Massage_connection

Table 4.13 Muscles That Move the Thigh Name Origin Insertion Action Nerve Supply Muscle Diagram Posteriorly located muscles Iliotibial tract and gluteal tuberosity of Gluteus Iliac crest of ilium; femur Extends and laterally rotates L5, S1–S2 maximus sacrum; coccyx and thigh; lower fibers assist in thoracolumbar fas- adduction of the hip joint; cia; sacrotuberous upper fibers assist in abduc- ligament tion; by its insertion into the iliotibial tract, helps to stabi- lize the knee in extension O Gluteus maximus I Gluteus Anterior iliac crest; Greater trochanter of Abducts and medially rotates L4–L5, S1 O medius lateral surface be- femur thigh; the anterior fibers me- tween superior and dially rotate and may assist in Gluteus inferior gluteal flexion of the hip joint; the medius lines posterior fibers laterally rotate and may assist in extension I Gluteus Lateral surface of il- Greater trochanter of Abducts and medially rotates L4–L5, S1 Chapter 4—Muscular System minimus ium between infe- femur thigh; may assist in flexion of rior and anterior the hip joint gluteal lines O Gluteus 277 minimus I Continued

Table 4.13 278 The Massage Connection: Anatomy and Physiology Muscles That Move the Thigh (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram Laterally located muscles Tensor fasciae Iliac crest and area Iliotibial tract Flexes: abducts; medially ro- L4–L5, S1 latae between anterior il- tates thigh; tenses the fascia iac spines lata; and may assist in knee extension Tensor O fasciae latae Obturators Margins of obturator Trochanteric fossa of Laterally rotates thigh Externus: (externus foramen femur L3–L4 and Internus: internus) L5, S1 I O Obturator internus Posterior view

O I Obturator Externus Posterior view Piriformis Anterolateral surface Greater trochanter of Laterally rotates and adducts L5, S1–S2 of sacrum; sacro- femur thigh tuberous ligament Piriformis O I Anterior view Chapter 4—Muscular System Continued 279

Table 4.13 280 The Massage Connection: Anatomy and Physiology Muscles That Move the Thigh (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram Medially located muscles Adductor Inferior ramus of Linea aspera of femur Adducts; flexes and medially ro- L2–L3 brevis pubis tates thigh O Adductor Inferior ramus of Linea aspera of femur Adducts; flexes and medially L2–L4 (obtu- I longus pubis rotates thigh rator nerve) Adductor brevis Anterior view O Adductor I longus Anterior view

Adductor Inferior ramus of pu- Linea aspera of femur; Adducts, flexes (anterior), ex- L2–L4 magnus bis; lower part of adductor tubercle tends (posterior) thigh tuberosity of ischium O Adductor magnus I I Anterior view Pectineus Superior surface of Pectineal line inferior to Flexes, adducts thigh L2–L3 pubis lesser trochanter O Pectineus I Anterior view Chapter 4—Muscular System Continued 281

Table 4.13 282 The Massage Connection: Anatomy and Physiology Muscles That Move the Thigh (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram L2–L3 Gracilis Inferior ramus of pu- Anterior surface of Adducts thigh; flexes leg; assists bis and ischium tibia, inferior to me- in medial rotation of thigh dial condyle when legs flexed O Gracilis I Anterior view Anteriorly located muscles Iliacus (part Iliac fossa of ilium; Distal to lesser Flexes thigh; L2–L3 O of iliopsoas) ala of sacrum and trochanter tendon With insertion fixed: Flexes (femoral adjacent ligaments fused with that of lumber spine nerve) Iliacus psoas I Anterior view

Psoas major Body and transverse Tendon fuses with ilia- Flexes thigh; L1–L3 (part of process of T12–L5 cus and inserts distal With insertion fixed: Flexes iliopsoas) to lesser trochanter lumbar spine O Psoas minor T12–L1 vertebral Ilium inner surface (il- Flexion of trunk and lumbar L1 O bodies and inter- iopectineal eminence spine vertebral disks and line) Psoas major (Iliacus and psoas are together known as the iliopsoas.) I Anterior view Chapter 4—Muscular System O Psoas minor I Continued 283

Table 4.13 284 The Massage Connection: Anatomy and Physiology Muscles That Move the Thigh (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram Lateral rotation of thigh L5, S1 Rotators Gemellus Dorsal aspect of the Medial aspect of the superior spine of the greater trochanter ischium Gemellus O superior I Gemellus Superior aspect of Medial aspect of the Lateral rotation of thigh L5, S1 Posterior view inferior the tuberosity of greater trochanter the ischium I O Gemellus inferior Posterior view Obturator (see laterally located externus muscles) Obturator (see laterally located internus muscles)

Table 4.14 Muscles That Move the Leg Name Origin Insertion Action Nerve Supply Muscle Diagram Extends leg; flexes thigh Anteriorly located muscles First into patella then via patellar ligament Rectus Anterior inferior iliac to tibial tuberosity L2–L4 femoris spine; superior ac- (one of etabular rim O quadriceps femoris) Rectus femoris I Vastus Anterolateral surface First into patella then Extends leg L2–L4 intermedius of femur along via patellar ligament (one of linea aspera (distal to tibial tuberosity quadriceps half) femoris) O Chapter 4—Muscular System Vastus intermedius I Continued 285

Table 4.14 286 The Massage Connection: Anatomy and Physiology Muscles That Move the Thigh (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram Extends leg L2–L4 Rotators O Vastus later- Anterior and inferior First into patella then alis (one of to greater via patellar ligament Vastus quadriceps trochanter, along to tibial tuberosity lateralis femoris) linea aspera (distal half) Vastus medi- Entire length of linea Tibial tuberosity Extends leg L2–L4 I alis (one of aspera quadriceps O femoris) Vastus medialis (The four muscles described above are together known as the quadriceps femoris. All four insert into the patella and then into the tibial tuberosity as the patellar ligament.) I

Posteriorly located muscles Biceps Ischial tuberosity Head of fibula, lateral Flexes the knee joint; the long L5, S1–S2 femoris (long head) and condyle of tibia head extends and assists in (part of linea aspera of fe- lateral rotation at the hip hamstrings) mur (short head) joint O Biceps femoris I Posterior view Semimembra- Ischial tuberosity Posterior surface of the Flexes leg, extends, adducts, L5, S1–S2 nosus (part medial condyle of and medially rotates thigh of ham- tibia strings) O Semimembranosus I Chapter 4—Muscular System Posterior view Continued 287

Table 4.14 288 The Massage Connection: Anatomy and Physiology Muscles That Move the Thigh (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram L5, S1–S2 Semitendi- Ischial tuberosity Posteromedial surface Flexes and medially rotates leg, nosus (part of tibia extends, adducts, and medi- of ham- ally rotates thigh strings) (The three muscles—rectus femoris, semimembranosus, and semitendinosus—are called hamstrings.) O Sartorius Anterior superior il- Medial surface of tibia, Flexes and assists in medial ro- L2–L3 Semitendinosus iac spine near tibial tuberosity tation of leg; flexes and later- ally rotates thigh I Posterior view O Sartorius I Posterior Anterior Medial view

Popliteus Proximal shaft of Lateral condyle of femur Laterally rotates femur; medi- L4–L5, S1 tibia ally rotates tibia and flexes knee joint; With the insertion fixed: Laterally rotates femur O on the tibia and flexes the knee joint; helps reinforce Popliteus posterior ligaments of the knee joint I Chapter 4—Muscular System 289

Table 4.15 290 The Massage Connection: Anatomy and Physiology Muscles That Move the Foot and Toes Name Origin Insertion Action Nerve Supply Muscle Diagram Posteriorly located muscles Plantar flexion; flexion of leg at knee joint Gastrocnemius Femoral condyles Posterior surface of cal- S1–S2 and posterior sur- caneus face of femur OO Gastrocnemius I Soleus Head and proximal Posterior surface of cal- Plantar flexion S1–S2 shaft of fibula; pos- caneus teromedial shaft of tibia O O Soleus I

Peroneus Lateral condyle of Base of 1st metatarsal Everts foot; plantar flexion at L5, S1 longus tibia; head of fibula ankle; supports longitudinal arch of foot O Peroneus longus Peroneus Lateral margin of Base of 5th metatarsal Everts foot; assists in plantar L5, S1 I brevis fibula (middle) flexion of ankle Lateral view O Peroneus brevis I Chapter 4—Muscular System Lateral view Continued 291

Table 4.15 292 The Massage Connection: Anatomy and Physiology Muscles That Move the Foot and Toes (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram L5, S1–S2 Flexor digito- Posteromedial sur- Inferior surface of pha- Plantar flexion of toes 2–5; as- rum longus face of tibia langes of toes 2–5 sists in plantar flexion of an- kle joint and inversion of foot O Flexor digitorum longus Flexor hallucis Posterior surface of Inferior surface of distal Flexes great toe; assists in flex- L5, S1–S2 I longus tibia phalanx of great toe ion of the metatarsopha- langeal joint, plantar flexion O of the ankle joint, and inver- sion of foot Flexor hallucis longus I

Anteriorly located muscles Tibialis Lateral condyle and Base of 1st metatarsal Dorsiflexes foot, assists in inver- L4–L5 O anterior proximal shaft of sion of foot tibia Tibialis anterior I Extensor Lateral condyle of Superior surfaces of Extends toes 2–5; assists in dor- L5, S1 O digitorum tibia; anterior sur- phalanges of toes 2–5 siflexion of ankle joint and longus face of fibula eversion of foot. Extensor digitorum longus Lateral Medial Chapter 4—Muscular System I 293 Anterior view Continued

Table 4.15 294 The Massage Connection: Anatomy and Physiology Muscles That Move the Foot and Toes (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram L5 Extensor hal- Anterior surface of Superior surface of dis- Extends great toe; assists in in- lucis longus tibia tal phalanx of great version of foot and dorsiflex- toe ion at ankle joint O Extensor hallucis longus Lateral Medial I Anterior view

Table 4.16 Intrinsic Muscles of the Toes Name Origin Insertion Action Nerve Supply Muscle Diagram L5, S1 Extensor digi- Anterior superolat- Medial part of the mus- Extends the metatarsopha- I torum brevis eral surface of the cle (extensor hallucis langeal joints of digits 1–4 calcaneum brevis): dorsal aspect and assists in extending the of the base of the interphalangeal joints of dig- proximal phalanx of its 2–4 the great toe tendons to the 2nd, 3rd, and Extensor 4th toes: into the lat- digitorum eral aspect of the cor- brevis responding extensor digitorum longus tendons O Extensor hal- Distal part of supe- Dorsal surface of base Extends metatarsophalangeal L4–L5, S1 Dorsal surface of the foot lucis brevis rior and lateral sur- of proximal phalanx joint of great toe faces of calcaneus, of great toe I lateral talocal- caneal ligament, and apex of infe- rior extensor reti- naculum Extensor Chapter 4—Muscular System hallucis brevis O Dorsal surface of foot Continued 295

Table 4.16 296 The Massage Connection: Anatomy and Physiology Intrinsic Muscles of the Toes (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram S1–S2 Abductor Medial process of the Medial aspect of the Abducts and assists in flexion of I hallucis calcaneal tuberos- base of the proximal metatarsophalangeal joint of ity; flexor retinacu- phalanx of the great the great toe, assists with ad- lum and plantar toe duction of foot aponeurosis Flexor hallucis Medial part of the Medial and lateral as- Flexes the metatarsophalangeal S1–S2 Abductor brevis plantar surface of pects of the base of joint of the great toe hallucis the cuboid bone the proximal phalanx and adjacent part of the great toe O of the lateral cuneiform bone Plantar surface I Flexor hallucis brevis O Plantar surface

Flexor digito- Medial process of the Medial and lateral as- Flexes proximal interphalangeal S1–S2 I rum brevis calcaneal tuberos- pects of the middle joints, and assists in flexion ity; plantar fascia phalanges of the lat- of metatarsophalangeal joints eral four toes of 2nd–5th digits. Flexor digitorum brevis O Plantar surface Flexor digiti Medial plantar aspect Lateral side of the base Flexion of the metatarsopha- S2–S3 minimi of the base of 5th of proximal phalanx langeal joint of the 5th toe brevis metatarsal; sheath of 5th toe of peroneus longus I Flexor digiti minimi O Plantar surface Chapter 4—Muscular System Continued 297

Table 4.16 298 The Massage Connection: Anatomy and Physiology Intrinsic Muscles of the Toes (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram S2–S3 Lumbricalis First lumbricalis: me- Medial aspects of dorsal Flexes metatarsophalangeal dial aspect of flexor digital expansions on joints and assists in extend- digitorum longus the proximal pha- ing interphalangeal joints of I tendon; 2nd to 4th langes of the lateral 2nd–5th digits lumbricalis: adja- four toes cent sides of the flexor digitorum longus tendons O Lumbricals Abductor Medial process of cal- Medial aspect of base of Abducts and assists in flexion of S1–S2 Plantar surface hallucis caneal tuberosity; the proximal phalanx the metatarsophalangeal joint I flexor retinaculum of the great toe of the great toe and plantar aponeurosis Abductor hallucis O Plantar surface

Abductor digiti Medial and lateral Lateral aspect of base of Abducts and flexes the 5th toe S1–S3 minimi processes of the the proximal phalanx calcaneal tuberos- of the 5th toe ity; the bone be- tween the tuberosi- I ties; plantar fascia Abductor O digiti minimi Plantar surface Dorsal Adjacent sides of First interosseous: me- Abducts the 2nd, 3rd, and 4th S2–S3 interossei metatarsal bones dial aspect of the base toes; flexes the metatarsopha- of the proximal pha- langeal joints lanx of 2nd toe; 2nd I to 4th interossei: lat- eral aspects of the O Dorsal bases of the proximal phalanges of the 2nd, interossei 3rd, and 4th toes; dor- sal digital expansions Dorsal surface Chapter 4—Muscular System Continued 299

Table 4.16 300 The Massage Connection: Anatomy and Physiology Intrinsic Muscles of the Toes (Continued) Name Origin Insertion Action Nerve Supply Muscle Diagram S2–S3 Plantar Bases and medial as- Medial aspects of the Adducts 3rd–5th toe; flexes the interossei pects of the 3rd, bases of the proximal metatarsophalangeal joints 4th, and 5th phalanges of the metatarsal bones 3rd–5th toes; dorsal digital expansions I O Plantar interossei Plantar surface

CHAPTER 5 Nervous System Objectives On completion of this chapter, the reader should be able to: • Identify the parts of a typical neuron and the functions of the different components. • Classify the neurons according to structure and function. • Explain how myelin sheaths are formed in the peripheral nervous system (PNS) and central ner- vous system (CNS). • List the different types and functions of neuroglia. • Explain how a resting membrane potential is created in an excitable cell. • Describe how an action potential is generated and propagated. • Name the factors that can affect the speed of transmission in a neuron. • Describe the structure of a synapse. • Describe how transmission occurs across synapses. • Give examples of neurotransmitters and their role in synaptic transmission. • Describe the process of neuron regeneration. • List the factors that affect regeneration. • Classify sense organs. • Describe how various sensations are perceived. • Define pain. • Compare acute and chronic pain. • Describe, in brief, some theories used to describe pain. • Explain the mechanism of pain and pain responses. • Define visceral pain. • Describe referred pain. • Explain the possible process of referred pain. • Identify the body areas where various visceral organs tend to produce referred pain. • List the various strategies available for managing acute pain. • List the various strategies available for managing chronic pain. • Identify, given a diagram, the various structures on a transverse section of the spinal cord. • Explain the role of white mater and gray mater of the spinal cord. • Describe the different types of information carried by a typical spinal nerve. • List the structures innervated by a typical spinal nerve. • Define a dermatome. • Explain the basis of the dermatomal pattern. • List the various nerve plexus found in the body. • Describe the course of major nerves arising in the brachial and lumbosacral plexus. • Describe a reflex arc. • List the components of a reflex arc. • List the different reflexes and the situations in which each type comes into play. 301

302 The Massage Connection: Anatomy and Physiology • Explain how higher centers modify the response produced by a reflex. • Describe the structure of a muscle spindle and explain its role in stretch reflexes. • Trace the path taken by various sensations from the point of stimulus to the brain. • Identify, given diagrams, the major regions of the brain and describe their functions. • Identify, given a diagram, the motor, sensory, and association areas of the brain. • Name the location and functions of the limbic system, thalamus, hypothalamus, basal ganglia, cerebellum, vestibular apparatus, reticular formation, pons, and medulla. • Name the different cranial nerves and the primary destinations and functions of each, with spe- cial reference to the olfactory, trigeminal, facial, and vagus nerves. • Trace the major motor pathways from the brain to the skeletal muscle. • Describe the process of voluntary muscle control and the structures involved. • Describe the process of posture control and the structures involved. • Identify the protective covering of the brain. • Explain the formation, circulation, and function of the cerebrospinal fluid. • Describe, in brief, the blood supply to the nervous system. • Compare the structure and functions of the sympathetic and parasympathetic system. • Name the neurotransmitters involved in these systems and the effects they have on various target organs. • Explain the importance of dual innervation of organs in the body. • Explain the importance of autonomic tone. • Describe the interacting levels of control in the autonomic nervous system. • Describe age-related changes in the nervous system. • Describe the possible effects of massage on the nervous system. • Explain the role of massage on pain management. T he nervous system and the endocrine system help ternal and external environment, and the nerves con- nect the sensors to the brain and spinal cord and take coordinate the various activities of the body that are commands to tissue from the spinal cord to produce altered, according to the changing environment and a response. situations, to maintain homeostasis. The nervous sys- tem helps initiate changes quickly, with responses Classically, the brain and spinal cord are known as lasting for a short duration; the endocrine system the central nervous system (CNS); the rest of the brings slower changes of longer duration. However, nervous system is the peripheral nervous system both the nervous and endocrine systems work in an (PNS). The CNS (see Figure 5.1) helps integrate, integrated and complementary manner. process, and coordinate the sensory input and motor commands. For example, when you see a car hurtling The nervous system is also responsible for the com- directly toward you, you jump out of its path. The plex processes of intelligence, learning, memory, com- CNS processes the input (the sight of a car) and, munication, emotion, and other higher functions. based on past experiences and learning (processing and integration), decides that you need to jump out This chapter describes the structure of the nervous of the way. It commands the relevant muscles to con- system and the physiologic mechanisms involved in tract and move the body and, perhaps, yell at the carrying out its functions. same time (coordination). Of course, more than this happens inside your body—your heart beats faster, Organization of the your palms sweat, and your blood pressure increases. Nervous System The parts of the nervous system are referred to ac- The nervous system includes all neural tissue present cording to function. The sensors that sense changes in in the body and accounts for just 3% of total body the internal and external environment are the recep- weight. The functional unit of the nervous system is tors. The nerves that carry impulses from the recep- the neuron. The neurons are supported and pro- tors to the CNS are the sensory nerves, or afferents. tected by specialized tissue known as neuroglia. The The nerves that carry impulses from the CNS to the organs of the nervous system are formed by the neu- muscles or glands are the motor nerves, or efferents. rons, neuroglia, connective tissue, and blood vessels. The nerves that carry impulses to and from the brain are the cranial nerves; those that carry impulses to The nervous system consists of the brain and and from the spinal cord are the spinal nerves. spinal cord, enclosed in the skull and vertebrae, re- spectively. The sensors sense the changes in the in- The organs that respond to impulses from the CNS are the effectors. The nerves that go to skeletal mus-

Chapter 5—Nervous System 303 Receptors Afferents Sensory information Special sensory Vision, hearing, smell, Central Nervous System balance, taste Brain and spinal cord Somatic sensory Skin, muscle, joints Viseral sensory Internal organs Effectors Somatic Processes, intergrates, nervous system and coordinates sensory Skeletal muscle information and motor commands Cardiac Efferents muscle Motor commands Smooth Autonomic muscle nervous system Glands FIGURE 5.1. Schematic Representation of the Nervous System cle comprise the somatic nervous system. Reaction divisions: the sympathetic and parasympathetic di- may take place before the stimuli reach the conscious visions (see page •• for details). level; this response is known as a reflex. STRUCTURE OF THE NEURON The part of the nervous system responsible for au- The structure of the neuron (see Figure 5.2), the func- tomatic, involuntary regulation of smooth muscle, tional unit of the nervous system, varies from site to cardiac muscle, and glands is the autonomic ner- vous system. This system, described later, has two

304 The Massage Connection: Anatomy and Physiology site. Typically, a neuron has a cell body/soma, or Flow From Axon to Cell Body perikaryon, with a nucleus and cytoplasm, along with the organelles normally found in a cell. The Many viruses enter the CNS at the synaptic level. They prominent rough endoplasmic reticulum is known as are absorbed into the synaptic knob and transported to Nissl bodies. However, most neurons do not have a the cell body that invariably lies in the spinal cord or centriole and lose the ability to multiply. Nuclei are brain. A good example of such transport is the chicken- clusters of cell bodies of neurons in the CNS (excep- pox virus, which causes shingles, or herpes zoster. In tion, basal ganglia). These clusters in the PNS are this condition, the virus in the nerve cell body becomes known as ganglia. activated, and the condition presents as a rash along the distribution of the nerve to the skin. Many processes lead off from the soma. The axon is long and helps conduct impulses away from the THE SYNAPSE cell body. The axon may have many branches, known as collaterals. The collaterals help the cell commu- A synapse (see Figure 5.3) is the region where neurons nicate with more than one neuron. The dendrites are communicate with each other. The axon of the neuron, highly branched processes from the cell body that which brings impulses to the synapse, is the presy- take impulses to the soma. The presence of numerous naptic neuron. The neuron, which receives the im- dendrites enables many neurons to have an effect on one cell. Dendrites Nucleus Nucleolus Axon Nissl bodies Cell body Unmylelinated (soma) region Myelin sheath Axon Mylelinated Neurolemma region Schwann cell Node of nucleus Ranvier Neurolemma Schwann Collateral cell nucleus Axon Muscle fiber Neuromuscular junction Unmyelinated fiber Myelinated fiber FIGURE 5.2. The Structure of a Typical Neuron. The arrows indicate direction of conduction.

Chapter 5—Nervous System 305 Presynaptic neuron Anatomic Classification Axodendritic Bipolar Neurons synapse These neurons have two processes extending from ei- ther end of the cell body, the dendrite and the axon (see Figure 5.4). This type of neuron is rare and is found in the retina of the eye. Synaptic Unipolar Neurons knob The cell body in this type lies to one side, with a sin- gle process leading off from one side of the body. This process divides at once into two processes: the axon and the dendrite. Sensory neurons are of this type. Axoaxonic Multipolar Neurons synapse This is the most common neuron, with the cell body Axosomatic having several dendrites and one axon. All the neu- synapse rons motor to the skeletal muscles are of this type. Postsynaptic neuron Functional Classification FIGURE 5.3. A Synapse The neurons may be also classified according to func- pulse, is the postsynaptic neuron. The end of the tion. These are the sensory, motor, and interneu- axon of the presynaptic neuron is enlarged into a bulb, rons, or association neurons. Those that take im- the synaptic knob, or terminal. The synaptic knob pulses to the CNS are known as the sensory neurons usually has neurotransmitters packaged in small or afferent fibers. Of the sensory neurons, the vis- structures called synaptic vesicles (Figure 5.9A; page ceral afferents innervate the organs, and the so- ••). When the presynaptic neuron is stimulated, it re- matic afferents carry impulses from the surface of leases neurotransmitters into the gap between the two the body. Sensory neurons may be named according neurons. These then become attached to receptors on to the type of information they sense. Those afferents the cell membrane of the postsynaptic neuron, pro- that sense information in the external environment ducing electrical changes. are called exteroceptors. Afferents sensing changes in the inside of the body are called interoceptors. A synapse may be at a dendrite (axodendritic), on the soma (axosomatic), or along the length of the Drugs Acting at the Synaptic Level axon (axoaxonic). Rarely, a synapse may exist be- tween two dendrites (dendrodendritic). The neuro- A familiar example is nicotine, the ingredient in ciga- transmitters are actually manufactured in the soma rettes. It binds to receptor sites and stimulates the post- and transported down the axon. Transport of sub- synaptic membrane, producing responses similar to that stances can also occur in the opposite direction, from resulting from stimulation of nicotinic receptors. An- the end of axon to the soma. other example is pesticide use. Because animals also use acetylcholine as a neurotransmitter (the pesticide, Neurons, in addition to communicating with each preventing the action of acetylcholinesterase, the en- other, can communicate with another cell type. This zyme that destroys acetylcholine in the synaptic cleft), communication is known as the neuroeffector prolongs the action of acetylcholine at the neuromuscu- junction. Such communications are seen between lar junction. As a result, muscles go into prolonged con- the neuron and muscle—neuromuscular junction traction, jeopardizing respiratory movements. (described in page ••)—and between neuron and glands—neuroglandular junction. Neurons also in- Many drugs produce an effect by altering synaptic nervate fat cells. transmission. They may (1) reduce or increase synthesis of neurotransmitters, (2) alter the rate of release of neu- CLASSIFICATION OF NEURONS rotransmitters, (3) alter the rate at which neurotransmit- ters are removed from the synaptic cleft, and (4) prevent Neurons are classified in many ways, according to binding of neurotransmitters to the receptors on the their anatomic structure and function. postsynaptic membrane.

306 The Massage Connection: Anatomy and Physiology Dendrites Axon Cell body Axon hillock Axon Cell body Unipolar neuron Bipolar neuron Multipolar neuron FIGURE 5.4. Anatomic Classification of Neurons. Unipolar Neuron (e.g., sensory neurons); Bipolar Neuron (e.g., neurons in the retina of the eye); and Multipolar Neuron (e.g., motor neurons to skeletal muscle). The arrows indicate the direction of transmission of nerve impulses. Those that monitor position and movement of skele- NEUROGLIA tal muscles and joints are the proprioceptors. Neuroglias are the supporting cells. They are five times The neurons that take impulses away from the more abundant than neurons. There are four types of CNS are the motor neurons or efferents. Somatic glial cells in the CNS: the ependymal cells, astro- motor neurons innervate skeletal muscles. Visceral cytes, microglia, and oligodendrocytes and two motor neurons are part of the autonomic nervous types in the PNS: Schwann cells, or neurolemmo- system that innervates various organs of the diges- cytes, and satellite cells, or ganglionic gliocytes. tive, cardiovascular, respiratory, reproductive, and re- nal systems. The ependymal cells line the cavities in the brain and spinal cord and are responsible for producing, Interneurons are neurons situated between a sen- circulating, and monitoring the cerebrospinal fluid— sory and motor neuron or between any two neurons. Most in number, they are responsible for distribution Multiple Sclerosis and Demyelination of sensory information to different areas of the CNS and play an important role in the coordination of mo- In certain conditions, such as multiple sclerosis (MS), tor activity. the myelin sheaths in both the CNS and PNS are slowly destroyed, with resultant reduction in rate of conduction Other and destruction of axons of both sensory and motor neurons. The axons are said to be demyelinated. Neurons are also classified on the basis of myelination (i.e., according to the presence or absence of myelin sheaths as myelinated or nonmyelinated neurons).

Chapter 5—Nervous System 307 the fluid inside and around the CNS that cushions multilayered membranous wrapping around the and protects the brain. axon is known as myelin. Each oligodendrocyte puts out many processes and each process wraps around The astrocytes, as the name suggests, are star- an axon. Thus, each oligodendrocyte forms myelin shaped. They are present between the blood capillaries around many axons (or many oligodendrocytes help and the brain and spinal cord, monitoring the sub- myelinate one axon). Not all neurons have this wrap- stances that enter and leave the brain and preventing ping; those that do are said to be myelinated and the sudden changes in the environment around the CNS. others are unmyelinated. This is the blood-brain barrier, and the astrocytes are responsible for its creation. The astrocytes also help at Now imagine many thin strips of paper (processes the time of tissue injury. In addition, they take up neu- of different oligodendrocytes) wrapped around the rotransmitters from the synapses, break them down pencil (axon), separated by small gaps between the and release the products, and make them available to strips. This is how an axon is typically myelinated. neurons to produce more neurotransmitters. The net- The gaps between the myelin, known as nodes of work of astrocytes, located over the entire brain and Ranvier or internodes, help speed the impulses that spinal cord, form a supporting framework for the travel down it, and the myelin helps insulate the axon CNS. The astrocytes are the most abundant of the neu- from the surrounding interstitial fluid. The myeli- roglial cells. nated neurons conduct impulses much faster than those that are unmyelinated. The myelinated axons Microglia are small cells similar to the monocytes appear white, and the regions dominated by this type and macrophages in the blood. They engulf dead cells of axon tend to be white (white mater). The areas and cellular remnants in the CNS. with cell bodies and/or unmyelinated neurons appear gray (gray mater). It is believed that the gray col- Oligodendrocytes are neuroglia with long, slender oration is a result of the presence of Nissl bodies. processes that come in contact with cell bodies and axons in the CNS. The processes form thin sheaths in Neuroglial cells also exist in the PNS. One type, the the region where they contact an axon. This sheath is Schwann cells or neurilemmal cells, surround ax- wound around the axon, serving as insulation. Be- ons and protect them from the surrounding intersti- cause the sheath around the axon is made of many tial fluid. They are similar in function to the oligo- layers of the cell membrane, it is composed of 80% dendrocytes of the CNS—formation of myelin (see lipids and 20% protein. Figure 5.5). Such protection is important as impulses are produced and propagated by ions flowing in and To visualize the sheath, place a pencil on one end out of the neurons (see below). While all axons are of a thin strip of paper and roll the paper around the protected by Schwann cells, some axons have the cell pencil. The pencil represents the axon, and the paper represents the process of an oligodendrocyte. This Process of Schwann cell: oligoden- cytoplasm drocyte nucleus Oligodendrocyte Node of Ranvier Axon: Neurolemma Myelin sheath axolemma Myelin sheath neurofibril B A Cut axon FIGURE 5.5. The Formation of Myelin Sheaths. A, CNS (by oligodendrocytes); B, PNS (by Schwann cells)

308 The Massage Connection: Anatomy and Physiology membrane of many Schwann cells wrapped around The resting membrane potential is caused by the them in segments, with small gaps between the distribution of ions inside and outside the neurons. wrappings of any two Schwann cells. These are the The inside of the cell contains large, negatively myelinated axons. charged organic particles, and the movement of smaller positively and negatively charged inorganic One type, satellite cells or ganglionic gliocytes, molecules occurs only through special channels. The surround the collections of cell bodies of neurons cell membrane also has active pumps that use energy (ganglions) lying outside the CNS. to pump ions in and out; therefore, at rest, the inside is maintained more negative than the outside. PRODUCTION AND PROPAGATION OF IMPULSES At rest, the inside of the neuron (intracellular fluid) Impulse formation is a complex process and is re- has more potassium ions (Kϩ) and proteins (Pr Ϫ), and lated to the properties of the cell membrane (review the outside has more sodium (Naϩ) and chloride ions the section on cell membrane, page ••, if necessary). (ClϪ). This is mainly because the cell membrane is not The neurons communicate with each other by chang- freely permeable to the ions. If it was, the ions would ing the electrical potential inside the cell. This is diffuse in and out to equalize the composition in and achieved by movement of ions in and out of the cell out of the cell. In this case, because of semiperme- and is determined by the permeability of the nerve ability, the ions only move in and out of the cell cell membrane. The changes in the neuron (at rest through channels on the cell membrane specific for and when stimulated) have been studied using each ion. minute electrodes that penetrate inside the neuron. Membrane Channels Resting Membrane Potential If two electrodes are placed on the surface of the cell Membrane channels (see Figure 5.7), which are actu- membrane of a neuron and connected to a measuring ally proteins, (see page ••) remain closed, partly device (see Figure 5.6), no electrical changes are de- closed, or fully opened and are affected by many fac- tected. However, if one of the electrodes is pushed tors. Some channels are operated by changes in volt- into the cell and the other placed on the surface, the age (voltage-gated channels). At a particular voltage recording device will show that the inside of the cell specific to the channel, the channel may be open, al- is negative to that of the outside. This is known as the lowing its particular ions to move freely along the resting membrane potential, or transmembrane concentration gradient. At other voltages, the chan- potential. nel may be closed, shutting off entry or exit of that ion. Some channels open fully at a positive voltage, mV others at a negative voltage. +80 +40 Other than voltage-gated channels, certain chan- 00 rmp Depolarization nels are operated by hormones and other chemicals -40 Hyperpolarization (ligand-gated channels). These channels open when -80 the chemical binds to receptor sites on the cell mem- brane. Other channels are regulated mechanically Recording (mechanically-regulated channels). electrodes The resting membrane potential in a neuron is Axon about -70 millivolts (mV). The concentration differ- ence of ions (chemical gradient), as well as the dif- FIGURE 5.6. Recording Electrical Changes Across Cell Membrane ference in electrical charges (electrical gradient), serve as a force to reinforce or oppose movement of ions when the channels open. For example, when the sodium channels open, sodium tends to move into the cell (along the con- centration gradient). The electrical gradient also helps as sodium is positively charged and the inside is negative (remember that opposite charges attract). Movement of sodium will occur into the cell as long as the channels are open and the gradient exists. When the potassium channels are open, potassium tends to move from inside the cell to the outside along the chemical gradient. Because the inside is negative, the electrical gradient will tend to oppose it.

Chapter 5—Nervous System 309 Ions Ligand Ions Plasma membrane Pore Cytosol Cytosol Cytosol A Voltage-gated channel B Ligand-gated receptor C Mechanically-regulated channel FIGURE 5.7. Membrane Channels. A, Voltage-Gated Channels; B, ligand-gated channel; C, mechanically- gated channel At rest, there is a leak of sodium into the cell and At the same time, voltage-gated potassium channels potassium out of the cell. More potassium leaves the open and potassium rushes in. The resultant reduc- cell than sodium enters cell. This is one of the rea- tion in movement of the positively charged sodium sons why the inside is negative at -70 mV. To combat into the cell and movement of positively charged the leak, a pump on the cell membrane, the sodium- potassium out of the cell is responsible for repolar- potassium (Na-K) pump or sodium-potassium ization. The sodium-potassium pump is important in ATPase, constantly pushes sodium out of the cell the generation of action potentials as it helps bring and brings potassium into the cell, using energy (see the ionic concentrations across the cell back to its page ••). original state. Action Potential If the stimulus is given again, another action po- tential results. In some neurons, many action poten- When a cell is stimulated, sodium channels open and tials are produced continuously as long as the stimu- sodium diffuses into the cell along its electrochemi- cal gradient. The inside becomes less negative and This Needs A Title this is known as depolarization. In some cells, if potassium channels open instead, potassium moves In Summary: out of the cell and the inside becomes more negative, • the resting membrane potential is about -70 mV known as hyperpolarization. Soon after the stimuli • when the stimulus is given to the neuron, some depo- are removed, the cell returns to its original state and this is known as repolarization (see Figure 5.8). larization occurs in the area of the stimulus • if the depolarization reaches threshold level (i.e., When a neuron is sufficiently stimulated to depolar- ize it to a threshold value of about -60 to -55 mV, many about -60 mV), voltage-gated sodium channels open voltage-gated sodium channels are opened and sodium • positively charged sodium rushes in, rapidly depolariz- rushes in, further depolarizing the cell. This depolar- ization is rapid and propagated throughout the cell ing the neuron along the axon. An action potential or nerve impulse • this depolarization is propagated to the rest of the cell is a rapid change in potential that is propagated along • during the rapid depolarization, voltage-gated sodium the cell. The direction of propagation of action poten- tials is from the dendrite or cell body down an axon. channels close and voltage-gated potassium channels open As a result of opening and closing of other voltage- • sodium does not move into the cell as rapidly as be- gated channels, the depolarization does not last long fore; however, positively charged potassium moves and the cell is repolarized to reach its original resting out, making the inside negative until it reaches the potential. The voltage-gated sodium channels close resting potential when the potential becomes more and more positive. • Na-K pump actively pumps sodium out and brings potassium in, using ATP for energy and bringing the ionic concentrations back to normal.

310 The Massage Connection: Anatomy and Physiology Na+ Cl Resting state (unstimulated state) Na+ Cl Excited state (stimulated state) Na+ Action potential Resting potential Stimulus Return to resting state Nerve impulse Site of summation All-or-none action potentials with of multiple stimuli transient reversal of polarity FIGURE 5.8. Recording of Electrical Changes That Occur at Rest and on Stimulation lus remains (i.e., the stimulus does not have to be cause it to fire an action potential. However, no ac- given over and over again) and the generation of ac- tion potential will be produced in those neurons with tion potentials stops after the stimulus is removed. a threshold of more than -55 mV (e.g., 50 mV). In the body, the strength of the stimulus is trans- DIFFERENCES IN PROPAGATION OF lated as more frequent action potentials generated ACTION POTENTIAL IN MYELINATED per second and not as increase in amplitude of the AND UNMYELINATED AXONS action potential (i.e., the higher the strength, the greater the frequency). The action potential in an unmyelinated neuron trav- els slowly along the axon because every region of the The threshold level at which an action potential axon has sodium and potassium channels. In a myeli- can be produced varies from neuron to neuron. For nated cell, the myelin sheath serves as insulators, pre- example, when a weak stimulus is used, only those venting movement of ions through the membrane. neurons that have a low threshold will be stimulated. Ions move only through the numerous channels lo- For example, if a neuron has a threshold of -55 mV, a cated in the nodes and the action potential is propa- stimulus that changes the membrane potential to -55 mV from the neuron’s resting potential of -70 mV will

Chapter 5—Nervous System 311 gated from one node of Ranvier to another, literally effects, it can be seen that the synapse is the region jumping from node to node across the myelin. Hence, where there is possibility of modifying the message. propagation is rapid. This is known as saltatory con- duction. It should be noted that jumping is only a Whether a neurotransmitter is inhibitory or stimu- metaphor. Actually, the action potential in one node latory is dependent on the type of receptors they bind depolarizes the membrane at the next node to thresh- to in the postsynaptic membrane. For example, the old and a new action potential is produced there. Ac- neurotransmitter acetylcholine released by nerves in tion potential is also faster in thicker axons. The rate the neuromuscular junction causes skeletal muscle to of conduction ranges from 1.0 m/sec in thin, un- contract. The same acetylcholine released by the myelinated fibers to 100 m/sec (225 miles per hour) nerves to cardiac muscle has an inhibitory effect. in thick, myelinated fibers. Soon after neurotransmitters are released into the When the action potential reaches a synapse, it synaptic cleft, they are removed quickly from the causes the release of neurotransmitters into the area by enzymes, which break them up, or by reup- synaptic cleft. The neurotransmitters, in turn, pro- take back into the presynaptic neuron. Or, the neuro- duce electrical changes in the postsynaptic neuron. transmitter diffuses into the intercellular fluid. For example, acetylcholine is broken down by the en- SYNAPTIC TRANSMISSION zyme acetylcholinesterase to acetic acid and choline. Choline enters the presynaptic neuron and is recy- For the neurotransmitters to have an effect on the cled. The neurotransmitters epinephrine and norepi- postsynaptic neuron, sufficient amounts of neuro- nephrine are removed in an unchanged form from transmitters must be released. The number of synap- the synapse by reuptake. Quick removal of neuro- tic vesicles that fuse with the cell membrane of the transmitters is important to enable the postsynaptic axon terminal, to be released into the synaptic cleft by neuron to respond to another stimulus again. exocytosis, depends on the frequency of action poten- tials. With greater frequency, more vesicles release the EXAMPLES OF NEUROTRANSMITTERS neurotransmitters contained within them. The neuro- transmitters become attached to receptors on the post- There are many neurotransmitters in the nervous sys- synaptic membrane that open chemical-gated sodium tem. Some examples of common neurotransmitters are channels (see Figure 5.9A). If sufficient channels open, norepinephrine, dopamine, serotonin, ␥-aminobutyric they depolarize the neuron to reach threshold poten- acid (GABA), glutamate, glycine, enkephalins, endor- tial and produce an action potential. This is an exam- phins, substance P, nitric oxide (yes, a gas!), among ple of a stimulatory neurotransmitter. The potential many others. Some neurotransmitters are predomi- changes that occur at the nerve junction are known as nant in certain areas of the nervous system. If produc- excitatory postsynaptic potential (EPSP). tion of these neurotransmitters is affected, the func- tioning of this region of the nervous system is affected. Certain neurotransmitters become attached to re- ceptors that open chemical-gated potassium channels Many drugs affect the nervous system at the synapse or chloride channels in the postsynaptic neuron. In level. For example, symptoms of strychnine poisoning this case, instead of depolarization, the neuron be- (spasm of skeletal muscles) is a result of the blocking comes hyperpolarized (the inside becomes more neg- of glycine receptors. Glycine is the neurotransmitter in ative as positively charged potassium channels move neurons that inhibits motor neurons to muscle. If these out). As a result, it becomes more difficult for action neurons don’t function, the motor neurons fire contin- potentials to be produced. Such neurotransmitters uously, causing muscles to spasm. Similarly, cocaine are known as inhibitory neurotransmitters. The po- causes euphoria by blocking dopamine removal from tential changes that occur at the nerve junction are certain areas of the brain. Increased levels of dopamine known as inhibitory postsynaptic potential (IPSP). in these synapses result in change in feeling. Valium From the description of stimulatory and inhibitory (diazepam), an antianxiety drug, enhances the effects of GABA (an inhibitory neurotransmitter). Viagra RECEPTORS (sildenafil) produces its action by facilitating the action of nitric oxide, and Prozac (fluoxetine), the drug pre- You may recall that the term receptor was used to denote scribed for attention deficit disorder (ADD) and de- proteins located on the cell membrane that bind to spe- pression, acts by slowing down the removal of sero- cific hormones and chemicals. Although these endings of tonin in synapses. the sensory neurons are known as sensory “receptors,” their structure is different. ELECTRICAL SYNAPSES Most synapses are chemical synapses; however, more recently, electrical synapses (Figure 5.9B) have been

312 The Massage Connection: Anatomy and Physiology Axon terminal (presynaptic element) Secretory granules Mitochondria Synaptic Synaptic Postsynaptic dendrite cleft vesicles A Receptors Cell 1 Fusion and exocytosis cytoplasm (Neurotransmitter released) Connexons 3.5 nm 20 nm Cell 2 Ions and Channels formed Intercellular space cytoplasm Plasma membranes small molecules by pores in each membrane of adjacent cells B FIGURE 5.9. Synapses. A, Structure of a Chemical Synapse; B, Structure of an Electrical Synapse discovered in the brain. Such synapses also exist be- presence of gap junctions, transmission of impulses is tween smooth muscle cells, between cardiac cells, and faster than in chemical synapses. between glial cells. In the region of such a synapse, there are gap junctions (see page ••) present between SUMMATION adjacent cells. These junctions allow ions to move in As already mentioned, each neuron can have many both directions and are a route of communication of synapses. Therefore, the potential changes that occur impulses from one cell to another. As a result of the

Chapter 5—Nervous System 313 in it depend on the net effect of all synapses. Certain that impulses generated can converge on one neuron synapses may produce inhibitory effects and others or diverge to many neurons or even have a feedback may produce stimulatory effects. What happens in on the neuron that originally generated the impulse. the postsynaptic neuron depends on which effect is All these possibilities help the body better coordinate predominant. its activities. For some different ways that impulses can be modified, see Figure 5.10. For example, if action potentials arrive rapidly in a synapse that has a stimulatory effect, the potential in A diverging arrangement allows for a wide distri- the postsynaptic neuron may reach threshold quickly bution of a specific input. For example, sensory input and produce an action potential. At the same time, if is distributed to other neurons in the spinal cord and action potentials arrive in a synapse that produces an the brain. Parallel processing allows the informa- inhibitory effect, the postsynaptic membrane will be- tion to be processed by different neurons at the same come hyperpolarized, making it difficult for action time and produce a response in different regions of potential to be generated. This mechanism of inte- the body. For example, if you encounter a grizzly bear grating the effects of two or more neurons by the face-to-face on a hike, I do not know what you would postsynaptic neuron is known as summation. do, but I would scream and run at the same time (but, please don’t do that if it actually happens). Par- FACTORS THAT AFFECT NEURAL FUNCTION allel processing would help me do that. Other factors that affect neuronal functioning are the A converging arrangement helps more than one changes in the extracellular environment and the neuron have an effect on a postsynaptic neuron. For metabolic demands of the neuron. Neurons are very example, if you are carrying a hot plate, your initial sensitive to pH. If the pH becomes too high (more al- response may be to drop the plate. But if the plate kaline), they start discharging action potentials spon- contained something you did not want to loose, you taneously. If the pH becomes too low, the opposite could force yourself to carry it to the nearest table happens and the nervous system shuts down. without it dropping. It is convergence that makes this possible. The motor neurons to your muscles have As can be expected, fluctuating levels of ions, espe- synapses with sensory neurons conveying tempera- cially sodium, potassium, and calcium, have a marked ture sensation, as well as neurons, from your brain. effect on impulse production. Similarly, an increase in The activity of the motor neuron depends on the in- body temperature makes neurons more excitable. tegrated effect of all the neurons synapsing with it. In this way, the motor neuron can be inhibited or stim- Neurons require energy for manufacturing neuro- ulated, according to situations and altering the activ- transmitters and for maintaining ionic composition. ity of the presynaptic neurons. They can be easily injured if metabolic demands are not met. Serial processing affects only one neuron. This arrangement is seen in the way sensations are con- FUNCTIONAL ORGANIZATION OF NEURONS veyed to the brain. This helps the brain discern the exact region from which the sensation originated. The body has about 10 million sensory neurons, 20 For example, all sensations initiated in the left big toe billion interneurons, and one-half million motor neu- reach the “toe area” in the brain on the right side rons. These neurons are arranged in so many ways (Figure 5.35, page ••). Divergence Convergence Reverberation Parallel processing Serial processing Presynaptic neurons Synapse Postsynaptic neuron FIGURE 5.10. Different Ways of Modifying Impulses

314 The Massage Connection: Anatomy and Physiology NEUROTROPHINS and those leading from and to the spinal cord are the spinal nerves. Chemicals that promote neuron growth—neurotrophins— have been discovered during our quest to help those per- Tracts are bundles of the axons of neurons having sons with nerve injuries. Nerve growth factor is one of the same function and destination. For example, them. Neurotrophins have important functions in the fetal, the spinothalamic tract carries pain impulses from the as well as the adult, brain. They help promote nerve body to the thalamus. Many tracts, if present in the growth in the fetus. In adults, they are needed for regener- same region of the CNS, are referred to as columns. ation after injury and for maintenance of neurons. For example, the sensations of touch and pressure are carried by neurons whose axons lie in the posterior as- By providing the appropriate environment with the pect of the spinal cord, the dorsal column. help of neurotrophins, limited regeneration has been achieved in experiments done on animals with spinal REGENERATION AND DEGENERATION cord injury. OF NEURONS Reverberation helps neurons down the circuit to Effect of Pressure on Neurons initiate an impulse in the presynaptic neuron. Al- though the Figure shows a simplified version, more Neurons generally have a limited capacity to regener- complicated circuits, with many neurons involved, ate. For most neurons, cell division stops at birth. Al- are seen in the CNS. These circuits continue on and though the whole neuron cannot be replaced if dam- on until the neurons are inhibited or fatigued. An ex- aged, it is possible for the dendrites and axons to ample of such circuits is the respiratory center, which regenerate if the cell body is intact. helps with repetitive activities such as breathing. If pressure is applied to the axon of a neuron, the STANDARD TERMS AND GROUPING lack of oxygen and blood supply reduces its ability to conduct. If the pressure is released after a few hours, Anatomically, the neurons are arranged in a system- the neurons recover in a few weeks. atic and logical manner in the brain and spinal cord, with neurons having the same or similar functions Cut Injury invariably grouped together. Many standard terms describe these areas and groupings: More severe pressure will present with the same symptoms as a cut to the nerve. If the axon of a neu- Ganglia. A collection of cell bodies of neurons ron is cut, the part of the axon distal to the cut de- (e.g., Preganglionic nerves of the sympathetic and generates and is phagocytized by the Schwann cells parasympathetic nerves synapse with postganglionic that surround it. This process is known as wallerian neurons in a region located outside the spinal cord degeneration. Macrophages come to the area and re- and brain). The collection of cell bodies of the post- move the debris. The Schwann cells, however, do not ganglionic neurons of one region is known as gan- degenerate. Instead, they multiply along the path of glia. Another example is the dorsal root ganglion, a the original axon. The axon stump connected to the collection of the cell bodies of the unipolar sensory cell body grows with multiple small branches into the neurons that lies just outside the spinal cord. injury site, guided by the cellular cord of multiplying Schwann cells. It is believed the Schwann cells secrete Centers, located in the CNS, are collections of cell chemicals that attract the growing axon. If the axon bodies of neurons having the same function. For ex- grows in the right direction, it may reach its original ample, the vasomotor center in the brain has cell synaptic contacts and recover. If it grows into the bodies of neurons involved in regulating the activities of the smooth muscles in the walls of blood vessels. If Thoracic Outlet Syndrome the boundary of a center can be distinctly made out in anatomic sections of the brain, it is referred to as Thoracic outlet syndrome includes conditions that pro- the nucleus. The hypothalamus, for example, has duce symptoms of pressure on structures such as nerves many nuclei, some controlling sleep, some appetite. (in the brachial plexus) and blood vessels that exit from the thorax (posterior to the clavicle) to enter the limbs. Nerve. A nerve is a collection of axons of motor Cervical ribs, malaligned ribs, spasm of neck muscles neurons, dendrites of sensory neurons, and axons/ (scalenes), or other muscles such as the pectoralis minor dendrites of autonomic fibers bundled together by lying close to the structures passing through the outlet can connective tissue in the PNS. A specific nerve may or cause this syndrome. It is characterized by edema, numb- may not contain all three types of nerve fibers (i.e., ness, tingling sensations, or weakness of the upper limbs. motor, sensory, and autonomic). Nerves leading to or from the brain and brain stem are the cranial nerves,

Chapter 5—Nervous System 315 Neurofibromatosis sues. In a cutting-type injury, if the cut ends are in close contact, chance of recovery is high. However, Neurofibromatosis is an inherited disorder that affects the alignment of the cut ends also matters. It is im- many systems. It is characterized by soft, multiple, ab- portant for the axon to grow back into the same area normal growths along the distribution of peripheral occupied by the original axon. If many adjacent ax- nerves. It is a result of abnormal multiplication of ons are cut, the nerve must be aligned so that the Schwann cells around peripheral nerves. right axon grows into the right area. For example, if an axon innervating skeletal muscle grows into the wrong direction, normal function does not return. area occupied by a sensory nerve, the neuron eventu- Chances of recovery are high if the cut ends of the ally dies. axon are in close contact. The rate of growth of axons is about 1 mm to 2 mm per day and recovery depends Injury in the CNS on the distance the axon has to regrow to reach the structure it originally innervated (see Figure 5.11). Damaged neurons in the CNS have greater difficulty recovering. This is because, invariably, many neurons Many factors affect the chance of recovery. If the are involved. Also, the astrocytes (neuroglia) form axon is cut close to the cell body, the cell body may scar tissue, which makes it difficult for axons to grow die, with no chance of recovery. If a crushing type in- back. In addition, chemicals that inhibit neuron jury has occurred, partial or, often, full recovery en- growth may be liberated in the area. After a spinal cord injury, many cells not directly injured die by Proximal end Distal end apoptosis (cell suicide). The reason for this is not clear. Although it was believed that nerves do not Schwann cell Endoneurium multiply, it was recently discovered that new neurons Myelin are formed in certain areas of the brain (such as the Fragmented Schwann cell area for learning). This discovery is encouraging, es- Axon pecially for those with injury to the CNS. axon Droplets of myelin Droplet Ischemic Injury A Site of nerve of myelin Fragment Although the brain accounts for only 2% of the body weight, it accounts for 18% of the energy consumption of axon at rest. Neurons rely solely on aerobic metabolism for their survival. Because they do not have stored glyco- lesion Endoneurium gen, oxygen and glucose must be continuously sup- plied by the blood. Therefore, injury to neurons occurs Macrophage if their blood supply is cut off for even a few seconds. The injury is in proportion to the duration of inter- B Schwann cell Macrophage ruption of the blood supply. Stroke is a result of im- pairment of blood supply to areas of the brain. C Multiple fine axon New sprouts or filaments Band fiber axon Having considered the functional unit of the ner- filament vous system: the neuron and its structure and classi- fication; how impulses are generated, propagated, New and communicated to other neurons; and how the axon neurons recover from injury, the structures involved in sensations are addressed. Schwann cell Isolated Paralysis D New myelin sheath Paralysis of isolated muscle groups usually indicates a Single enlarging Schwann cell lesion of one or more peripheral nerves. If the lesion is axon filament in an individual peripheral nerve, there will be weak- ness or paralysis of the muscle or group of muscles and FIGURE 5.11. Regeneration of a Cut Axon impairment or loss of sensation in the distribution of the nerve in question.

316 The Massage Connection: Anatomy and Physiology Sensory Nervous System Leprosy, or Hansen’s Disease SENSE ORGANS AND In this disorder, a bacteria—Mycobacterium leprae— INITIATION OF IMPULSES invades the Schwann cells in cutaneous nerves and pro- duces inflammation. It results in reduction or loss of The neurons that convey information about the in- sensation in patchy areas of the skin. ternal and external environment—the sensory or af- ferent neurons—detect the actual changes in the en- the eye is light. Sound will have no effect on them, al- vironment by means of sensory receptors, which though, if pressure is applied over the eye, flashes of are located at that end of unipolar neurons. Sensory light may be seen. As the sensory receptors are spe- receptors are transducers that convert different cialized to respond to one type of energy, it follows forms of energy into action potentials. The endings of that there are many different kinds of receptors. sensory nerves alone may have transducer function or they may be surrounded by other non-neural cells Although one learns in school that there are five that produce action potentials in the neuron. In the senses, the body is able to sense many different latter case, it is known as a sense organ. senses. Table 5.1 lists some of the senses the body Some different forms of energy that receptors con- Table 5.1 vert into action potentials are mechanical (touch, pres- sure), thermal (degrees of warmth and cold), electro- Receptors/Sense Organs and Sensations magnetic (light), and chemical energy (taste, smell, oxygen content in blood, and carbon dioxide content). Sensation Receptor Sense Organ Each receptor responds maximally and is sensitive to one type of energy. The particular form of energy to Vision Rods and cones in Eye which the receptor responds is its adequate stimulus. retina For example, the adequate stimulus for receptors in Hearing Ear Smell Hair cells Nose CLASSIFICATION OF SENSORY NEURONS Taste Olfactory neurons Tongue Acceleration Taste receptors Ear (vestibular ap- Sensory neurons can be classified by the: Hair cells • origin of stimulus (e.g., near or far away) Touch-pressure paratus) • type of adequate stimulus (e.g., touch, sound) Warmth Nerve endings Various • threshold of stimulus required for perception (e.g., low Cold Nerve endings Various Pain Nerve endings Various threshold; high threshold) Joint position Free nerve endings • rate of adaptation (e.g., rapid, slow) Nerve endings Various • anatomic structure (e.g., free nerve ending, hair cells) and movement • type of sensory information they deliver to the brain Muscle length Nerve endings Muscle spindle Muscle tension Nerve endings Golgi tendon organ (e.g., proprioceptors [sense of body position], nocicep- Arterial blood Nerve endings Stretch receptors tors [sense of pain]). pressure Nerve endings in aortic arch Terms to describe sense: and carotid sinus Chemoreceptors are receptors stimulated by a change in Venous pressure Nerve endings Stretch receptors the chemical composition of the environment. in walls of great Cutaneous senses are senses with receptors are located on Inflation of lung Neurons in vein the skin. hypothalamus Stretch receptors Exteroceptors are concerned with events near at hand. Temperature of in lung Interoceptors are concerned with the internal environment. blood in head Nerve endings? Nociceptors are pain receptors often referred to as noci- Aortic and carotid ceptors because they are often stimulated by noxious or Oxygen content Cells in different parts bodies damaging stimuli. in blood of brain (e.g., thirst Proprioceptors give information about the body in space center) at any given instant. Osmotic pressure Special senses: smell, taste, vision, hearing, and rotational in plasma Cells in hypothalamus and linear acceleration. (hunger center) Teleceptors are receptors concerned with events at a Glucose level in distance. blood Visceral senses are senses that perceive changes in the in- ternal environment.

Chapter 5—Nervous System 317 Hairy skin Glabrous (hairless) skin Merkel’s Epidermis disk Epidermal- dermal border Free nerve Dermis ending Meissner’s corpuscle Hair follicle receptor Pacinian corpuscle Ruffini’s ending FIGURE 5.12. Anatomic Structure of Certain Cutaneous Receptors possesses. Some senses listed are complex; for exam- Temperature Receptors ple, a number of different receptors can sense differ- ent taste sensations: bitter, sweet, salt, and sour. There are two types of temperature receptors; one that responds maximally to temperatures slightly CUTANEOUS RECEPTORS above body temperature (warmth) and one that re- sponds to temperatures slightly below body tempera- There are many different types of nerve endings on ture (cold). These are actually two degrees of warmth the skin. Some are free nerve endings, some have a because cold is not a form of energy. capsule around them, and others have expanded tips of nerve endings. Some nerve endings are found There are 4 to 10 times more cold receptors than wound around hair follicles (see Figure 5.12). Any warm receptors. Cold receptors respond to tempera- given receptor signals or responds to only one kind of tures from 10–40°C (50–104°F), and warm receptors cutaneous sensation. There are four different cuta- respond from 30–45°C (86–113°F). With time, be- neous senses: touch-pressure, pain, cold, and warmth. tween 20–40°C (68–104°F), the receptors adapt and conscious perception of temperature diminishes. At Touch Receptors temperatures above and below this, the receptors do not adapt. At temperatures above 45°C (113°F), the Touch receptors are present over the entire body, but tissue becomes damaged and the sensation is that of are more numerous in the skin of the fingers and lips, pain. with relatively fewer receptors in the skin of the trunk. Many are located around hair follicles. The hair acts Itch and Tickle as a lever and slight movements of the hair magnify the effect on the receptors. Mild stimulation, especially if produced by something that moves across the skin, causes itch and tickle sen- Proprioceptors HAVE YOU BEEN CONFUSED BY TEMPERATURE CHANGES? Awareness of the body in space is a result of impulses from receptors located in and around joints (joint re- We are often unable to identify the water temperature ceptors), within skeletal muscle, and between ten- when we run our bath water. This is probably because dons and muscles (see Figures 4.12 and 4.13, page both cold and warm receptors are stimulated between the ••). A conscious picture of the position of the body is temperatures 30–40°C (86–104°F), and the degree to a result of integration of impulses generated by these which they are stimulated determines if the water is cold receptors and those from the eyes, muscle spindles, or hot. skin, and other tissue.

318 The Massage Connection: Anatomy and Physiology sations. Free nerve endings of slow conducting, un- many action potentials at greater frequency are gen- myelinated fibers seem to carry these sensations and erated. In this way, by differences in the frequency of repetitive, mechanical, local stimuli and/or chemicals the action potentials, the brain is able to discern the such as histamine stimulate these receptors. Itch intensity of the stimulus applied. could possibly be a fifth cutaneous sense. If a stimulus is applied for a prolonged period, the Complex sensations, such as the ability to sense vi- frequency of the action potentials generated declines. bration; discriminate two stimuli applied close to This phenomenon is adaptation. The degree to each other (2-point discrimination); and identify which receptors adapt varies with sense organs. In objects based on size, shape, consistency, and texture receptors that do not adapt quickly, the action poten- by handling the object without looking at it (stereog- tials continue for as long as stimuli are applied. These nosis), are a result of the integration of the various are the slow adaptors or tonic receptors. Certain cutaneous senses and require an intact cerebral cor- receptors trigger action potentials at the beginning tex. A pattern of rhythmic pressure stimuli is inter- and end of the application of stimulus, the rapidly preted as vibration. adapting receptors or phasic receptors. Both types are valuable for survival. Pain and cold receptors are Pain Receptors or Nociceptors slow adapting and help warn the body regarding in- jury. Similarly, the stretch receptors that regulate The stimuli for pain, pathways, and perception of blood pressure are slow adaptors. This is because the pain is complex. Refer to page •• for a detailed dis- blood to the brain must be constantly monitored. cussion of pain. PERCEPTION OF SENSATIONS AN OVERVIEW OF ELECTRICAL AND IONIC EVENTS IN RECEPTORS Doctrine of Specific Nerve Energies The receptors convert the adequate stimuli into ac- According to the explanation above, all stimuli seem tion potentials, similar to the electrical changes to be converted to action potentials. If so, how does (EPSP and IPSP) that occur at the synapse. However, the body know what the original stimulus was? This the electrical changes here are known as generator doctrine, stated by Muller, explains how this hap- potentials or receptor potential. For example, if pens. The sensation perceived depends on which part pressure is applied over a receptor that responds to of the brain they activate. The pathway taken by ac- this stimuli and the electrical changes are recorded tion potentials generated by a sense organ is specific using microelectrodes inside and outside the nerve, from sense organ to the brain. The sensation that is the inside of the nerve ending which is originally neg- perceived is that for which the receptor is specialized, ative depolarizes. This is caused, in most cases, by no matter where along the pathway the activity is ini- opening of sodium channels on the cell membrane of tiated. For example, the sense of pressure is per- the nerve endings and sodium rushing in. This seems ceived as coming from the hand irrespective of where to trigger an action potential down the nerve. It has the sensory nerve is stimulated: pressure receptors in been shown that if the stimuli are more intense, the hand, nerve at the elbow, the axilla, in the poste- rior aspect of the spinal cord where it travels, or even in the brain where it finally arrives. HOW WE SENSE Law of Projection • Each sense organ or sensory receptor is specialized to This law explains that wherever the nerve is stimu- convert (transduce) one form of energy into action po- lated in a sensory pathway, the sensation is projected tentials in the sensory nerves. (perceived) in the area of the body where the sensa- tion normally originates. For example, if, during • Different forms (modalities) of sensation are identified brain surgery on a conscious patient, the brain re- by the fact that they are transmitted to the brain by dif- ceiving area for impulses from the right hand is stim- ferent nerve pathways and synaptic connections (al- ulated, the patient reports sensation in the right hand though all of them are in the form of nerve impulses). and not in the hand where the actual stimulation was. • The intensity of the stimulus is identified by (1) the Intensity Discrimination change in the frequency of the action potential pro- duced, and (2) the number of receptors stimulated. Again, if all stimuli are converted to action potentials, how does the body perceive variations in intensity? • The area/region of the body stimulated is identified by impulses from these areas reaching a specific region in the brain (stimuli to right leg reaches area representing the right leg in the cerebral cortex).

Chapter 5—Nervous System 319 STIMULATION OF THE POSTCENTRAL unit. The sensory units tend to overlap the areas sup- GYRUS plied by other sensory units. Studies show that stimulation of specific areas in the sen- As the stimulus is increased, more and more sen- sory cortex, using fine electrodes, can elicit pure sensa- sory units are stimulated because the stimuli affect a tions of pain, cold, warmth, and touch in the representa- large area. As a result, more pathways are affected, tive part of the body! and the brain perceives more stimuli intensity. There are two ways by which intensity is transmitted Why Are Some Areas More to the brain. One is by altering the frequency of stim- Sensitive Than Others? ulation. The other is by the number of receptors that have been stimulated. Some areas, such as the face, fingertips, and toes, are more sensitive than others because there are more Lateral Inhibition sensory neurons innervating a unit area of skin, each with a smaller receptive area. In areas such as the Another mechanism the CNS uses to better perceive back, the receptive field of each sensory neuron is sensations is lateral inhibition, in which the neuron large and the number of innervating nerves is less that is most stimulated inhibits surrounding neurons (see Figure 5.13). That is why a light touch on the via interneurons. For example, when the sound pro- face feels good. But the touch has to be more intense, duced by striking middle C reaches the ear, the neu- covering a larger area, in regions such as the back. rons that respond to this particular pitch are most This is one reason why fingertips are used on the face stimulated. When responding by production of ac- and the entire palm is used to massage the back. tion potentials, they inhibit surrounding neurons that respond to the next closest pitch. This way, the sound See pages •• and •• for details of some special of middle C is heard sharply. sensory organs (smell, taste, vestibular apparatus). Sensory Unit THE SPINAL CORD, SPINAL NERVES, AND DERMATOMES This term is applied to a single sensory neuron and all its peripheral branches. The number of branches From the area of supply, the peripheral branch of the varies, and branches are numerous, especially in the sensory neuron continues toward the CNS. As a re- skin. The receptive field of a sensory unit is the area sult of the way the body is developed in the embryo, from which a stimulus produces a response in that there is a specific pattern in the way nerves from dif- ferent regions of the skin converge to the CNS. Thirty-one spinal nerves and certain cranial nerves are responsible for all body sensations. Figure 2.6, Smaller receptor field Larger Sensory neurons receptor field Sensory neurons FIGURE 5.13. Receptive Fields

320 The Massage Connection: Anatomy and Physiology TEST YOUR SKIN SENSITIVITY You can crudely test the sensitivity of different areas of the body by using a paper clip. Bend the clip so that the two ends are level and about a few millimeters apart. Now, ask your colleague to close her or his eyes and then touch a region of the body so that both points of the paper clip touch the body at the same time. Ask her or him to say if the touch was felt as one or two stimuli. If it was felt as one stimulus, spread the paper clip apart a little more and repeat until it is perceived as two sepa- rate stimuli. This gives you an idea of the size of the receptive field of a sensory neuron in that area. If both points of the pa- per clip stimulate the same receptive field, the sensation is felt as one stimulus. If each point stimulates a different receptive field, the person feels it as two. Repeat this in different regions of the body, and determine which areas can sense stimuli as two, even when the tips are close together. These areas are more sensitive because they have more sensory units per unit area of skin, with each receptive field being smaller. Care should be taken that the test is done on bare skin as the stimuli spreads to other areas if it is performed over clothing. Also, if both tips do not touch the body at exactly the same time, it may be felt as two stimuli. Receptive fields Skin surface The sensation is felt as two points as two receptive fields are stimulated Perception Sensory of two points neurons of touch Skin surface The sensation is felt as one point as only one receptive field is stimulated Perception Sensory neuron of one point of touch page ••, shows the area of the skin and the spinal brain via cranial nerve V (trigeminal nerve). It is of cord segment to which sensory nerves go from spe- interest that the anal region lies in the dermatome of cific areas of the body. The region of the body wall the sacral nerves; the most distal segment of the supplied by the cutaneous branches of a single pair of spinal cord. In the embryo, this is the tail region, and spinal nerves (right and left) is known as a der- the lower limb develops from the lumbar and upper matome. Although the dermatomes appear to have sacral region. distinct borders, there is some overlap between adja- cent areas. The sensations from the face reach the The cell bodies of the sensory nerves are located close to the spinal cord at the location where they en-

Chapter 5—Nervous System 321 Dura mater ter. This collection of cell bodies in each segment is Spinal cord known as the dorsal root ganglion and is seen as a Ventral root Arachnoid slight enlargement just outside the spinal cord (see Dorsal root Pia mater Figure 5.15). A similar ganglion is present in the Subarachnoid Spinal nerve route of cranial nerves that contain branches of sen- space sory neurons. Dorsal root Transverse ganglion process Sensory neurons, as already mentioned, are unipo- lar, and the other branch leading off the cell body Anterior Vertebral (axon) enters the spinal cord where it immediately Body of foramen synapses or travels up (and/or down) before synaps- vertebra ing. The pathway of the sensory impulses in the CNS is described on page ••. Lumbar vertebra ANATOMIC STRUCTURE OF THE SPINAL CORD The spinal cord in an adult is about 45 cm (18 in) long and 14 mm (0.55 in) wide. It lies in the spinal canal of the vertebral column, extending inferiorly as far as vertebra L1 and L2 (see Figure 5.14). It is shorter than the vertebral column because its growth does not match that of the vertebral column during growth of the fetus and young child. DERMATOMAL PATTERNS Posterior An easy way to remember the dermatomal pattern is to FIGURE 5.15. Meninges and Other Structures Protecting the Spinal imagine the body with the arm and legs stretched out to Cord the sides at 180°. The dermatomes seem to stretch across as transverse strips. The spinal cord has 31 pairs of spinal nerves leaving through the intervertebral foramen located between the vertebrae. Some nerves are large because they sup- ply a larger area of the body. Large nerves are seen in the lower neck region, supplying the arms, forearms, and hands. Such nerves are also seen in the lumbar and sacral regions, supplying the thighs, legs, and feet. The cord is enlarged slightly in the cervical (cervical en- largement) and lumbosacral (lumbosacral enlarge- ment) regions, as it has to accommodate the cell bod- ies of a greater number of neurons. The lower end of the spinal cord becomes conical and tapers into the conus medullaris region. A thin, fibrous tissue ex- tends from its tip to the sacral region, the filum termi- nale. This fibrous tissue gives longitudinal support to the spinal cord. A segment is the part of the spinal cord that cor- responds to a single pair of spinal nerves. Therefore, there are 31 segments in the spinal cord: 8 cervical, Dermatomal Pattern BELL-MAGENDIE LAW All sensory neurons enter the spinal cord in the dorsal re- gion, and all motor neurons leave ventrally. This principle is known as the Bell-Magendie law.

322 The Massage Connection: Anatomy and Physiology 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Specific spinal nerves are denoted with the first letter 1st cervical nerve of the spinal segment and the number. For example, 1st cervical vertebra (C1) the spinal nerve that is the eighth nerve arising in the Cervical plexus cervical region is referred to as C8. Although the 7th cervical vertebra (C7) spinal cord is much shorter than the vertebral col- 8th cervical nerve umn, ending close to the second lumbar vertebrae, 1st thoracic vertebra (T1) the lower lumbar and sacral segments of the spinal 1st thoracic nerve cord exist, although at a more superior location. Be- cause the spinal cord is shorter than the vertebral Brachial plexus column, the spinal nerves from the lower lumbar and sacral regions course downward to exit through the Dura mater correct intervertebral foramen. These nerves com- bined make the lower end of the spinal cord look like Cervical a horse's tail and are referred to as the cauda equina. enlargement Conus The spinal nerves are easily indicated in relation to medullaris the vertebra. Each nerve caudal to the vertebra takes 12th thoracic its name in relation to the vertebra preceding it (i.e., vertebra (T12) the nerve between thoracic vertebrae 1 and 2 is T1). 12th thoracic Because the first spinal nerve exits between the skull nerve and the first cervical vertebrae, cervical spinal nerves 1st lumbar are indicated in relation to the vertebrae following it vertebra (L1) (i.e., the nerve lying between the skull and the first 1st lumbar nerve vertebra is C1). Cauda equina Lumbar plexus Each spinal nerve is attached to the cord by two 5th lumbar roots—the dorsal root (sensory root) and the ventral vertebra (L5) root (motor root). The dorsal root is enlarged to form 5th lumbar nerve the dorsal root ganglion, which contains the unipolar 1st sacral vertebra sensory neurons with their single axon and two (S1) branches; the peripheral branch and the central 1st sacral nerve branch. Two-thirds of the central branches terminate at the dorsal horn (see below) they entered. The re- Sacral plexus maining one-third ascends up the spinal cord to synapse with neurons located in the lower part of the FIGURE 5.14. Structure of the Spinal Cord—Coronal Section medulla. The peripheral branch passes via the spinal nerves to sensory receptors in the body. Distal to the dorsal root ganglia, the sensory and motor roots are bound together to form a spinal nerve. Thus, spinal nerves contain both motor and sensory nerves and are considered mixed nerves. The distribution of the spinal nerves to different parts of the body is consid- ered later. Protection of the Spinal Cord The vertebral column and its surrounding ligaments, tendons, and muscles separate the spinal cord from the external environment. The spinal cord must also be protected from damaging contact with the bony wall of the vertebral canal. The spinal meninges (Fig- ure 5.16) provide physical stability and, along with the cerebrospinal fluid, help absorb shock. Blood ves- sels that supply the spinal cord pierce through the meninges. Like the meninges around the brain, the spinal meninges have three layers: the pia, arach- noid, and dura mater. The spinal meninges are con-

Chapter 5—Nervous System 323 Meningitis LUMBAR PUNCTURE OR SPINAL TAP Meningitis is inflammation of the meninges. Usually In this procedure, a needle is inserted into the subarach- caused by infection, it affects blood and cerebrospinal noid space to take samples of CSF. To reduce the chance fluid circulation and destroys nerve tissue, leading to se- of injury to the spinal cord, the lower lumbar region is vere headache and motor and sensory complications. used because the spinal cord ends at around L2. tinuous with the cranial meninges at the foramen mater. The cell bodies of neurons and short nerve magnum. fibers are located in the gray mater. The white mater contains neuroglia and fiber tracts. The gray mater The dura mater is a tough fibrous sheath that is appears as if it has many horns. Depending on the lo- the outer covering of the spinal cord. Its collagen cation, the horns are called the posterior or dorsal fibers are oriented longitudinally. At the foramen gray horns, anterior or ventral gray horns, and lat- magnum, it fuses with the periosteum of the occipi- eral gray horns. The cell bodies of nerves supplying tal bone. Distally, it forms a cord that surrounds the skeletal muscles (motor neurons) are located in the filum terminale to form the coccygeal ligament. ventral horn. The lateral horn contains cell bodies of This way, it provides longitudinal stability to the the autonomic nerves, and the dorsal horn has cell spinal cord. Laterally, the dura fuses with the con- bodies of nerves that receive impulses from the spinal nective tissue surrounding the spinal nerves as they nerves. At the center of the gray mater is the central exit through the intervertebral foramen. The space canal through which cerebrospinal fluid flows. between the dura and the vertebral canal is the epidural space, containing loose connective tissue, Functions of the Gray Mater adipose tissue, and blood vessels. of the Spinal Cord The arachnoid mater is the membrane lying deep The gray mater of the spinal cord has two functions. to the dura. A potential space—the subdural space— First, synapses relay signals between the periphery separates it from the dura. The arachnoid is lined by and the brain in both directions. It is in the dorsal squamous epithelium. From its inner surface, deli- horns that sensory signals are relayed from the sen- cate, loose collagen and elastic fibers extend between sory roots of the spinal nerves to other parts of the the epithelium and the inner layer pia. This space, the CNS. It is mainly in the ventral and lateral horns that subarachnoid space, is filled with cerebrospinal motor signals are relayed from the neurons descend- fluid (CSF). The CSF, discussed on page ••, serves as ing from the brain to the motor nerves and auto- a shock absorber and a medium that transports dis- nomic nerves. solved gases, nutrients, chemical messengers, and waste products. Second, the gray mater of the cord integrates some motor activities. For example, if you touch a hot ob- The pia mater is firmly bound to the spinal cord. ject with your hand, the hand is withdrawn within a Collagen and elastic tissue from this layer extend lat- erally from either side of the spinal cord as the den- Posterior horn ticulate ligament. This ligament, after piercing the arachnoid, becomes attached to the dura, giving the Posterior root Posterior median sulcus spinal cord lateral stability. of spinal nerve Posterior funiculus Sectional Anatomy of the Spinal Cord Spinal Lateral ganglion horn If a transverse section is made of the spinal cord (see Figure 5.16) and viewed, two areas are noted. Around Lateral the center, there is a gray area—the gray mater. Sur- funiculus rounding it is a white area known as the white Spinal nerve Epidural Block Anterior root of Anterior horn spinal nerve Anterior funiculus Anesthetics are injected into the epidural space at spe- Central canal cific points to control pain in regions supplied by spinal Anterior median nerves that exit from there. This procedure is known as fissure an epidural block. FIGURE 5.16. Structure of the Spinal Cord—Transverse Section

324 The Massage Connection: Anatomy and Physiology few seconds. This reflex occurs even without the sig- white, and the branch is known as the white ramus. nals reaching the brain. Other similar reflexes that The axons synapse with postganglionic neurons lo- occur at the level of the cord are contraction of the cated in the sympathetic ganglion that runs along the extensors when standing, stretch reflexes that cause side of the vertebral column. From the sympathetic the muscle to contract when stretched and, in lower ganglion, axons of postganglionic fibers that inner- animals, the scratch reflex. vate glands and smooth muscles in the body wall and limbs join the spinal nerve again as the gray ramus. Function of the White Mater Other postganglionic fibers of the sympathetic sys- tem that innervate the organs in the thoracic and ab- The white mater surrounding the gray mater is dominal cavity form networks and nerves in the tho- white because of the myelinated fibers. It contains rax and abdomen. bundles of axons that carry impulses across spinal segments, as well as to and from the brain. The The spinal nerve then branches into the dorsal ra- horns of the gray mater divide the white mater into mus and ventral ramus, distal to the white and gray three distinct columns. The two dorsal, or poste- rami. The dorsal ramus of each spinal nerve carries rior white columns, lie between the dorsal gray sensory and motor innervation to the skin and the horns. The two lateral white columns lie on each muscles of the back. The larger ventral ramus sup- side of the cord lateral to the gray mater. Two ven- plies the structures on the body wall, the limbs, and tral, or anterior white columns, lie anterior to the the ventrolateral areas of the body. ventral gray horns. Axons from neurons having the same function, conduction speed, diameter, and NERVE PLEXUS myelination are bundled together as tracts in these columns. Tracts carrying sensory information to the The arrangement of spinal nerves in the segments brain are the ascending tracts, and those from the controlling the muscles of the neck and limbs is a lit- brain are the descending tracts, conveying com- tle complicated because, in the embryo, small mus- mands from the brain to the spinal cord. cles supplied by nerves in the ventral rami fuse to form larger muscles. These larger muscles (now iden- The spinal cord has two central depressions anteri- orly and posteriorly. The anterior is deep and narrow Dorsal ramus Spinal cord and is known as the anterior median fissure. The White ramus posterior depression is the posterior median sulcus. Sympathetic ganglion Ventral ramus DISTRIBUTION OF SPINAL NERVES Sympathetic To review, each of 31 spinal nerves are attached to the nerve spinal cord via the dorsal and ventral roots. The dor- sal root is enlarged to form the dorsal root ganglion. Anterior cutaneous Lateral Close to the intervertebral foramen, the dorsal and branches cutaneous ventral roots are bound together to form the spinal branches nerve. FIGURE 5.17. Transverse Section of Body, Showing Peripheral As the spinal nerve continues to the periphery, it is Distribution of Spinal Nerves. Dorsal ramus (to skeletal mus- surrounded by layers of connective tissue. The outer- cles, smooth muscles, and glands, etc., of back; from receptors most layer, or epineurium, consists of a dense net- of back); ventral ramus (to skeletal muscles, smooth muscles, work of collagen fibers. The nerve is divided into and glands, etc., of body wall, limbs; from receptors of body many bundles of axons by the perineurium, which wall, limbs); and sympathetic nerve (to smooth muscles, glands, consists of collagen fibers that extend inward from and visceral organs in the thoracic and abdominopelvic cavity; the epineurium. Individual axons are surrounded by from receptors of organs). delicate connective tissue fibers, the endoneurium. The blood vessels travel along the connective tissue layers, delivering nutrients and oxygen to individual axons, Schwann cells, and connective tissue and re- moving waste products. Figure 5.17 shows the distribution of a typical spinal nerve. The first branch of the spinal nerve in the thoracic and upper lumbar regions (T1–L2) car- ries preganglionic axons of the sympathetic nervous system (see page ••). These myelinated fibers appear

Chapter 5—Nervous System 325 Cervical Plexus and Injuries The plexus lies in relation to the first four cervical vertebrae, venterolateral to the levator scapulae mus- Injury to the spinal cord above the origin of the phrenic cle and scalenus medius and deep to the sternoclei- nerve (nerve to the diaphragm, C3, 4, 5) will result in domastoid muscle. Impingement of the plexus results respiratory arrest because impulses cannot be sent to in headaches, neck pain, and breathing difficulties the diaphragm. and is most often a result of pressure on the nerves by the suboccipital and sternocleidomastoid muscles or tified as single muscles) have compound nerve supply by shortening of the connective tissue located in the (i.e., they are supplied by nerves originating from base of the skull. more than one spinal segment). The ventral rami also blend their fibers with that of adjacent ones and a The Brachial Plexus complex interwoven network of nerves is found. These networks are termed nerve plexus, and there This plexus (Figures 5.14 and 5.18) is formed by the are three major ones: the cervical plexus, brachial ventral rami of spinal nerves C5–T1. It innervates the plexus, and lumbosacral plexus (Figure 5.14). shoulder girdle and upper limb. The ventral rami combine and divide in a specific manner as they pass The Cervical Plexus from the vertebral column and neck region into the upper limbs. The cervical plexus consists of the ventral rami of spinal nerves C1–C5 (Figure 5.14). Nerves from here Initially, the ventral rami (roots) combine to form innervate muscles of the neck, shoulder, and the di- three large trunks—superior, middle, and inferior. aphragm (phrenic nerves C3–5). The sensory compo- These trunks divide into anterior and posterior di- nent of this plexus can be visualized in Figure 2.6, visions. All the divisions pass under the clavicle and page ••, showing the dermatomal pattern. It supplies over the first rib into the axilla (armpit), where they the skin of the ear, neck, and upper chest. fuse to form the three cords—lateral, medial, and posterior cords. The cords align around the axillary artery in the axilla and give rise to the major nerves Posterior scapular nerve C5 Long thoracic nerve Suprascapular nerve Subclavian nerve C6 Posterior cord C7 Lateral pectoral nerve C8 Lateral cord Musculocutaneous nerve T1 Axillary nerve Contribution Radial nerve from T2 Thoracodorsal nerve Medial cord Subscapular nerve Median nerve Medial pectoral Ulnar nerve nerve Medial antebrachial cutaneous nerve Medial brachial cutaneous nerve Roots Anterior divisions Trunks Cords Posterior divisions Terminal branches FIGURE 5.18. Brachial Plexus

326 The Massage Connection: Anatomy and Physiology The brachial plexus, being more accessible than the others, is prone for injury in the neck and axilla. Injury to Superior Roots (C5, C6) of the Some nerves are also prone for damage, especially Brachial Plexus those branches that lie superficially. It is important for therapists to understand the relationship of the If the head is pulled forcefully from the shoulder (or brachial plexus in the neck and axilla. In the neck, the vice versa), there is potential for injury to the superior brachial plexus lies in the posterior triangle and is roots. Such trauma may occur when a person falls on covered by skin, platysma, and deep fascia. The roots the shoulder or during delivery when unusual force is lie between the scaleni anterior and medius. The used to extract the head after the shoulder has been de- plexus then becomes dorsal to the clavicle and sub- livered. In a person with such an injury, the limb on the clavius and superficial to the first digitation of the affected side is characteristically positioned. There is serratus anterior and the subscapularis to enter the adduction and medial rotation of the shoulder, exten- axillary region. Impingement of the brachial plexus is sion of the elbow, pronation of the forearm, and flexion most often a result of the scalenes, pectoralis minor, of the wrist. Sensation may be lost on the lateral side of and subclavius. the arm. This condition is known as Erb-Duchene palsy or Waiter’s tip/Porter’s tip position. The major nerves of the lateral cord are the muscu- locutaneous nerve (supplies the flexors of the arm) and the median nerve (supplies most muscles of the anterior forearm and certain muscles of the hand). The major nerve of the medial cord is the ulna nerve (supplies the anteromedial muscles of the forearm and most muscles of the hand). The axillary (supplies the deltoid and teres minor) and radial nerves (supplies the muscles on the posterior aspect of the arm and forearm) are the major nerves of the posterior cord. The muscles supplied and the area of skin innervated by each of these five nerves is given in Table 5.2. Waiter’s Tip/Porter’s Tip Position Courses of the Major Nerves Sometimes, the long thoracic nerve (C5–C7) that sup- plies the serratus anterior muscle may be injured, result- The axillary nerve (C5–C6) curves around the upper ing in winging of the scapula where the medial border and anterior aspect of the humerus to supply the del- and inferior angle of the scapula move away from the toid and teres minor muscles and the skin of the thoracic cage. In this condition, it is difficult to abduct shoulder and the shoulder joint (see Figure 5.19). the shoulder beyond 90°. The musculocutaneous nerve (C5–C7) curves lat- Winging of the Scapula erally through the deep portions of the anterior arm and then continues superficially down the lateral sur- of the upper limb. Branches from these nerves supply face of the forearm to provide sensory innervation (see the skin and muscles of the upper limb. Figure 5.20). The biceps brachii, coracobrachialis, and brachialis are some major muscles innervated. Roots (C5–T1) → Trunks (superior, middle, infe- rior) → Divisions (anterior, posterior) → Cords (lat- The radial nerve (C5–C8, T1) curves posteriorly eral, medial, posterior) → Nerves. and laterally behind the humerus and enters the forearm over the lateral epicondyle of the humerus (see Figure 5.21). Thereafter, it follows the lateral border of the radius and finally continues into the posterior portions of the thumb and first three fin- gers. The principal movements controlled by the ra- dius are (1) extension of the elbow, (2) supination of the forearm and hand, (3) extension of the wrist, fin- gers, and thumb, and (4) abduction of the thumb. The median nerve (C6–C8, T1) passes down the anteromedial portion of the arm, then distally in the anterolateral portions of the forearm, into the lateral palm of the hand, and into the anterior compartment of the thumb and first two fingers and lateral half of the third finger (see Figure 5.22). The median nerve innervates approximately the lateral two-thirds of the