days or Birdsong after age 60 days has little effect (Marler & Peters, 1987, 1988). However, a bird As mentioned, certain areas of the brain that listens to a live tutor can learn its make it possible for a bird to sing. But song during a much longer sensitive how does it know what to sing? The period, lasting until at least age 100 answer is more complicated for some days (Baptista, 1985; Baptista & species than it is for others (Nottebohm, Petrinovich, 1984). 1980b). This is a most unusual type of Pigeons, doves, chickens, and learning. A male sparrow that learns turkeys make only simple coos and his song during the first two months cackles, which ornithologists (bird or so of life cannot begin to practice biologists) do not classify as songs. the song until he reaches sexual These species do not need to learn their maturity the following spring-more calls. In fact, even if they are deafened than half a year later! As spring early in life, they develop normal calls. approaches, the young sparrow (Note that these observations do not begins to sing. At first, his song is like explain how pigeons and the others the babbling of a human infant, a acquire their calls. They merely indicate disorganized mixture of many that we should look for the answers in sounds. As time passes, the sparrow the embryology of the nervous system, eliminates some of his sounds and not in the individual's experience. ) In rearranges the others into an order other species, however, each bird has to that comes closer and closer to learn its song by listening to others of its matching what he heard the previous species. For example, in several summer. For example, male song sparrow species, a male reared in iso- sparrows at first produce songs of six lation from the sounds of his own or more notes at a time before species will develop a distinctly eventually settling on the three-note abnormal song. To develop a normal call that is typical of their species song, a male must hear the song of his (Marler & Peters, 1981, 1982). own species, either from adult males or from tape recordings (Marler & Peters, What about the female? In nature, 1977). He can learn the song from a females do not sing, but they do learn tape recording only during a sensitive their song. A female that is injected period early in life. The sensitive period with enough testosterone will sing her varies from species to species; in song species' song if she was exposed to it during the sensitive period (Marler, 1970). Like a male, she sings an abnormal song if she never heard her
97 species' song. changes are mostly slow and gradual, In short, sparrows (and numerous we can see in each species certain remnant characteristics of its ancestors. other songbirds) ordinarily hear their In some cases, we can roughly song early in life. During that time, they reconstruct how a particular feature may form a template (or model) of what their have evolved. song should sound like. Later, if and when their testosterone levels are high For example, two species of birds enough to enable them to sing, they that appear to be closely related engage in an apparent trial-and-error (based on their anatomy) generally process to match their own song to the produce similar vocalizations. To the template. trained ear, the song of each species of sparrow is recognizable and the A song sparrow song of each species of warbler is learns its song by recognizable. Yet it is also possible to imitating others of hear an unfamiliar species and say, its species. (Ed \"That's probably some kind of Reschke.) sparrow,\" or \"That sounds like a warbler of some sort.\" Even though An each species has evolved a unique Evolutionary song, it also retains general features Explanation of characteristic of its ancestors. Birdsong Through natural selection, each species In certain cases, biologists can use evolves adaptations that distinguish it birdcalls and birdsongs to infer some- from other species. But because such thing about the evolutionary relationship among various species. For example, the many species of sandpipers emit similar calls, presumably because they share a common ancestor. Two species-dunlins and Baird's sandpipers- give their calls in distinct pulses. (Other sandpipers do not.) This resemblance implies that these two species are more closely related than the others, that they share a recent evolutionary link (Kroodsma & Miller, 1982).
98 The similarity of the dunlin's song (top) to that of a Baird's sandpiper (below) suggests
99 A song sparrows. He would gain no Functional advantage if his song were easily Explanation confused with those of other species of Birdsong (Miller, 1982). If a bird species has genes that enable it Finally, a functional explanation can to learn its song and to sing it at the apply to some of the physical charac- appropriate time (spring), then natural teristics of birdsongs. A singing bird selection must have favored those gains an advantage genes for certain reasons. What might those reasons be? The song of a male by being heard throughout the bird serves two functions. First, it territory he can defend. A song heard attracts females of the species and more widely offers no advantage; it primes them to engage in reproductive might even be harmful if it attracts cats behaviors. For example, a female and other predators. For this reason, canary that hears a large number and most birds produce songs with notes variety of canary songs is quick to ranging from 1 kHz (kilohertz) upward- respond to a male sexually, quick to lay frequencies that do not carry well over eggs, and likely to lay a large number of long distances in a forest (Konishi, eggs (Kroodsma, 1976). Second, a 1969). male's song alerts other males to his presence and announces that he is In summary, when biological defending that territory. In this way, he psychologists ask why an animal shows may deter competition for his nesting a certain behavior, they look for several site and the female with which he kinds of answers: mates. . Physiological (How does the brain generate the behavior?) Given these principles, we can . Ontogenetic (How did the capacity for understand why males sing frequently this behavior develop?) during the breeding season and rarely . Evolutionary (How did this capacity during fall and winter. They sing only evolve from the capacities of related species?) when they are defending a territory . Functional (Why did this capacity and seeking a mate. (Can you imagine evolve? What good does it do for the any disadvantages to a bird that sings animal? ) out of season?) Note the relationship between We can also see why each species evolution and physiology. The bird has sings its own song: A male song the physiology that it does because sparrow, for example, gains an evolution has favored that physiology. advantage by attracting female song Note also that we do not have to sparrows and driving away other male assume that the bird knows why it sings
100 at all, much less why it sings at a particular time and place. Evolution has equipped it with the tendency to sing in certain ways and at certain times, but we need not assume that the bird knows its evolutionary history or the purposes its song may serve. (Similarly, we humans do not have to be consciously aware of the ultimate functions of all our own behaviors. When people are asked why they play, laugh, have sexual relations, or take care of babies, they sometimes reply that they simply \"like to\" or \"want to\" do these things. Evolution presumably constructed us to \"like\" certain activities because those activities increase our chance of passing on our genes.)
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