KATCH AND KATCH - Essentials of Exercise Physiology

11C h a p t e r The Neuromuscular System and Exercise CHAPTER OBJECTIVES • Identify the major structural components of the central • Draw and label a skeletal muscle fiber’s ultrastructural nervous system that control human movement. components. • Diagram the anterior motoneuron and discuss its role • Describe the sequence of chemical and mechanical in human movement. events during skeletal muscle contraction and relaxation. • Draw and label the basic components of a reflex arc. • Contrast slow- and fast-twitch muscle fiber • Define motor unit, neuromuscular junction, autonomic characteristics including subdivisions. nervous system, excitatory postsynaptic potential, and inhibitory postsynaptic potential. • Outline muscle fiber-type distribution patterns among • Explain factors associated with neuromuscular fatigue. diverse groups of elite athletes. • Describe the function of muscle spindles and Golgi • Explain how exercise training modifies muscle fibers tendon organs. and fiber types. 337

•338 SECTION IV The Physiologic Support Systems Part 1 Neural Control of spheres. The outer portion of the brain, thecerebral cortex Human Movement or gray matter (gray because nerve fibers lack a whit myelin coating), consists of a series of folded convolutions. Similarities exist between the most advanced supercom- The bottom panel in Fig. 11.2C depicts the four lobes of puter and the brain’s highly sophisticated and intricate the cerebral cortex ( occipital, parietal, temporal, and multiple-layer system of neurons and their interconnec- frontal) and the sensory and motor areas and cerebellum. tions to the muscular system. Not surprisingly, the inte- grative and organizational complexity of the human The bony skull and a composite of four tough mem- nervous system far exceeds the capacity of many clusters branes called meninges, which contain a jelly-like cushioning of supercomputers. Interactive neural control mechanisms substance, surround the brain to protect it from external selectively process bits of sensory input in response to trauma as occurs in sports-related traumatic brain injuries ever-changing internal and external stimuli. Human move- (www.headinjury.com/sports.htm). ments that require little force, and sophisticated move- ments that require large force, depend on the coordinated Central Nervous System—The Spinal Cord reception and integration of sensory neural input to trans- mit and coordiante signals to the effector organs—the Figure 11.3A illustrates the spinal cord (about 45 cm long muscles. and 1 cm in diameter) encased by 33 vertebrae (seven cer- vical, 12 thoracic, five lumbar, five sacral, and four co This chapter describes the neural control of human cygeal). The 12 pairs of peripheral nerves, grouped into movement that includes: cervical, thoracic, lumbar, and sacral sections according to their location along the spine, exit the spinal cord through 1. Structural organization of the neuromotor system, a small hole or foramen at the juncture between each pair with emphasis on the central and peripheral of vertebrae (Fig. 11.3C). nervous systems This unique anatomical design allows extreme vertebral 2. Neuromuscular transmission movement without affecting spinal nerve function. Inter- 3. Sensory input for muscular activity vertebral discs separate adjacent vertebrae and under 4. Motor unit type, function, and activation normal circumstances provide a cushioning surface. Unfor- tunately, a disc can bulge into the space occupied by that NEUROMOTOR SYSTEM segment’s spinal nerve, compressing it and causing pain in an area the nerve innervates (e.g., lower back, buttocks, or ORGANIZATION full length of the leg). This unfortunate cascade of events can cause loss of motor control. If the condition persists with The human nervous system consists of two major compo- significant muscle weakness (e.g., inability to raise an nents: (1) the central nervous system (CN S), which lower the body vertically off the ball of one foot), surgical includes the brain and spinal cord, and (2) the peripheral repair or removal of the offending disc often relieves the nervous system (PN S) composed of cranial and spinal pressure and pain, but this is not a foolproof solution. nerves. Figure 11.1 presents an overview of these two components of the human nervous system. When viewed in cross-section ( Fig. 11.3B), the spinal cord shows its H-shaped core of gray matter. The limbs of Central Nervous System—The Brain this core, the ventral (anterior) and dorsal (posterior) horns, contain principally three types of nerves: Figure 11.2A illustrates a lateral view of the brain’s six main divisions: 1. Interneurons 2. Sensory neurons 1. Medulla oblongata 3. Motoneurons (motor neurons) 2. Pons 3. Midbrain Motor or efferent neurons conduct impulses outward from 4. Cerebellum the brain or spinal cord. They exit the cord through the 5. Diencephalon ventral root to supply extrafusal and intrafusal skeletal 6. Telencephalon muscle fibers. Sensory or afferent neurons enter the spinal cord via the dorsal root. An area of white matter that con- Each of the 12 cranial nerves originates in one of these tains ascending and descending nerve tracts within the anatomic areas. Figure 11.2B shows a superior view of the cord surrounds the gray core. The ascending nerve tracts brain. The longitudinal fissure runs down the midline an within the spinal cord transmit sensory information from separates the brain’s right and left sides, which are called peripheral sensory receptors to the brain. Tracts of nerve hemispheres. Below the fissure, a large tract of nerve fibe tissue descend from the brain and terminate at neurons in (corpus callosum, not shown) connects the two hemi- the spinal cord. One key tract of neurons, the pyramidal tract, transmits impulses downward through the spinal cord. By direct routes and interconnecting spinal cord neu- rons, these nerves eventually excite motoneurons that control skeletal muscles. Extrapyramidal tract nerves orig- inate in the brain stem and connect at all levels of the

•Chapter 11 The Neuromuscular System and Exercise 339 Brain Peripheral Central Nervous Nervous Spinal cord System System Vertebral Skin Somatic sensory column fiber Spinal nerve Cardio- vascular Visceral sensory fiber PSayrmaspyamthpaetthicetmicomtoortfoirbefirber temof s omatic Muscle Mneortvoor ufisbseyrs Nervous System Central Nervous System (CNS) Peripheral Nervous System (PNS) • Brain (including retinas) • Cranial nerves III—XII • Spinal cord • Spinal nerves • Integrative/control centers Afferent Division (sensory) Efferent Division (motor) • Somatic and visceral neurons • Motor neurons • Conducts impulses from • Conducts impulses from the receptors to CNS CNS to effectors Autonomic Nervous System Somatic Nervous System • Involuntary • Voluntary • Conducts impulses from the CNS • Conducts impulses from the CNS to cardiac muscle, smooth to skeletal muscles muscles, and glands Sympathetic Parasympathetic Figure 11.1 The two divisions of the human nervous system. The central nervous system (CNS) contains the brain (including the retinas), spinal cord, and integrating and control centers; the cranial nerves and spinal nerves make up the peripheral nervoussystem (PNS). The PNS further subdivides into the afferent (sensory) and efferent (motor) divisions. The efferent division consists ofthe somatic nervous system and autonomic nervous system (sympathetic and parasympathetic divisions).

•340 SECTION IV The Physiologic Support Systems A Diencephalon Midbrain Telencephalon Pons Thalamus Epithalamus B Longitudinal Cerebellum Medulla oblongata fissure Spinal cord Motor Central sulcus cortex Sensory cortex Left Right hemisphere hemisphere C Parietal lobe Sensory cortex Frontal lobe Occipital lobe Motor cortex Vestibular area Taste area Visual area Auditory area Cerebellum Temporal lobe Figure 11.2 (A) Principal six divisions of the brain, lateral view. (B) Superior view of the brain. (C) Four lobes of the cerebral cortex.

•Chapter 11 The Neuromuscular System and Exercise 341 spinal cord. These neurons control posture and provide a continual background uestions & Notes Qlevel of neuromuscular tone in contrast to discrete movements stimulated by the pyramidal tract nerves. List the 2 major components of the human nervous system. Brain Neurotransmitters Nerves communicate by releasing at their ter- 1. minal ends chemical messengers called neurotransmitters that diffuse across the synapse or junction between one nerve end and the cell body of another 2. nerve. The neurotransmitter combines with a targeted receptor molecule on the postsynaptic membrane to facilitate depolarization or, in some instances, hyper- polarization. Many of the neurons of the CNS particularly in the brain release or respond to these neurotransmitters. Three important brain neurotransmitter List the 2 parts of the autonomic nervous categories include: system. 1. Monoamines: Modified amino acids include epinephrine 1. norepinephrine, serotonin, histamine, and dopamine. 2. Neuropeptides: Short chains of amino acids include arginine, 2. vasopressin, and angiotensin II (also act as hormones [see Chapter 12]). Enkephalins and endorphins (sometimes called opioid neurotransmitters) represent other neuropeptides that produce a general sense of well-being. Release of endogenous opioid List the 2 parts of the somatic nervous neurotransmitters with exercise contributes to the exercise “high.” system. 3. Nitric oxide (NO): Neurons in the CNS and other cell types contain NO 1. receptors that serve as signalling molecules in the cardiovascular system. 2. Peripheral Nervous System The PNS consists of 31 pairs of spinal nerves (eight cervical, 12 thoracic, fiv The number of cranial nerves ϭ ________ lumbar, five sacral, and one coccygeal) and 12 pairs of cranial nerves. Number identify these nerves (e.g., C1 is the first nerve from cervical region). Carefu List the 4 brain lobes. experiments have tracked the exact location of the spinal nerves and mapped 1. the muscles they innervate. Injury to a specific spinal cord area produces pre 2. dictable neurologic consequences. For example, quadriplegia almost always 3. results from damage to the upper thoracic vertebra and corresponding descend- ing nerve tract. The PNS includes afferent nerves that relay sensory information from muscles, joints, skin, and bones toward the brain and efferent nerves that transmit information away from the brain to glands and muscles. The somatic and autonomic nervous systems consist of efferent neurons. Somatic Nervous System The somatic nervous system innervates 4. skeletal muscle (voluntary muscle). Somatic efferent nerve firing excites muscl State the primary function of intervertebral activation, and autonomic nerve firing discussed in the next section can excit discs. or inhibit activation. List 3 types of neurons in the human Autonomic Nervous System Efferent nerves of the autonomic nerv- nervous system. ous system activate the viscera and other tissues on a subconscious level. Auto- 1. nomic nerves innervate smooth muscle (involuntary muscle) in the intestines, 2. sweat and salivary glands, myocardium, and some endocrine glands. The heart 3. and intestines display automatic excitability, but one can also exert conscious control over these tissues under some circumstances. For example, individuals who practice yoga or meditation can modify their heart rate and regional blood flow on command. In hypnosis (from the Greek word “sleep”) a state of height ened awareness and focused concentration can manipulate pain perception, access repressed material, and “reprogram” some behaviors. Some champion weight lifters apply self-hypnosis before attempting heavy lifts to focus all their muscular efforts on the lift without the possible distraction of discomfort in attempting the lift (just prior to the lift as the muscles tense and prepare for an all-out effort). This self-induced “trance” blocks out superfluous neural inpu that might interfere with a maximal effort.

•342 SECTION IV The Physiologic Support Systems A B Spinal Cord Ventral View Cerebrum Dorsal root White Gray Dorsal ganglion matter matter root Cerebellum Lower brainstem Spinal (medulla) nerve Peripheral nerves Motor unit 2 Spinal cord Impulse Motoneuron Motor axon unit 1 Ventral root Axonal terminals at neuromuscular junctions Muscle fibers C Spinal cord Nerve root Spinous process Superior articular Spinal cord process Vertebral foramen Intervertebral disc Spinal nerve Lumbar vertebrae Vertebral body Cervical vertebra Figure 11.3 (A) Human spinal cord showing the peripheral nerves. (B) Ventral view of a spinal cord section to illustrate dorsal and ventral root neural pathways and nerve impulse direction. (C) Junction of two lumbar vertebral bodies and a cross-section through one cervical vertebra.

•Chapter 11 The Neuromuscular System and Exercise 343 Conscious modulation of aspects of the autonomic nervous system offers uestions & Notes Qalternative treatment in medicine, such as control of hypertension and stress- related disorders through biofeedback techniques and applies to certain sports. Name the 2 divisions of the autonomic Competitors in archery and other target-shooting events consciously modify nervous system. their cardiovascular and respiratory patterns so normal breathing and pulse rate 1. temporarily “stop” during the crucial steadying phase of performance. The autonomic nervous system functions as a unit to maintain constancy in the internal environment. Figure 11.4 illustrates the sympathetic and parasym- pathetic divisions of the autonomic nervous system. Sympathetic nerve fiber 2. mediate excitation and parasympathetic activation inhibits excitation except for vagal parasympathetic excitation of gastrointestinal motility and tone and pan- creatic insulin secretion. In contrast to the somatic nervous system, some cell bodies or ganglia of sympathetic and parasympathetic neurons exist outside the CNS. Describe the function of sympathetic nerve Sympathetic Nervous System Sympathetic nerve fibers supply the heart fibers smooth muscle, sweat glands, and viscera. These neurons exit the spinal cord and enter a series of ganglia near the cord’ssympathetic chain. The nerves ter- minate relatively far from the target organ in adrenergic fibe endings that release norepinephrine. Excitation of the sympathetic nervous system occurs during fight-or-flight situations that require whole-body arousal for emerge Describe the function of parasympathetic cies. Autonomic sympathetic stimulation accelerates breathing and heart rate nerve fibers almost instantaneously; the pupils dilate, and blood flows from the skin t deeper tissues in anticipation of a perceived bodily challenge. Parasympathetic N ervous System Parasympathetic nerve fibers exit th Indicate areas of the body innervated by brain stem and sacral segments of the spinal cord to supply the thorax, the sympathetic and parasympathetic abdomen, and pelvic regions. Parasympathetic nerve endings release acetyl- nervous system. choline (ACh; cholinergic fiber ). The postganglionic parasympathetic nerve fibers located close to the organs they innervate produce effectsopposite of sym- Sympathetic: pathetic fibers. For example, parasympathetic neural stimulation via the vagu nerve slows heart rate while sympathetic stimulation accelerates heart rate. Parasympathetic: Most organs receive simultaneous sympathetic and parasympathetic stimu- lation. Both systems maintain a constant degree of activation calledneural tone. Depending on physiologic need, one system becomes more active while the other becomes inhibited. Dual innervation of this type permits a finer level o control at the end organs. This can be likened to hot and cold faucets being open at the same time; minor adjustments in both faucets rapidly and precisely change the temperature compared with alternately turning each of the faucets on or off. Autonomic Reflex Arc For Your Information Figure 11.5 illustrates a typical neural arrangement for a monosynaptic refle INNERVATION RATIO arc in the spinal cord. Sensory input (e.g., a knee tap with a percussion refle hammer and subsequent excitation of muscle spindles within the quadriceps The finger contains 120 motor units muscle) initiates transmission of afferent impulses to the spinal cord via the that control 41,000 muscle fibers. In sensory (dorsal) root. This in turn stimulates the anterior motoneuron to the contrast, the medial gastrocnemius quadricep femoris to contract and extend the lower leg, counteracting the initial muscle (calf ) has 580 motor units that stretch. The “knee-jerk” reaction only takes a few milliseconds because the trig- innervate 1,030,000 fibers. The ratio gered impulse has to make a return trip via the spinal cord without going to the of muscle fibers per motor unit aver- brain. A delay or absence of the stretch reflex can indicate possible neurologi ages 340 for finger muscles and 1800 or neuromuscular dysfunction to spinal nerves and their innervations or for the gastrocnemius muscle. injuries to the knee and leg. In a polysynaptic reflex arc the nerves synapse i the cord through interneurons that distribute information to various cord lev- els. The impulse then passes over the motor root pathway through anterior motoneurons to the effector organ.

Sympathetic division Parasympathetic division Dilates Constricts pupil pupil Inhibits Stimulates salvation salvation Cranial Constricts Relaxes Constricts Cranial Cervical blood vessels airways airways Cervical Thoracic Thoracic Accelerates Slows Lumbar heartbeat heartbeat Lumbar Stimulates Inhibits Stimulates secretion by digestion digestion sweat glands Celiac Stimulates gall ganglion bladder to release bile Stimulates glucose pro- duction and release Sacral Sacral Stimulates Dilates blood secretion of vessels in gut epinephrine and norepinephrine Inferior Relaxes urinary Stimulates urinary Noradrenergic neurons mesenteric bladder bladder to contract Postganglionic ganglion Stimulates Stimulates penile Cholinergic neurons ejaculation erection Preganglionic Postganglionic Comparison of effects of sympathetic and parasympathetic activation on end organs End organ Sympathetic effects Parasympathetic effects Skeletal muscle Increase blood flow Decrease blood flow Ventilation Increase Decrease Sweat glands Increase perspiration No effect Heart Increase force and contraction rate Decrease force and contraction rate GI tract motility Decrease Increase Eyes Dilate pupils Constrict pupils Secretion of digestive juices Decrease Increase Blood pressure Increase mean pressure Decrease mean pressure Airways Increase diameter Decrease diameter Figure 11.4 The sympathetic and parasympathetic divisions of the autonomic nervous system: comparisons of effects of activation of each. The preganglionic inputs of both divisions use acetylcholine (Ach; colored red) as a neurotransmitter. The postganglionic parasympathetic innervation of the visceral organs also uses Ach, but postganglionic sympathetic innervation uses norepinephrine (NE; colored blue), with the exception of innervation of the sweat glands, which use Ach. The adrenal medulla receives pregangilonic sympathetic innervation and secretes epinephrine into the bloodstream when activated. In general, sympathetic stimulation produces catabolic effects that prepare the body to “fight” or “flee,” and parasympathetic stimulation produces anabolic responses that omote normal function and conserve energy. (Modified from Bear, M.F., et al.:Neuroscience: Exploring the Brain, 3rd ed. Baltimore: Lippincott Williams & Wilkins, 2006.) 344

•Chapter 11 The Neuromuscular System and Exercise 345 White Dorsal horn Questions & Notes matter Ventral horn Draw and label a typical autonomic refle Sensory neuron Gray arc in the spinal cord. (afferent fiber) matter Muscle Alpha spindle motor neuron (efferent fiber) Extensor muscles Synapse Tendon Briefly describe the main difference between a simple and complex spinal reflex Leg extension Figure 11.5 Schematic of the patella tendon stretch reflex (also called the patella For Your Information reflex). Firm percussion or tapping of the patellar tendon with the reflex hamm shown at the left draws the patella momentarily down, stimulating the muscle spindles’ TYPES OF MOTONEURONS afferents and Golgi tendon organs (GTOs) by altering the stretch and length of the muscle that provokes a preprogrammed reflex contraction. The “knee-jerk” reactio The large diameters of anterior only takes a few milliseconds because the triggered impulse only has to make a return motoneurons, termed type A ␣ fibers, trip to the spinal cord without going to the brain. A delay or absence of the stretch range between 8 and 20 ␮m (1 ␮m ϭ reflex can indicate possible neurologic or neuromuscular dysfunction to spinal nerve one-millionth [0.000001] of a meter). and their innervations or injuries to the knee and leg. The diagram only shows one side Diameters of other smaller type A of the spinal nerve complex. fibers (␥ efferent motoneurons) do not exceed 10 ␮m. Their conduction Another example of a simple reflex arc occurs when a person accidentall velocities equal about half of ␣ fibers. touches a hot object. Stimulation of pain receptors in the fingers fires senso ␥ Efferent fibers connect with propri- information over afferent fibers to the spinal cord to activate efferent moto oreceptors (special stretch sensors) in fibers to remove the hand from the hot object. Concurrently, the signal trans skeletal muscle to detect minute mits via interneurons up the cord to the sensory area in the brain that actually changes in muscle fiber length. “feels” the pain. The various operational levels for sensory input, processing, and motor output, including the reflex action just described, explain how th hand withdraws from the hot object before the person perceives pain. Refle actions in the spinal cord and other subconscious areas of the CNS control many muscle functions. These reflex actions even operate for people who hav had their spinal cords severed above the level required for the reflex Complex Reflexes Complex spinal reflexes that involve multiple synapses and muscle groups als exist. Consider the situation of stepping on a tack with the left foot. Almost simultaneously as the tack pierces the skin, the right leg straightens to remove weight from the injured foot, which lifts off the ground.Figure 11.6 illustrates the neural and motor pathways activated in this complex action, termed the crossed-extensor reflex in the following five-step sequence: Step 1. The tack stimulates pain receptors in the skin. The receptors transmit the message to the spinal cord via the sensory nerve. Step 2. Sensory neurons branch to each side of the cord to activate interneurons in the gray matter. Step 3. Interneurons synapse with motoneurons, innervating both flexo and extensor muscles in each leg.

•346 SECTION IV The Physiologic Support Systems Crossed-Extensor Reflex Extensor Flexor Interneurons Flexor Extensor (relaxation) (contraction) (contraction) (relaxation) To brain + + + + + Stimulation Inhibition Tack pierces left foot, Right leg extended left leg withdrawn Figure 11.6 Crossed-extensor reflex in both legs represents a more complex reflex with multiple synapses and muscle group Step 4. Inhibition and stimulation of appropriate leg messages have mastered the proper sequencing of finge flexor and extensor muscles cause concurrent rapi and hand movements so the desired outcome occurs essen- extension of the uninjured limb and flexion t tially automatically. Perfecting a sports skill, no matter how remove the injured limb. simple it may appear like swinging a baseball bat to contact a “fast” pitch or kicking a soccer ball with just the right Step 5. Interneuron connections simultaneously speed into the right or left side of the net, requires hundreds activate neural pathways to transmit information to or even thousands of practice hours to engrain the move- appropriate sensory areas of the brain where the ment until it becomes automatic and flawless in execution pain is “felt.” Unfortunately, improper practice also can automate a task to engrain less than optimal neuromuscular actions. Most Learned Reflexes The knee-jerk and crossed-exten- individuals who practice the golf swing, for example, do so by reinforcing poor habits. It starts with the grip and the sor reflexes occur automatically and require no learning first 6 inches of the takeaway in the backswing. Setting u Practice facilitates other more complex reflex patterns suc with an improper grip, followed by a rapid cocking of the as most sports performances and occupational tasks. Con- wrists at the start of the backswing, fuels a recipe for disas- sider a trained office worker who types 90 words a minute ter. This means that continual “poor” practice reinforces At an average of five letters per word, this requires six t nonoptimal mechanics and poor shots. Instead of hitting eight keystrokes per second. For this person, the sight of a one ball after another, hours on end, the aspiring golfer word to type initiates a series of rapid hand and finge must practice correct swing mechanics under the watchful movements that require little conscious effort. A beginning eye of a trained professional, ideally with video feedback. typist, in contrast, proceeds slowly since thought must be The adage “practice makes perfect” should be amended to directed to the position of each key along the keyboard and “perfect practice leads to more perfect performance.” proper execution and sequencing of wrist and finger move ments. As neuromuscular pathways become “ingrained” Nerve Supply to Muscle The terminal branches of through proper or meaningful practice the typing move- ments progressively become reflex actions as the beginne one neuron innervate at least one of the body’s approximately approaches expert status. People who routinely send text

•Chapter 11 The Neuromuscular System and Exercise 347 250 million muscle fibers. About 420,000 motor nerves exist yet a single nerv uestions & Notes Qusually supplies numerous individual muscle fibers. In general, the branches of nerve within a muscle pass to specific localized groups of motor units. In multipl The ratio of muscle fibers to neuron group muscles, for example, the quadriceps with four separate muscles act in con- relates to a muscle’s particular cert to perform the main muscle action with no single muscle responsible for the _____________ _____________. full movement. The ratio of muscle fibers to nerves generally relates to a muscle’s par ticular movement function . Delicate, precise eye muscle movements require one neuron to control fewer than 10 muscle fibers (ratio of 10:1). The ratio in the lar ynx is an even lower 1:1. For less complex movements of the large leg muscles, a motoneuron may innervate as many as 3000 muscle fibers (ratio of 3000:1) A basic rule states that less complex movements like forearm supintion and Describe the anatomy of a motor unit. elbow flexion have a higher ratio of muscle fibers to motor nervesComplex eye and hand movements that require more specialized movements have a much lower ratio. The next sections review how information processed in the CNS activates specific muscles to cause an appropriate, specialized motor response Describe an anterior motoneuron. THE MOTOR UNIT A motor unit describes skeletal muscle fibers and their corresponding innervat Describe the main function of the axon’s ing anterior motoneuron. The motor unit thus represents the funtional unit of myelin sheath. movement. A whole muscle contains many motor units, each containing a sin- gle motoneuron and its composite muscle fibers The muscle fibers belonging to a particular motor unit are scattered ove subregions of the muscle; fibers from one motor unit are interspersed amon fibers of other motor units. The consequence of this disperson results in force being spread over a large muscle area to minimize mechanical stress. Motor Unit Anatomy Anterior Motoneuron Figure 11.7 illustrates that an anterior (alpha) motoneuron with its three main parts: cell body, axon, and dendrites. The cell’s unique architectural design permits the transmission of electrochemical impulses from the spinal cord to muscle. The cell body, located within the spinal cord’s gray matter, houses the control center, the structures involved in replicating and transmitting the genetic code. The axon extends from the cord and delivers an impulse to the muscle fibers it innervates. Short neural branche called dendrites receive impulses through numerous spinal cord connections and conduct them toward the cell body. Nerve cells conduct impulses in one direction only, akin to a one-way street, down the axon away from the stimulation point. As the axon approaches the muscle, it branches with each terminal branch to innervate a single muscle fiber A whole muscle contains numerous motor units, each with a single motoneuron and its complement of muscle fibers The myelin sheath encircles the axon of nerve fibers that are either long i length or large in diameter. A large part of this sheath acts as an electrical insu- lator that envelops the axon akin to the plastic coating around a copper electri- cal wire. The lipid-protein membrane myelin consists of approxomately 75% lipids (cholesterol and phospholipid) and 25% proteins. Myelin’s main func- tion increases the speed of neural impulses along the myelinated fiber. Thi occurs because myelin increases electrical resistance across the cell membrane by a factor of 5000 and decreases capacitance by 10-fold this number. Fiber myelination thus keeps the electrical current from leaving the bare axon while at the same time allowing a high signal transmission speed. The fat-containing molecules inhibit the propagation of electricity to make the signals jump from one section of myelin to the next. In the PNS, specializedSchwann cells encase the bare axon and then spiral around it. Myelin forms a large part of this sheath to insulate the axon. A thinner membrane, the neurilemma, covers the myelin

•348 SECTION IV The Physiologic Support Systems Alpha motoneuron (cell body) Dendrites Nerve trunk Motor unit Axon Bare hillock axon Direction of Nerve propagation of fibers action potential. Node of Vein Ranvier Artery Terminal Motor Impulse branches endplate Myelin Neurilemma sheath Figure 11.7 The anterior ␣-motoneuron consists of a cell body, axons, and dendrites. The roundinset at the bottom right illustrates a node of Ranvier that permits impulses to jump from one node to the next as the electrical current travels toward the terminabl ranches at the motor end-plate. sheath. Nodes of Ranvier interrupt the Schwann cells and cord, and optic nerves. This interferes with vision, sensa- myelin every 1 or 2 mm along the axon’s length. The tion, and body movements. myelin sheath insulates the axon to ion flow allowing th nodes of Ranvier axon to depolarize along their axon seg- Neuromuscular Junction (Motor End-Plate) ments. The alternating sequence of myelin sheath and node of Ranvier (termed saltatory conduction ) allows Figure 11.8 highlights the microanatomy of the neuromus- impulses to “jump” from node to node (similar to signals cular junction or motor end-plate that provides interface progressing from one telephone pole to the next) as elec- between the end of a myelinated motoneuron and a muscle trical current travels toward the terminal branches at the fiber. It functions to transmit nerve impulses to muscle fiber motor end-plate. N erve conduction in this manner Each muscle fiber usually has one neuromuscular junction accounts for the higher transmission velocity in myeli- The inset table in Figure 11.8 lists representative values for nated compared with unmyelinated fibers. Degenertion o ionic concentrations across the motoneuron membrane. the myelin sheath produces peripheral neuropathy and dis- ease. In multiple sclerosis, an autoimmune disease that The terminal portion of the axon forms several smaller affects approximately 200 people each week in the United axon branches whose endings, called presynaptic termi- States, destruction of the myelin sheath surrounding nerve nals, lie close to but not in contact with the muscle fiber’ fibers adversely affects neural pathways to the brain, spina plasma membrane orsarcolemma. The synaptic gutter, the region of the postsynaptic membrane, contains infoldings that increase its surface area. Between the synaptic gutter

•Chapter 11 The Neuromuscular System and Exercise 349 Action potential Axon Sarcolplasm of Synaptic vesicles muscle fiber containing acetylcholine T tubule Sarcolemma Mitochondrion Synaptic knob Sarcoplasmic Presynaptic reticulum membrane Neuromuscular junction Postsynaptic membrane Synaptic cleft Ionic concentrations (mM • L–1) Myofibril across neuron membrane Myofilament Ion Extracellular Intracellular Sodium (Na+) 150 15 Chloride (Cl–) 110 10 Potassium (K+) 150 5 Figure 11.8 Microanatomy of the neuromuscular junction, including details of the presynaptic and postsynaptic contact area between the motoneuron and the muscle fiber it innervates. Theinset table shows representative values for ionic concentrations across the motoneuron membrane. and the presynaptic terminal of the axon lies thesynaptic cleft, the region where uestions & Notes Qneural impulse transmission occurs. Excitation Excitation normally occurs only at the neuromuscular junction. The Describe the primary function of the neurotransmitter ACh provides the chemical stimulus to change an electrical neuromuscular junction. neural impulse into a chemical stimulus at the motor end-plate. Acetylcholine, released from small, saclike vesicles within the terminal axon, increases the postsynaptic membrane’s permeability to sodium and potassium ions. This spreads the impulse over the entire muscle fiber as a distinct wave of depolar Describe the role of acetylcholine in ization. As depolarization progresses, the muscle fiber’s contractile machiner neuromuscular excitation. primes for its major function—to contract or shorten. The enzyme cholinesterase, purified and crystallized from electric eels i 1968 and concentrated at the borders ofthe synaptic cleft,degrades acetylcholine within 5 millisecond (ms) of its release from the synaptic vesicles. This action immediately repolarizes the postsynaptic membrane. The axon resynthesizes acetylcholine from acetic acid and choline, byproducts of cholinesterase action, so the entire process can repeat with the arrival of successive nerve impulses.

•350 SECTION IV The Physiologic Support Systems Table 11.1 Characteristics and Correspondence Between Motor Units and Muscle Fiber Types MOTOR UNIT FORCE CONTRACTION FATIGUE MOTOR UNIT MUSCLE DESIGNATION PRODUCTION SPEED RESISTANCEa SAGb FIBER TYPE Fast fatigable (FF) High Fast Low Yes Fast glycolytic (FG) Fast Moderate Yes Fast oxidative-glycolytic (FOG) Fast fatigue-resistance (FR) Moderate Slow High No Slow oxidative (SO) Slow (S) Low aHow much the muscle tension declined with repetitive stimulation. bUnder repetitive stimuli, some motor units respond smoothly with a systematic increase in tension, but others first increase te sion and then decrease or “sag” slightly in response to the same tetanic stimulus. These sag characteristics can classify the different motor units. Only the slow (S) motor units do not exhibit sag, which probably relates more to their lower force-generating capabilities than their fatigue characteristics. Modified from Lieber, R.L.: Skeletal Muscle Structure, Function, and Plasticity, 3rd ed. Baltimore: Lippincott Williams & Wilkins, 2010. Facilitation A motoneuron generates an action potential containing a single motoneuron; thus, it is difficult t when its microvoltage decreases sufficiently to reach it identify which fibers belong to which motor unit withou threshold for excitation. With a subthreshold action poten- ambiguity. The classic motor unit physiology experiments tial, the neuron does not discharge, but its resting mem- performed in the late 1960s and early 1970s were on iso- brane potential still lowers, temporarily increasing its lated cat hindlimb motor units using intracellular motoneu- “tendency” to fire. A neuron fires when many subthresho ron stimulation to measure different electrophysiologic excitatory impulses arrive in rapid succession, a condition properties of the motoneuron and mechanical properties of termed temporal summation. Spatial summation describes the motor units within the whole muscle. This research, the simultaneous stimulation of different presynaptic ter- still used today, describes motor unit differences based on minals on the same neuron. The “summing” of each excita- the physiologic properties of the respective muscle fiber tory effect often initiates an action potential. innervated. Table 11.1 and Figure 11.9 describe and illus- trate the three physiologic and mechanical properties of Removing inhibitory neural influences becomes impor motor units and the muscle fibers they innervate tant under certain exercise conditions. In all-out strength and power activities like maximal bench press or vertical 1. Twitch (speed of contraction) characteristics leap, disinhibition and maximal activation of all motoneu- 2. Tension-generating (force) characteristics rons required for a movement enhances performance.Effec- 3. Neuromuscular fatigability tive disinhibition fully activates muscle groups during maximal lifting, an effect that accounts for the rapid, highly specifi Twitch Characteristics Early motor unit studies strength increases observed during the first few days and week of resistance training. Enhanced neuromuscular activation revealed that in response to a single electrical impulse, accounts for considerable improvements in muscular some units developed high-twitch tensions but others strength without concurrent increases in muscle size. CNS develped “relatively” low-twitch tensions whereas others excitation (also called “neuronal facilitation”) explains why generated intermediate tension. The differences between intense concentration, or “psyching,” can substantially motor units were judged on a relative rather than absolute “supercharge” maximal strength and power efforts. basis, sometimes making comparisons between studies difficult. Motor units with low-twitch tensions also tende Inhibition Some presynaptic terminals generate inhibitory to have “slower” contraction times but those with higher impulses by releasing chemicals that increase postsynaptic tensions tended to have “faster” contraction times. membrane permeability to potassium and chloride ions. The efflux of positively charged potassium ions or influx of neg Tension-Generating Characteristics Different tively charged chloride ions increases the membrane’s rest- ing electrical potential to create an inhibitory postsynaptic motor units and the muscles they innervate can develop potential (IPSP) that makes the neuron more difficult to fir different amounts of tension based on many factors. For No action potential occurs when a motoneuron encounters example, fast motor units and their corresponding fast excitatory and inhibitory influences or encounters a larg muscle fibers are able to generate more tension than IPSP. For example, one can usually override or inhibit the slow motor unit and its corresponding slow muscle fiber reflex to pull the hand away when removing a splinter Alternatively, perhaps fast and slow fibers generate th same tension, but fast motor units simply innervate a The neurotransmitter ␥-aminobutyric acid (GABA) and greater number of fibers then slow motor units. Or fas glycine exert inhibitory effects. Neural inhibition serves and slow units have the same number of fibers of equa protective functions and reduces the input of unwanted intrinsic strength, but fast fibers are larger and therefor stimuli to produce smooth, purposeful responses. generate more tension. Each of these possibilities is sup- ported by research, but current thinking indicates that fast Motor Unit Physiology and slow muscle fibers within a motor unit have about th same specific tension capacity but that fiber size a Identifying Muscle Fibers with Motor Units innervation ratio differ between motor unit types. Differ- ences in tension generation are govered by three factors: A whole muscle contains many possible motor units the all-or-none principle, graduation of force principle, and level of motor unit recruitment patterns.

•Chapter 11 The Neuromuscular System and Exercise 351 All-or-N one Principle If a stimulus triggers an action potential in the Questions & Notes motoneuron, all of the accompanying muscle fibers contract synchronously. single motor unit cannot generate strong and weak contractions; either the Describe 2 situations when temporal impulse elicits a contraction or it does not. Once the neuron fires and the impuls summation could enhance exercise reaches the neuromuscular junction, the muscle cells always contract to the performance. fullest extent in accord with the all-or-none principle first described by Henr Pickering Bowditch in 1871, American physiologist at Harvard’s Medical School 1. and Department of Anatomy, Physiology, and Physical Training (see Chapter 1). 2. Gradation of Force Principle The force of muscle action varies from slight to Describe what happens when a motor maximal in one of two mechanisms: neuron encounters both excitatory and inhibitory influences but the IPSP is larger 1. Increasing the number of motor units recruited 2. Increasing the frequency of motor unit discharge Activation of all motor units in a muscle generates considerable force compared with activating only a few units. Total tension also increases if repetitive stimuli Spinal cord Peripheral nerve Fast fatigable (FF) 50 g 100% Skeletal muscle 0 2 4 6 60 min 100 msec Twitch Rate of fatigue Fast fatigue resistant (FR) 20 g 100% Fast glycolytic fibers 0 100 msec 2 4 6 60 min Twitch Rate of fatigue Slow (S) Fast oxidative glycolytic fibers 50 g 100% 0 200 msec 2 4 6 60 min Twitch Rate of fatigue Slow oxidative fibers Figure 11.9 Schematic reprsentation of the anatomic, physiologic, and histochemical properties of the three motor unit types. Fast fatigable (FF) motor units (top) have large axons that innervate many large muscle fibers. The units generate large tensions but fatigu rapidly (see the tension and fatigue graphs to the left). Fast-fatigue resistant (FR) units (middle) have modertely sized axons that inner- vate many muscle fibers. The units generate moderte tensions and do not fatigue much. Slow units (S) bottom) composed of small axons innervate few small fibers. These units generate low forces but maintain force for a prolonged time period

•352 SECTION IV The Physiologic Support Systems reach a muscle before it relaxes. Blending recruitment of twitch, fatigue-resistant motor units serves as a built-in motor units and their firing rate permits a wide variety o recuperative period so performance continues with mini- graded muscle actions. These range from the delicate touch mal fatigue. This becomes possible because motor units of an eye surgeon reparing a retinal blood vessel to the share the burden of “cooperation” during multiple move- maximal effort in throwing a baseball from deep left field t ments and changing exercise intensities. throw out a runner rounding third base and heading to home plate. The golf swing provides a good example of Neuromuscular Fatigability Fatigability represents the force gradation. Tension in the fingers, hands, arms, an decline in muscle tension or force capacity with repeated legs continually adjusts during the backswing, swing initi- stimulation during a given time period . Resistance to ation and acceleration, club-ball contact, and follow- fatigue (maintenance of muscle tension with repeated through. Similarly, the seemingly simple task of writing stimulation) represents another important quality to dis- with a pen involves innumerable highly complex and coor- tinguish differences in motor units. dinated neuromuscular forces and actions. Think about picking up a grape and bringing it to your mouth. Without These four factors can decrease a muscle’s force- exquisite neuromuscular control, the fingers would liter generating capacity: ally crush the grape when grasped, and your hand and arm, without exhibiting precise control and coordination over 1. Exercise-induced alterations in levels of CNS neu- their movements, might thrust what remains of the grape rotransmitters serotonin, 5-hydroxytryptamine forcibly into your nose or eye, totally missing your mouth (5-HT), dopamine, and ACh, including the neuro- not to mention the pain of the jab! modulators ammonia and cytokines secreted by immune cells. The latter alter one’s psychic or per- Motor Unit Recruitment Low-force muscle actions ceptual state to disrupt ability to exercise. activate only a few motor units, whereas higher force actions progressively enlist more units.Motor unit recruit- 2. Reduced glycogen content in active muscle fiber ment describes the process of adding motor units to during prolonged exercise. Such “nutrient-related increase muscle force. Motoneurons with progressively fatigue” occurs despite availability of sufficient oxy larger axons become recruited as muscle force increases. gen and fatty acid substrate for adenosine This response, termed the size principle, provides an triphosphate (ATP) regeneration through aerobic anatomic basis for the orderly recruitment of specifi metabolic pathways. Depletion of phosphocreatine motor units to produce a smooth action. (PCr) and decline in the total adenine nucleotide pool (ATP ϩ Adenosine diphosphate [ADP] ϩ All of a muscle’s motor units do not fire at the sam Adenosine monophosphate [AMP]) accompanies the time. If they did, it would be virtually impossible to control fatigue state during prolonged submaximal exercise. muscle force output. From the standpoint of neuromotor con- trol, selective recruitment and firing pattern of the fast- an 3. A lack of oxygen and increased levels of blood and slow-twitch motor units that control movement (and perhaps muscle lactate relate to muscle fatigue in short- other stabilizing regions) provide the mechanism to produce term, maximal exercise. The dramatic increase in the desired coordinated response. [Hϩ] in the active muscle disrupts the intracellular environment and the process of energy transfer. In accordance with the size principle, slow-twitch motor units with low activation thresholds become selectively 4. Fatigue at the neuromuscular junction does not recruited during light to moderate effort. Activation of more allow the action potential to traverse from the powerful, higher threshold, fast-twitch units progresses as motoneuron to the muscle fiber force requirements increase. Sustained, submaximal jog- ging, cycling, cross-country skiing on a level grade, and lift- If muscle function declines during prolonged sub- ing a light weight at slow speed involve selective recruitment maximal exercise, additional motor unit recruitment of slow-twitch motor units. Rapid, powerful movements occurs to maintain the crucial force output necessary to such as sprint running, cycling, and swimming progressively provide a relatively constant level of performance. Dur- activate fast-twitch, fatigue-resistant motor units up through ing all-out exercise that presumably activates the avail- the fast-twitch fatigable units at peak force. able number of motor units, fatigue occurs when accompanied by an objectively measured decline in The differential control of the motor unit firing patter neural activity to those motor units. Similar to the dim- distinguishes specific athletic groups and skilled fro ming of a light bulb, reduced neural activity probably unskilled performers. Weight lifters, for example, gener- indicates that failure in neural or myoneural transmis- ally demonstrate a synchronous pattern of motor-unit fir sion produces fatigue in activities that involve maximal ing (i.e., many motor units recruited simultaneously effort muscle actions. during a lift). Endurance athletes generally exhibit an asyn- chronous firing pattern (i.e., some motor units fire whi PROPRIOCEPTORS IN MUSCLES, others recover). The synchronous firing of fast-twitc motor units allows the weight lifter to generate high force JOINTS, AND TENDONS quickly for the desired lift. In contrast, for endurance ath- letes, the asynchronous firing of predominantly slow Muscles, joints, and tendons contain specialized sensory receptors sensitive to stretch, tension, and pressure. These proprioceptor end organs almost instantaneously relay

•Chapter 11 The Neuromuscular System and Exercise 353 critical information about muscular dynamics, limb position, and kinesthesia uestions & Notes Qand proprioception to conscious and subconscious portions of the central nerv- ous system. Proprioception allows continual monitoring of the progress of any Briefly describe the advantage of selectiv movement or sequence of movements and serves as the basis for modifying sub- motor recruitment. sequent motor actions. Individuals with chronic low back pain, or individuals who have undergone successful low back surgery, often temporarily lose full proprioception in the ankles, predisposing them to possible balance issues (e.g., an inability to easily balance with the eyes open or closed on one leg without swaying for more than 3 to 5 s). Describe the major factor(s) that Muscle Spindles distinguishes a skilled from an unskilled individual. Muscle spindles provide mechanosensory information about changes in muscle fibe length and tension . They primarily respond to muscle stretch through refle action by initiating a stronger muscle action to counteract the stretch. Structural Organization Figure 11.10 A–C illustrates the general loca- tion of the fusiform-shaped muscle spindle attached in parallel to regular muscle fibers (called extrafusal fiber ) and Golgi tendon end organs.Any elongation of the muscle stretches the spindle. The number of spindles per gram of muscle varies depending on the muscle group. More spindles exist in muscles that rou- tinely perform complex movements. The spindle contains two types of special- ized fibers with contractile capabilities calledintrafusal fibers Peripheral nerve Sensory neurons Gamma motoneuron (motor and sensory to contractile end of A nerve fibers) B to central, noncontractile muscle spindle portion of muscle spindle Individual Alpha motoneuron skeletal to muscle fibers muscle fiber Muscle Skeletal spindle muscle Bone Tendon Golgi tendon organ Sensory Golgi tendon neuron organ C Tendon (collagen) connects bone to muscle Figure 11.10 (A) General location of muscle spindles and Golgi tendon end organs. (B) Muscle spindle surrounded by skeletal muscle fibers. Two types of sensory neurons innervate the spindle’s central portion: (1) fast-adapting neurons with spiral endigs and (2) slow-adapting neurons with branched endings.␥ motoneurons innervate the contractile ends of muscle spindle cells, and ␣-motoneurons activate skeletal muscle cells. (C) Slow-adapting sensory neurons innervate Golgi tendon organs (see also Fig. 11.12).

•354 SECTION IV The Physiologic Support Systems BOX 11.1 CLOSE UP Proprioceptive Neuromuscular Facilitation Stretching Static stretching techniques include passive (relaxation Both PN F techniques use reciprocal inhibition, the of all voluntary and reflex muscular resistance followe isometric action of the antagonists muscle group being by passive assistance from another person or device dur- stretched to induce a reflex facilitation and contraction o ing voluntary movement), active assistive (involves the agonist. This suppresses the contractile activity in the assistance from another person as the segment moves antagonist muscle during the slow, static stretch phase. through its normal range of motion [ROM]), active (a Inhibition allows for an increased stretch of the antagonist muscle or joint actively moves through its ROM), and muscle and connective tissue harness. The technique proprioceptive neuromuscular facilitation (PNF; an induces additional inhibitory input to the antagonist inverse stretch reflex induces relaxation in a muscl through reciprocal inhibition, allowing for greater stretch prior to its being stretched, allowing for increased of the antagonist. stretch). PERFORMING PNF STRETCHES PROPRIOCEPTIVE NEUROMUSCULAR FACILITATION STRETCHING 1. Stretch the target muscle group by moving the joint to the end of its ROM (Fig. 1A). PNF stretching increases ROM by augmenting prior mus- cle relaxation through spinal reflex mechanisms usin 2. Isometrically contract the prestretched muscle group these techniques: against an immovable resistance (e.g., partner) for 5 to 6 seconds. 1. Contract–relax stretch (hold–relax stretch). This stretching technique involves a prior isometric action 3. Relax the contracted muscle group as the partner of the muscle group to be stretched followed by a stretches the muscle group to a new, increased ROM slow, static stretch (relaxation phase). (Fig. 1B). With CRAC, the opposing muscle group (agonist) contracts submaximally for 5 to 6 seconds 2. Contract–relax–contract stretch (hold–relax–contract to facilitate relaxation and produce further stretching stretch), also referred to as the contract–relax with of the muscle group. agonist contraction (CRAC) technique. This approach involves an isometric action of the muscle PNF Example: To stretch the hamstring and lower back group to be stretched; the relax stretching phase is muscles, the individual lies on the floor with the arm accompanied by a submaximal action of the opposing extended to the side ( Fig. 1A). The person contracts the (agonist) muscle group. lower back muscle isometrically as the partner offers resist- ance to horizontal extension (Fig. 1A). After the isometric action, the partner stretches the hamstrings to a new increased ROM (Fig. 1B). AB Figure 1 Proprioceptive neuromuscular facilitation (PNF) stretching technique: isometric phase (A); stretching phase (B).

•Chapter 11 The Neuromuscular System and Exercise 355 GUIDELINES FOR PROPER STRETCHING USING PNF 1. Determine the appropriate posture or position to 4. Exhale and feel the muscle being stretched and ensure proper position and alignment. relaxed to achieve further ROM. 2. Emphasize proper breathing. Inhale through the nose 5. Do not bounce or spring during stretching. and exhale during the stretch through pursed lips 6. Do not force a stretch during breath-holding. with the eyes closed to increase concentration and 7. Increasing stretching range during exhalation awareness of the stretch. encourages full-body relaxation. 3. Hold end-points progressively for 30 to 90 seconds 8. Slowly reposition from the stretch posture and allow followed by another deep breath. the muscles to recover to their natural resting length. REFERENCES Fasen, J.M., et al.: A randomized controlled trial of hamstring stretching: comparison of four techniques. J. Strength Cond. Res., 23:660, 2009. Kreun, M.K., et al.: The efficacy of two modified proprioceptive neuromuscular facilitation stretching techniques in subjects w h reduced hamstring muscle length. Physiother. Theory Pract., 26:240, 2010. Ryan, E.E., et al.: The effects of the contract-relax-antagonist-contract form of proprioceptive neuromuscular facilitation stretching on postural stability. J. Strength Cond. Res., 24:1888, 2010. Streepey, J.W., et al.: Effects of quadriceps and hamstrings proprioceptive neuromuscular facilitation stretching on knee movement sensation. J. Strength Cond. Res., 24:1037, 2010. SELECTED INTERNET SITES 1. www.thestretchinghandbook.com/archives/pnf-stretching.php (PNF stretching explained) 2. www.youtube.com/watch?vϭ791XXiYzNbE (PNF stretching for hamstrings) 3. www.canadaspace.com/crwb.php?qϭPNF+Stretching (resources for PNF stretching) Two afferent (sensory) and one efferent (motor) nerve fibers innervate th uestions & Notes Qspindles. The motor spindles consist of thin ␥ (gamma) efferent fibers tha innervate the contractile, striated ends of intrafusal fibers. These fibers, act What is the major function of a muscle vated by higher brain centers, maintain the spindle at peak operation at all mus- spindle? cle lengths. Stretch Reflex Muscle spindles lodged in parallel with the main fibers i List the 3 main components of the stretch reflex the belly of a muscle detect, respond to, and modulate changes in the length of the extrafusal muscle fibers. This provides an important regulatory contro 1. function for total body movement and maintenance of posture. Postural mus- cles continuously receive neural input to sustain their readiness to respond to 2. conscious (voluntary) movements. These muscles require continual subcon- scious activity to adjust to the pull of gravity in upright posture. Without this 3. monitoring and feedback mechanism, the body would literally collapse into a heap from the absence of tension in the neck muscles, spinal muscles, hip flex ors, abdominal muscles, and large leg musculature. The patella tendon stretch refle in Figure 11.5 serves as a fundamental controlling mechanism in human movement. The stretch reflex consists of three main components: 1. Muscle spindle that responds to stretch 2. Afferent nerve fiber that carries the sensory impulse from the spindle t the spinal cord 3. Efferent spinal cord motoneuron that activates the stretched muscle fiber This simplest autonomic monosynaptic reflex arc involves only one synapse Spindles lie parallel to the extrafusal fibers and stretch when these fibers elo gate as the relfex hammer strikes the patellar tendon. The spindle’s sensory receptors fire when its intrafusal fibers stretch, directing impulses through t dorsal root into the spinal cord to directly activate the anterior motoneurons.

•356 SECTION IV The Physiologic Support Systems The gray matter contains neuron cell bodies; the white mat- Golgi Tendon Organs ter carries longitudinal columns of nerve fibers. Stimulatio of a single ␣-motoneuron affects up to 3000 muscle fibers Golgi tendon organs (GTOs), named to honor Italian The reflex also activates interneurons within the spinal cor physician Camillo Golgi (1843–1926) who first identifi to facilitate the appropriate motor response. For example, these proprioceptors in 1898, connect in series to as many excitatory impulses activate synergistic muscles that support as 25 extrafusal fibers in contrast to muscle spindles tha the desired movement, and inhibitory impulses flow t lie parallel to extrafusal muscle fibers. These tiny sensor motor units that normally counter the movement. In this receptors also are located in ligaments of joints to primarily way, the stretch reflex acts as a self-regulating, compensatin detect differences in muscle tension rather than length. mechanism. This salient feature allows the muscle to adjust Figure 11.11 shows details of the GTOs that respond as a automatically to differences in load and length without feedback monitor to discharge impulses when muscle requiring immediate information processing through higher shortens or stretches. CNS centers. Humans are indeed fortunate to have this CNS system of checks and balances. Without it, the human body When activated by excessive muscle tension or stretch, could not perform relatively “simple” muscular movements Golgi receptors immediately transmit signals to cause refle like touching the tip of your index finger to the tip of you inhibition of the muscles they supply. This occurs because of thumb let alone the highly complex, coordinated movement an overriding influence of inhibitory spinal interneuron patterns such as smashing a volleyball over a net or trying to on the motoneurons supplying muscle. With extreme ten- hit a stationary golf ball 155 yards straight at the pin. sion or stretch, the Golgi “sensor” discharge increases to further depress motoneuron activity and reduce tension in Spinal cord Inhibitory interneuron Sensory nerve from tendon organ Alpha motor neuron Capsule Sensory fiber Collagen fibrils Golgi tendon organ Figure 11.11 The Golgi tendon organ (GTO). Excessive tension or stretch on a muscle activates the tendon’s Golgi receptors, which brings about a reflex inhibition of the muscles they supply. In this way, the GTO function as a protective sensory mechanism to detect and subsequently inhibit undue strain within the muscle-tendon structure.

•Chapter 11 The Neuromuscular System and Exercise 357 the muscle fibers. Ultimately, the GTOs protect muscle and its connective tissu Questions & Notes harness from injury by sudden, excessive load or stretch. What is the main function of pacinian Pacinian Corpuscles corpuscles? Pacinian corpuscles are small, ellipsoidal bodies located close to the GTOs and embedded in a single, unmyelinated nerve fiber. They are located in th subcutaneous tissue on the nerves of the soles of the feet and palms of the hands, in the mucous membranes, in male and female genital organs, and in close proximity with the nerves of joints. The ends of the corpuscles are cov- Describe the major function of Golgi ered by a sensitive receptor membrane whose sodium channels open with any tendon organs. membrane deformation or vibration. Several concentric capsules of connec- tive tissue with a viscous gel between them surround each corpuscle, which attaches to and encloses the termination of a single nerve fiber These sensitive sensory receptors respond to quick movement and deep pres- sure. Deformation or compression of the onionlike capsule by any mechanical stimulus transmits pressure to the sensory nerve ending within its core to change the electric potential of the sensory nerve ending. If this generator potential achieves sufficient magnitude, a sensory signal propagates down the myelinate axon that leaves the corpuscle and enters the spinal cord. Think of Pacinian cor- puscles as fast-adapting mechanical sensors. They discharge a few impulses at the onset of a steady stim- ulus and then remain For Your Information electrically silent or discharge another vol- CAMILLO GOLGI ley of impulses when In 1878, Camillo Golgi (1843–1926), an Italian neurohistochemist, discovered the minute tendon the stimulus ceases. organs that now bear his name using a silver nitrate stain described in his masterful text, On the Fine Pacinian corpuscles Anatomy of the Nervous System. Golgi received the Nobel Prize in Physiology or Medicine in 1906 with detect changes in Santiago Ramón y Cajal (1852–1934) for their insightful contributions about the structures of the movement or pressure nervous system. One of Golgi’s greatest contributions was his creative method of staining individual rather than the magni- nerve and cell structures using a weak solution of silver nitrate. This invaluable method traced the tiny tude of movement or processes and intricate structures of cells (nobelprize.org/nobel_prizes/medicine/laureates/ the quantity of pres- 1906/golgi-bio.html). sure applied. SUMMARY 6. The anterior motoneuron (cell body, axon, and dendrites) transmits the electrochemical neural 1. CNS neural control mechanisms finely regulate huma impulse from the spinal cord to the muscle. Dendrites movement. In response to internal and external stimuli, receive impulses and conduct them toward the cell bits of sensory input are automatically and rapidly body; the axon transmits the impulse in one direction routed, organized, and retransmitted to the muscles. only—down the axon to the muscle. 2. The cerebellum serves as the major comparing, 7. The neuromuscular junction provides the interface evaluating, and integrating center to fine tun between the motoneuron and its muscle fibers. AC muscular activity. release at this junction activates the muscle. 3. The spinal cord and other subconscious areas of the 8. Excitatory and inhibitory impulses continually CNS control numerous muscular functions. bombard synaptic junctions between neurons. These alter a neuron’s threshold for excitation by increasing 4. The reflex arc processes and initiates automati or decreasing its tendency to fire (subconscious) muscular movements and responses. 9. In all-out, high-power output exercise, a large 5. The number of muscle fibers in a motor unit depend degree of disinhibition benefits performance because on the muscle’s movement function. Intricate it allows for maximal activation of a muscle’s motor movement patterns require a relatively small units. fiber-to-neuron ratio, but for gross movements, single neuron may innervate several thousand muscle fibers

•358 SECTION IV The Physiologic Support Systems 10. Gradation of muscle force results from an interaction 12. Sensory receptors in muscles, tendons, and joints relay of factors that regulate the number and type of motor information about muscular dynamics and limb units recruited and their frequency of discharge. movement to specific portions of the CNS 11. In accordance with the size principle, light exercise 13. Golgi tendon organ receptors respond to quick predominantly recruits slow-twitch motor units movement and deep pressure. followed by activation of fast-twitch units when force output requirements increase. 14. Pacinian corpuscles detect changes in movement or pressure. THOUGHT QUESTIONS 1. Discuss why fatigue may not relate to”only” muscular 3. How might drugs that mimic neurotransmitters affect factors. physiologic response and performance in maximal exercise? 2. Discuss factors to explain why some individuals are “faster learners” of certain tasks. Part 2 Muscular System: Figure 11.12 shows a skeletal muscle cross-section that consists of thousands of cylindrical cells called fibers Organization and These long, slender multinucleated fibers lie parallel to on Activation another whose number largely becomes fixed by the sec ond trimester of fetal development, with the force of con- Skeletal muscles transform the chemical energy within traction occurring mainly along the fiber’s long axis ATP molecules into the mechanical energy of motion. Part 2 presents the architectural organization of skeletal muscle A fine layer of connective tissue, the endomysium, and focuses on its gross and microscopic structure. The wraps each fiber and separates it from neighboring fiber discussion includes the sequence of chemical and mechan- Another layer of connective tissue, the perimysium, ical events in muscular contraction and relaxation and the surrounds a bundle of up to 150 fibers to form afasciculus. differences in muscle fiber characteristics among elite per The epimysium surrounds the entire muscle with a fascia of formers in different sports. fibrous connective tissue. This protective sheath tapers a its distal end as it blends into and joins the intramuscular COMPARISON OF SKELETAL, tissue sheaths to form the dense, strong connective tissue CARDIAC, AND SMOOTH MUSCLE of tendons. Tendons connect each end of the muscle to the periosteum, the outermost covering of the skeleton. The Humans possess three types of muscle—cardiac, smooth, force of muscle action transmits directly from the muscle’s and skeletal, which each have functional and anatomical connective tissue harness to the tendons at their bony differences. Cardiac muscle occurs only in the heart. It points of attachment. shares several common features with skeletal muscle; both appear striated (striped) under microscopic examination The sarcolemma, a thin, elastic membrane that encloses and contract (shorten) in a similar manner. Smooth muscle the fiber’s cellular contents, lays beneath the endomysiu lacks a striated appearance but shares cardiac muscle’s and surrounds each muscle fiber. Sarcoplasm (the fiber’ characteristic of nonconscious regulation.Table 11.2 con- aqueous protoplasm) contains enzymes, fat and glycogen trasts the structural and functional characteristics of the particles, nuclei that contain the genes, mitochondria, and three types of muscle. other specialized organelles. The sarcoplasm includes an extensive interconnecting network of tubular channels GROSS STRUCTURE and vesicles called the sarcoplasmic reticulum. This OF SKELETAL MUSCLE highly specialized system enhances the cell’s structural integrity. It allows the wave of depolarization to spread Each of the more than 430 voluntary muscles in the body rapidly from the fiber’s outer surface to its inner environ contains various wrappings of fibrous connective tissue ment through the T-tubule system to initiate muscle action. The sarcoplasmic reticulum that surrounds each myofibril contains biologic “pumps” that take up C ϩϩ from the fiber’s sarcoplasm. This produces a calcium con centration gradient between the sarcoplasmic reticulum (higher Caϩϩ) and the sarcoplasm surrounding the fila ments (lower Caϩϩ).

•Chapter 11 The Neuromuscular System and Exercise 359 Table 11.2 Characteristics of the Three Types of Human Muscle TYPE OF MUSCLE CHARACTERISTICS SKELETAL CARDIAC SMOOTH Location Attached to bones Heart only Part of blood vessel structure: surrounds many internal hollow organs Function Movement Pumps blood Constricts blood vessels; moves contents Anatomical description Large cylindrical, Quadrangular cells of internal organs multinucleated cells Striated arranged in parallel Yes Small, spindle-shaped cells with long axis Initiation of action potential Spontaneous oriented in the same direction Yes Duration of electrical activity By neuron only (pacemaker cells) No Energy source Long (ϳ200 ms) Spontaneous Energy efficienc Short (1–2 ms) Aerobic Fatigue resistance Anaerobic, Aerobic Moderate Very long, slow (ϳ300 ms) Rate of shortening Low Low Aerobic Duration of action Low to high Moderate High Fast Short (ϳ300 ms); Very low As brief as 100 ms; Very slow summation and Very long; may be sustained indefinitel prolonged tetanus tetanus not possible Chemical Composition Questions & Notes Skeletal muscle contains about 75% water and 20% protein, with the remaining Discuss why the sarcoplasmic reticulum 5% comprising inorganic salts and high-energy phosphates, urea, lactate, cal- important. cium, magnesium, and phosphorus; enzymes and pigments; sodium, potas- sium, and chloride ions; and amino acids, fats, and carbohydrates. Blood Supply Intense dynamic muscle actions often require an oxygen uptake of 4000 mLиminϪ1 and higher, and the oxygen consumed by active muscle increases at least 70 times above its resting level to about 3400 mLиminϪ1. To accommodate the increased oxygen requirement, the local vascular bed redirects blood flo through active tissues similar to a traffic officer rerouting congested traffic. continuous, rhythmic running, swimming, and cycling, muscle blood flow flu tuates; it decreases during the shortening action and increases during muscle relaxation. Alternating contraction and relaxation provides a “milking action” to propel blood through the muscles back to the heart. Concurrently, the rapid dilation of previously dormant capillaries complements the pulsatile blood flow. Between 200 and 500 capillaries deliver blood to each square millimeter o active muscle cross-section, with up to four capillaries directly contacting each fiber. In endurance athletes, five to seven capillaries surround each fiber; t adaptation ensures greater local blood flow and adequate tissue oxygenatio when needed. Straining-type activities, such as trying to lift a heavy weight, present a some- what different picture. When a muscle contracts at about 60% of its force-gen- erating capacity, elevated intramuscular pressure begins to restrict the muscle’s blood supply. The muscle’s compressive force with a maximal isometric action literally retards blood flow. This changes the energy dynamics so the break down of stored intramuscular phosphagens and anaerobic glycolytic reactions now provide the energy stream to sustain the muscular effort. Muscle Capillarization Capillary microcirculation expedites removal of heat and metabolic byproducts from active tissues. A rich network of these tiny

•360 SECTION IV The Physiologic Support Systems Bone Tendon Muscle belly Epimysium (deep fascia) Perimysium Endomysium (between fibers) Capillary Fasciculus Endomysium Sarcoplasm Single Nuclei muscle fiber Sarcolemma A Myofibrils Mitochondrion A band I band Z line Sarcolemma Terminal Transverse Sarcoplasmic Nucleus reticulum cisternae tubule Triad of B the reticulum Figure 11.12 Cross-section of skeletal muscle structures and arrangement of connective tissue wrappings. (A) Endomysium covers individual fibers. Perimysium surrounds groups of fibers called fasciculi, and epimysium wraps the entire muscle in a sheath of onnec- tive tissue. The sarcolemma, a thin, elastic membrane, covers the surface of each muscle fiber. B) A cross-section of the sarcoplasmic reticulum and T-tubule system that surrounds the myofibrils. Note the close contact of the mitochondria and network of intracelular membranes and tubules.

•Chapter 11 The Neuromuscular System and Exercise 361 exchange vessels provides a large surface area to exchange not only metaboli- uestions & Notes Qcally generated heat but fluids, electrolytes, gases, and macromolecules as well In contracting muscles, microvessels immediately after stimulation exhibit Which cellular component gives the muscle increased flow of blood and perfused capillary surface area transport. In regard fiber its striated appearance to exercise training, electron microscopy reveals that the total number of capil- laries per muscle (and capillaries per mm2 of muscle tissue) averages about 40% higher in endurance-trained ath. letes than untrained counterparts. A positive association also exists between VO2max (maximal oxygen consumption) and the average number of muscle capillaries. Enhanced vascularization on the capillary Name the functional unit of the muscle level proves particularly beneficial during exercise that requires a high level o fiber steady-rate aerobic metabolism. ULTRASTRUCTURE OF SKELETAL MUSCLE Electron microscopy, laser diffraction, and histochemical staining techniques Name the sarcomere’s 2 contractile have revealed the ultrastructure of skeletal muscle (http://muscle.ucsd.edu/ proteins. musintro/jump.shtml). Figure 11.13A to 11.13F show the gross and subcellu- lar microscopic organization of skeletal muscle. 1. Each muscle fiber contains smaller functional units that lie parallel to th 2. fiber’s long axis. The myofibrils approximately 1 ␮ in diameter, contain even smaller subunits called myofilament that also run parallel to the myofibril’ long axis. The myofilaments consist mainly of the proteins actin and myosin that constitute about 84% of the myofibrillar complex The Sarcomere Describe the structural link between thin and thick myofilaments At low magnification under a light microscope, the alternating light and dar bands along the length of the skeletal muscle fiber appearstriated. Figure 11.14 illustrates the structural details of the myofibril’s cross-striation pattern. In th resting state, the length of each sarcomere averages 2.5 ␮m. Thus, a myofibri that is 15 mm long contains about 6000 sarcomeres joined end to end. The length of the sarcomere largely determines a muscle’s functional properties. The I band shows up as the lighter area and darker zone theA band. The Z line bisects the I band and adheres to the sarcolemma to stabilize the entire structure. The sarcomere, the repeating unit between two Z lines, comprises the functional unit of the muscle cell. The actin and myosin filaments within a sarcomere provide th mechanical mechanism for muscle action (i.e., contraction and relaxation). The position of the sarcomere’s thin actin and thicker myosin proteins over- laps the two filaments. The center of the A band contains theH zone, a region of lower optical density because of the absence of actin filaments in this region The M line bisects the central portion of the H zone and delineates the sarcom- ere’s center. The M line contains the protein structures that support the arrangement of myosin filaments Actin–Myosin Orientation Thousands of myosin filaments lie along the line of actin filaments in a musc fiber. Figure 11.15 illustrates the ultrastructure of actin–myosin orientation within a sarcomere at resting length. Six thin actin filaments, each about 50 angstroms (Å; 1 Å ϭ 100 millionths of a centimeter) in diameter and 1 ␮ long, surround a thicker myosin filament (150 Å in diameter and 1.5 ␮ long). This forms an impressive muscular substructure. For example, a myofibril tha is 1 ␮ in diameter contains about 450 thick filaments in the center of th sarcomere and 900 thin filaments at each end. Consequently, a single muscle fib 100 ␮ in diameter and 1 cm long contains about 8000 myofibrils, each with 450 sarcomeres. In a single muscle fiber, this translates to a total of 16 billion thick an 64 billion thin filaments in a single muscle fibe

362 • SECTION IV The Physiologic Support Systems UI � - b� nd --A- , .. - ... ' ---- ... ---- , --- ---- ;�� ���;�, Sarcolemma ----- Sarcomere unit ---_.: (plasma - .. - membrane) M band Z , :• line , Z line \\, v ;� � Actin (thin filament) � ;_________�=---�v �4]) T ponin complex Z line M band Z line , \" , .. , .. ... , .. ... , ... , ..... Figure 11.13 Gross and subcellular microscopic organization . . . . .. of skeletal muscle. (A) Individual fibers constitute the whol •° 0 • • • • •0. . .. . . . . muscle. (B) Fibers consist of myofibrils with actin and myosi ·....... protein filament subdivisions. C) to (F) Details of a single sar­ o • • • •. • . • • .. . . . . . . . .· . . . . . ·. . .. . .. comere with the actin and myosin filaments, a microscopic vie . . . . . . . .. of the sarcomere (note the two Z lines), and a cross-sectional ·. .. . . . . . . . . ·....... · . . . view of the filaments ...... . ... ... ... .... . .. . . . · · .. . . .. . . ... . . .· ·. .. . . . .. . . . . . .... .. .. . . . .. .

Chapter 11 The Neuromuscular System and Exercise • 363 Figure 11.1 6 details the spatial orientation of various proteins that form the Questions & Notes contractile filaments. Projections or crossbridges spiral around the myosin fil What activates crossbridge activity? ament in the region where the actin and myosin filaments overlap. Crossbridge List the 2 components of the Actin helix repeat at intervals of 450 A along the filament. Their globular, \"lollipop-like 1. heads extend perpendicularly to latch onto the thinner, double-twisted actin 2. strands to create structural and functional links between myofilaments. Th unique feature of myosin's two heads concerns their opposite orientation at the ends of the thick filament. ATP hydrolysis activates the two heads, placing the in an optimal orientation to bind actin's active sites. This pulls the thin fila ments and Z lines of the sarcomere toward the middle. Tropomyosin and troponin, the two most important core constituents of the actin helix structure, regulate the make-and-break contacts between myofila ments during muscle action. Tropomyosin distributes along the length of the actin filament in a groove formed by the double helix. It inhibits actin an myosin interaction (coupling) to prevent their permanent bonding. Troponin, which is embedded at fairly regular intervals along the actin strands, exhibits a high affinity for calcium ions (C )++ . Troponin which play a crucial role in :- Sarcomere :-' : I band A band I band H zone Thin filament: Thick filament: '. '. actin, troponin, tropomyosin myosin J ;- IP.£ XV2.Jl : :'-� it_-�.'� �� �. � Z line: a-actin -RJ2. J2.Jl.R JZ.iV'���=�!I!!»!I'l!lla I·'\\; •••.'\\; � i1h\" :)M d: C stripes:�----------v�--------� Elastic filaments: I :lNe lin ( - Connections between two M protein C protein titin sarcomeres from adjacent myofibrils: desmin Myomesin X protein c�o . M creatine kinase H protein Figure 11.14 (Top) Structural position of the filaments in a sarcomere. The Z line bounds a sarcomere at both ends. Bottom) Detailed view of a sarcomere, including the additional proteins nebulin (major regulator of force production with three subuni$ that lie in the groove of each actin filament that block myosin's binding site in the absence of ionic calcium) and titin (closely a;ociated with the myosin molecule, it appears to anchor the myosin network to the actin network).