
<p> Chapter 11 *Lecture PowerPoint Muscular Tissue </p><p>*See separate FlexArt PowerPoint slides for all figures and tables preinserted into PowerPoint without notes. </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Introduction </p><p>• Movement is a fundamental characteristic of all living organisms • Three types of muscular tissue—skeletal, cardiac, and smooth • Important to understand muscle at the molecular, cellular, and tissue levels of organization </p><p>11-2 Types and Characteristics of Muscular Tissue </p><p>• Expected Learning Outcomes – Describe the physiological properties that all muscle types have in common. – List the defining characteristics of <a href="/tags/Skeletal_muscle/" rel="tag">skeletal muscle</a>. – Discuss the possible elastic functions of the connective tissue components of a muscle. </p><p>11-3 Universal Characteristics of Muscle • Responsiveness (excitability) – To chemical signals, stretch, and electrical changes across the plasma membrane </p><p>• Conductivity – Local electrical change triggers a wave of excitation that travels along the muscle fiber </p><p>• Contractility – Shortens when stimulated </p><p>• Extensibility – Capable of being stretched between contractions </p><p>• Elasticity – Returns to its original resting length after being stretched 11-4 Skeletal Muscle • Skeletal muscle— voluntary, striated muscle attached to one or more bones Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Striations—alternating light and dark transverse Nucleus bands Muscle fiber – Results from an </p><p> overlapping of internal <a href="/tags/Endomysium/" rel="tag">Endomysium</a> contractile proteins </p><p>• Voluntary—usually Striations subject to conscious control © Ed Reschke • <a href="/tags/Muscle_cell/" rel="tag">Muscle cell</a>, muscle fiber Figure 11.1 (myofiber)—as long as 30 cm 11-5 </p><p>Skeletal Muscle </p><p>• Tendons are attachments between muscle and bone matrix – Endomysium: connective tissue around muscle cells – <a href="/tags/Perimysium/" rel="tag">Perimysium</a>: connective tissue around muscle fascicles – <a href="/tags/Epimysium/" rel="tag">Epimysium</a>: connective tissue surrounding entire muscle – Continuous with collagen fibers of tendons – In turn, with connective tissue of bone matrix </p><p>• Collagen is somewhat extensible and elastic – Stretches slightly under tension and recoils when released • Resists excessive stretching and protects muscle from injury • Returns muscle to its resting length • Contributes to power output and muscle efficiency </p><p>11-6 Microscopic Anatomy of Skeletal Muscle </p><p>• Expected Learning Outcomes – Describe the structural components of a muscle fiber. – Relate the striations of a muscle fiber to the overlapping arrangement of its protein filaments. – Name the major proteins of a muscle fiber and state the function of each. </p><p>11-7 The Muscle Fiber </p><p>• <a href="/tags/Sarcolemma/" rel="tag">Sarcolemma</a>—plasma membrane of a muscle fiber • <a href="/tags/Sarcoplasm/" rel="tag">Sarcoplasm</a>—cytoplasm of a muscle fiber • <a href="/tags/Myofibril/" rel="tag">Myofibrils</a>—long protein bundles that occupy the main portion of the sarcoplasm – Glycogen: stored in abundance to provide energy with heightened exercise – Myoglobin: red pigment; stores oxygen needed for muscle activity </p><p>11-8 The Muscle Fiber </p><p>• Multiple nuclei—flattened nuclei pressed against the inside of the sarcolemma – Myoblasts: stem cells that fuse to form each muscle fiber – Satellite cells: unspecialized myoblasts remaining between the muscle fiber and endomysium • May multiply and produce new muscle fibers to some degree • Mitochondria—packed into spaces between myofibrils </p><p>11-9 The Muscle Fiber </p><p>• <a href="/tags/Sarcoplasmic_reticulum/" rel="tag">Sarcoplasmic reticulum</a> (SR)—smooth ER that forms a network around each <a href="/tags/Myofibril/" rel="tag">myofibril</a>: calcium reservoir – Calcium activates the <a href="/tags/Muscle_contraction/" rel="tag">muscle contraction</a> process • <a href="/tags/Terminal_cisternae/" rel="tag">Terminal cisternae</a>—dilated end-sacs of SR which cross the muscle fiber from one side to the other • T tubules—tubular infoldings of the sarcolemma which penetrate through the cell and emerge on the other side • Triad—a T tubule and two terminal cisterns </p><p>11-10 The Muscle Fiber </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Muscle fiber </p><p>Nucleus </p><p>A band </p><p>I band </p><p>Z disc </p><p>Mitochondria Openings into transverse tubules </p><p>Sarcoplasmic reticulum </p><p>Triad: Terminal cisternae Transverse tubule </p><p>Sarcolemma Myofibrils Sarcoplasm Figure 11.2 </p><p>Myofilaments 11-11 <a href="/tags/Myofilament/" rel="tag">Myofilaments</a> </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Head Tail </p><p>(a) <a href="/tags/Myosin/" rel="tag">Myosin</a> molecule </p><p>Myosin head </p><p>Figure 11.3a,b (b) Thick filament • Thick filaments—made of several hundred myosin molecules – Shaped like a golf club • Two chains intertwined to form a shaftlike tail • Double globular head – Heads directed outward in a helical array around the bundle • Heads on one half of the thick filament angle to the left • Heads on the other half angle to the right • Bare zone with no heads in the middle 11-12 Myofilaments • Thin filaments – Fibrous (F) <a href="/tags/Actin/" rel="tag">actin</a>: two intertwined strands • String of globular (G) actin subunits each with an active site that can bind to head of myosin molecule – <a href="/tags/Tropomyosin/" rel="tag">Tropomyosin</a> molecules • Each blocking six or seven active sites on G actin subunits – <a href="/tags/Troponin/" rel="tag">Troponin</a> molecule: small, calcium-binding protein on each tropomyosin molecule </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Tropomyosin Troponin complex G actin </p><p>(c) Thin filament Figure 11.3c 11-13 Myofilaments </p><p>• Elastic filaments – <a href="/tags/Titin/" rel="tag">Titin</a> (connectin): huge, springy protein – Flank each thick filament and anchor it to the Z disc – Help stabilize the thick filament – Center it between the thin filaments – Prevent overstretching </p><p>11-14 Myofilaments </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Myosin head </p><p>(b) Thick filament </p><p>Tropomyosin Troponin complex G actin </p><p>Figure 11.3b,c (c) Thin filament • Contractile proteins—myosin and actin do the work • Regulatory proteins—tropomyosin and troponin – Like a switch that determines when the fiber can contract and when it cannot – Contraction activated by release of calcium into sarcoplasm and its binding to troponin – Troponin changes shape and moves tropomyosin off the active sites on actin 11-15 Myofilaments </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Thick filament Thin filament Bare zone </p><p>(d) Portion of a <a href="/tags/Sarcomere/" rel="tag">sarcomere</a> showing the overlap of thick and thin filaments Figure 11.3d 11-16 Myofilaments </p><p>• At least seven other Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p> accessory proteins in or Endomysium associated with thick or thin Linking proteins filaments Basal lamina – Anchor the myofilaments, </p><p> regulate length of Sarcolemma myofilaments, keep alignment for optimal contractile <a href="/tags/Dystrophin/" rel="tag">Dystrophin</a> effectiveness Thin filament Thick filament </p><p>Figure 11.4 </p><p>11-17 Myofilaments </p><p>• Dystrophin—most clinically important Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. – Links actin in outermost Endomysium Linking proteins myofilaments to transmembrane </p><p> proteins and eventually to Basal lamina fibrous endomysium </p><p> surrounding the entire muscle Sarcolemma cell Dystrophin – Transfers forces of muscle Thin filament contraction to connective tissue around muscle cell Thick filament – Genetic defects in dystrophin produce disabling disease muscular dystrophy Figure 11.4 </p><p>11-18 Striations </p><p>• Myosin and actin are proteins that occur in all cells – Function in cellular motility, mitosis, transport of intracellular material • Organized in a precise way in skeletal and <a href="/tags/Cardiac_muscle/" rel="tag">cardiac muscle</a> </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Sarcomere I band A band I band H band </p><p>Thick filament Thin filament M line Titin Elastic filament (b) Z disc Z disc </p><p>Figure 11.5b 11-19 Striations – A band: dark; A stands for anisotropic • Part of A band where thick and thin filaments overlap is especially dark – H band: middle of A band; thick filaments only – M line: middle of H band – I band: alternating lighter band; I stands for isotropic • The way the bands reflect polarized light – Z disc: provides anchorage for thin filaments and elastic filaments • Bisects I band </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Sarcomere I band A band I band H band </p><p>Thick filament Thin filament M line Titin Elastic filament (b) Z disc Z disc </p><p>Figure 11.5b 11-20 Striations </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Nucleus </p><p>Sarcomere </p><p>5</p><p>Z disc M line </p><p>4</p><p>H band </p><p>3</p><p>I band I band </p><p>A band 2</p><p>Individual myofibrils Individual </p><p>1</p><p>(a) </p><p>Visuals Unlimited Figure 11.5a </p><p>11-21 Striations </p><p>• Sarcomere—segment from Z disc to Z disc – Functional contractile unit of muscle fiber </p><p>• Muscle cells shorten because their individual <a href="/tags/Sarcomere/" rel="tag">sarcomeres</a> shorten – Z disc (Z lines) are pulled closer together as thick and thin filaments slide past each other </p><p>• Neither thick nor thin filaments change length during shortening – Only the amount of overlap changes </p><p>• During shortening dystrophin and linking proteins also pull on extracellular proteins – Transfers pull to extracellular tissue </p><p>11-22 The Nerve—Muscle Relationship </p><p>• Expected Learning Outcomes – Explain what a motor unit is and how it relates to muscle contraction. – Describe the structure of the junction where a nerve fiber meets a muscle fiber. – Explain why a cell has an electrical charge difference across its plasma membrane and, in general terms, how this relates to muscle contraction. </p><p>11-23 The Nerve—Muscle Relationship </p><p>• Skeletal muscle never contracts unless stimulated by a nerve </p><p>• If nerve connections are severed or poisoned, a muscle is paralyzed – Denervation atrophy: shrinkage of paralyzed muscle when connection not restored </p><p>11-24 Motor Neurons and Motor Units </p><p>• Somatic motor neurons—nerve cells whose cell bodies are in the brainstem and spinal cord that serve skeletal muscles </p><p>• Somatic motor fibers—their axons that lead to the skeletal muscle – Each nerve fiber branches out to a number of muscle fibers – Each muscle fiber is supplied by only one <a href="/tags/Motor_neuron/" rel="tag">motor neuron</a> </p><p>11-25 Motor Neurons and Motor Units </p><p>• Motor unit—one nerve fiber Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. and all the muscle fibers Spinal cord innervated by it </p><p>• Muscle fibers of one motor </p><p> unit Motor neuron 1 – Dispersed throughout the Motor muscle neuron 2 </p><p>– Contract in unison Neuromuscular – Produce weak contraction junction over wide area Skeletal muscle – Provides ability to sustain fibers long-term contraction as Figure 11.6 motor units take turns contracting (postural control) – Effective contraction usually requires the contraction of several motor units at once 11-26 Motor Neurons and Motor Units </p><p>• Average motor unit—200 muscle fibers for each motor unit </p><p>• Small motor units—fine degree of control – Three to six muscle fibers per neuron – Eye and hand muscles </p><p>• Large motor units—more strength than control – Powerful contractions supplied by large motor units (e.g., gastrocnemius has 1,000 muscle fibers per neuron) – Many muscle fibers per motor unit 11-27 The <a href="/tags/Neuromuscular_junction/" rel="tag">Neuromuscular Junction</a> </p><p>• Synapse—point where a nerve fiber meets its target cell • Neuromuscular junction (NMJ)—when target cell is a muscle fiber • Each terminal branch of the nerve fiber within the NMJ forms separate synapse with the muscle fiber • One nerve fiber stimulates the muscle fiber at several points within the NMJ </p><p>11-28 The Neuromuscular Junction </p><p>• Synaptic knob—swollen end of nerve fiber – Contains synaptic vesicles filled with acetylcholine (ACh) • Synaptic cleft—tiny gap between synaptic knob and muscle sarcolemma • Schwann cell envelops and isolates all of the NMJ from surrounding tissue fluid • Synaptic vesicles undergo exocytosis releasing ACh into synaptic cleft </p><p>11-29 The Neuromuscular Junction </p><p>• Synaptic vesicles undergo exocytosis releasing ACh into synaptic cleft </p><p>• 50 million ACh receptors—proteins incorporated into muscle cell plasma membrane – Junctional folds of sarcolemma beneath synaptic knob • Increase surface area holding ACh receptors – Lack of receptors leads to paralysis in disease myasthenia gravis </p><p>11-30 The Neuromuscular Junction </p><p>• Basal lamina—thin layer of collagen and glycoprotein separates Schwann cell and entire muscle cell from surrounding tissues – Contains acetylcholinesterase (AChE) that breaks down ACh after contraction causing relaxation </p><p>11-31 The Neuromuscular Junction Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Motor nerve fiber Myelin </p><p>Schwann cell Synaptic knob </p><p>Basal lamina Synaptic vesicles (containing ACh) </p><p>Sarcolemma Synaptic cleft Nucleus </p><p>ACh receptor </p><p>Junctional folds </p><p>Nucleus Mitochondria Sarcoplasm Myofilaments Figure 11.7b </p><p>(b) 11-32 The Neuromuscular Junction </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Motor nerve fibers </p><p>Neuromuscular junction </p><p>Muscle fibers </p><p>Figure 11.7a (a) 100 µm Victor B. Eichler 11-33 Electrically Excitable Cells </p><p>• Muscle fibers and neurons are electrically excitable cells – Their plasma membrane exhibits voltage changes in response to stimulation • Electrophysiology—the study of the electrical activity of cells • Voltage (electrical potential)—a difference in electrical charge from one point to another • Resting membrane potential—about −90 mV – Maintained by sodium–potassium pump </p><p>11-34 Electrically Excitable Cells </p><p>• In an unstimulated (resting) cell – There are more anions (negative ions) on the inside of the plasma membrane than on the outside – The plasma membrane is electrically polarized (charged) – There are excess sodium ions (Na+) in the extracellular fluid (ECF) – There are excess potassium ions (K+) in the intracellular fluid (ICF) – Also in the ICF, there are anions such as proteins, nucleic acids, and phosphates that cannot penetrate the plasma membrane – These anions make the inside of the plasma membrane negatively charged by comparison to its outer surface 11-35 Electrically Excitable Cells </p><p>• Stimulated (active) muscle fiber or nerve cell – Ion gates open in the plasma membrane – Na+ instantly diffuses down its concentration gradient into the cell – These cations override the negative charges in the ICF – Depolarization: inside of the plasma membrane becomes briefly positive – Immediately, Na+ gates close and K+ gates open – K+ rushes out of cell – Repelled by the positive sodium charge and partly because of its concentration gradient – Loss of positive potassium ions turns the membrane negative again (repolarization) </p><p>11-36 Electrically Excitable Cells </p><p>• Stimulated (active) muscle fiber or nerve cell (cont.) – <a href="/tags/Action_potential/" rel="tag">Action potential</a>: quick up-and-down voltage shift from the negative RMP to a positive value, and back to the negative value again – RMP is a stable voltage seen in a waiting muscle or nerve cell – Action potential is a quickly fluctuating voltage seen in an active stimulated cell – An action potential at one point on a plasma membrane causes another one to happen immediately in front of it, which triggers another one a little farther along and so forth </p><p>11-37 Neuromuscular Toxins and Paralysis </p><p>• Toxins that interfere with synaptic function can paralyze the muscles • Some pesticides contain cholinesterase inhibitors – Bind to acetylcholinesterase and prevent it from degrading Ach – Spastic paralysis: a state of continual contraction of the muscles; possible suffocation • Tetanus (lockjaw) is a form of spastic paralysis caused by toxin Clostridium tetani – Glycine in the spinal cord normally stops motor neurons from producing unwanted muscle contractions – Tetanus toxin blocks glycine release in the spinal cord and causes overstimulation and spastic paralysis of the muscles </p><p>11-38 </p><p>Neuromuscular Toxins and Paralysis </p><p>• Flaccid paralysis—a state in which the muscles are limp and cannot contract – Curare: compete with ACh for receptor sites, but do not stimulate the muscles – Plant poison used by South American natives to poison blowgun darts </p><p>• Botulism—type of food poisoning caused by a neuromuscular toxin secreted by the bacterium Clostridium botulinum – Blocks release of ACh causing flaccid paralysis – Botox cosmetic injections for wrinkle removal </p><p>11-39 Behavior of Skeletal Muscle Fibers </p><p>• Expected Learning Outcomes – Explain how a nerve fiber stimulates a skeletal muscle fiber. – Explain how stimulation of a muscle fiber activates its contractile mechanism. – Explain the mechanism of muscle contraction. – Explain how a muscle fiber relaxes. – Explain why the force of a muscle contraction depends on sarcomere length prior to stimulation. </p><p>11-40 Behavior of Skeletal Muscle Fibers </p><p>• Four major phases of contraction and relaxation – Excitation • The process in which nerve action potentials lead to muscle action potentials </p><p>– Excitation–contraction coupling • Events that link the action potentials on the sarcolemma to activation of the myofilaments, thereby preparing them to contract </p><p>– Contraction • Step in which the muscle fiber develops tension and may shorten </p><p>– Relaxation • When its work is done, a muscle fiber relaxes and returns to its resting length 11-41 Excitation </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Nerve signal 2+ Motor Ca enters nerve synaptic knob fiber Sarcolemma Synaptic Synaptic knob vesicles </p><p>ACh Synaptic cleft ACh receptors </p><p>1 Arrival of nerve signal 2 Acetylcholine (ACh) release </p><p>Figure 11.8 (1, 2) • Nerve signal opens voltage-gated calcium channels in synaptic knob • Calcium stimulates exocytosis of ACh from synaptic vesicles 11-42 • ACh released into synaptic cleft Excitation </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>ACh ACh K+ ACh receptor </p><p>Sarcolemma </p><p>Na+ </p><p>3 Binding of ACh to receptor 4 Opening of ligand-regulated ion gate; creation of end-plate potential </p><p>Figure 11.8 (3, 4) • Two ACh molecules bind to each receptor protein, opening Na+ and K+ channels • Na+ enters; shifting RMP goes from −90 mV to +75 mV, then K+ exits and RMP returns to −90 mV; quick voltage shift is called an end-plate potential (EPP) 11-43 Excitation </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>K+ Plasma membrane of synaptic knob </p><p>Na+ Voltage-regulated ion gates Sarcolemma Figure 11.8 (5) </p><p>5 Opening of voltage-regulated ion gates; creation of action potentials • Voltage change (EPP) in end-plate region opens nearby voltage- gated channels producing an action potential that spreads over muscle surface 11-44 Excitation–Contraction Coupling </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 11.9 (6, 7) </p><p>Terminal T tubule cisterna of SR </p><p>T tubule </p><p>Sarcoplasmic reticulum Ca2+ </p><p>Ca2+ </p><p>6 Action potentials propagated 7 Calcium released from down T tubules terminal cisternae </p><p>• Action potential spreads down into T tubules • Opens voltage-gated ion channels in T tubules and Ca+2 channels in SR • Ca+2 enters the cytosol 11-45 Excitation–Contraction Coupling </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Active sites 2+ Troponin Ca Tropomyosin Actin Thin filament </p><p>Myosin Ca2+ </p><p>8 Binding of calcium 9 Shifting of tropomyosin; to troponin exposure of active sites on actin </p><p>Figure 11.9 (8, 9) • Calcium binds to troponin in thin filaments • Troponin–tropomyosin complex changes shape and exposes active sites on actin 11-46 Contraction </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Myosin ATPase enzyme in myosin Troponin Tropomyosin head hydrolyzes an ATP molecule </p><p>ADP </p><p>Pi • Activates the head Myosin ―cocking‖ it in an 10 Hydrolysis of ATP to ADP + Pi; activation and cocking of myosin head extended position </p><p>– ADP + Pi remain attached </p><p>Cross-bridge: Actin • Head binds to actin Myosin active site forming a 11 Formation of myosin–actin cross-bridge myosin–actin cross- bridge Figure 11.10 (10, 11) 11-47 Contraction </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Myosin head releases </p><p>ADP and Pi, flexes pulling thin filament past thick— power stroke ATP </p><p>13 Binding of new ATP; • Upon binding more ATP, breaking of cross-bridge myosin releases actin and process is repeated </p><p>– Each head performs five ADPADP PPi i power strokes per second 12 Power stroke; sliding of thin – Each stroke utilizes one filament over thick filament molecule of ATP Figure 11.10 (12, 13) </p><p>11-48 Relaxation </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Figure 11.11 (14, 15) </p><p>AChE </p><p>ACh </p><p>14 Cessation of nervous stimulation and ACh release 15 ACh breakdown by acetylcholinesterase (AChE) </p><p>• Nerve stimulation and ACh release stop • AChE breaks down ACh and fragments reabsorbed into synaptic knob • Stimulation by ACh stops 11-49 Relaxation </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Terminal cisterna of SR </p><p>Ca2+ </p><p>Ca2+ </p><p>16 Reabsorption of calcium ions by Figure 11.11 (16) sarcoplasmic reticulum </p><p>• Ca+2 pumped back into SR by active transport • Ca+2 binds to calsequestrin while in storage in SR • ATP is needed for muscle relaxation as well as muscle contraction 11-50 Relaxation </p><p>• Ca+2 removed from troponin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. is pumped back into SR Ca2+ • Tropomyosin reblocks the ADP P Ca2+ active sites i </p><p>17 Loss of calcium ions from troponin • Muscle fiber ceases to produce or maintain tension </p><p>Tropomyosin • Muscle fiber returns to its </p><p> resting length ATP – Due to recoil of elastic 18 Return of tropomyosin to position components and blocking active sites of actin contraction of antagonistic Figure 11.11 (17, 18) muscles 11-51 The Length–Tension Relationship and Muscle Tone </p><p>• Length–tension relationship—the amount of tension generated by a muscle and the force of contraction depends on how stretched or contracted it was before it was stimulated </p><p>• If overly contracted at rest, a weak contraction results – Thick filaments too close to Z discs and cannot slide </p><p>• If too stretched before stimulated, a weak contraction results – Little overlap of thin and thick does not allow for very many cross-bridges to form 11-52 </p><p>The Length–Tension Relationship and Muscle Tone </p><p>• Optimum resting length produces greatest force when muscle contracts – Muscle tone: central nervous system continually monitors and adjusts the length of the resting muscle, and maintains a state of partial contraction called muscle tone </p><p>– Maintains optimum length and makes the muscles ideally ready for action </p><p>11-53 Length–Tension Relationship </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Optimum resting length (2.0–2.25µm) z z </p><p>Overly contracted z z Overly stretched z z </p><p>1.0 </p><p>0.5 </p><p> upon upon stimulation Tension (g) generated Tension 0.0 1.0 2.0 3.0 4.0 Sarcomere length (µm) before stimulation Figure 11.12 11-54 Rigor Mortis </p><p>• Rigor mortis—hardening of muscles and stiffening of body beginning 3 to 4 hours after death – Deteriorating sarcoplasmic reticulum releases Ca+2 – Deteriorating sarcolemma allows Ca+2 to enter cytosol – Ca+2 activates myosin-actin cross-bridging – Muscle contracts, but cannot relax </p><p>• Muscle relaxation requires ATP, and ATP production is no longer produced after death – Fibers remain contracted until myofilaments begin to decay </p><p>• Rigor mortis peaks about 12 hours after death, then diminishes over the next 48 to 60 hours </p><p>11-55 Behavior of Whole Muscles </p><p>• Expected Learning Outcomes – Describe the stages of a muscle twitch. – Explain why muscle does not contract in an all-or- none manner. – Explain how successive muscle twitches can add up to produce stronger muscle contractions. – Distinguish between isometric and isotonic contraction. – Distinguish between concentric and eccentric contraction. </p><p>11-56 Threshold, Latent Period, and Twitch </p><p>• The response of a muscle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. to weak electrical stimulus Relaxation Contraction phase seen in frog phase gastrocnemius—sciatic Latent </p><p> nerve preparation period Muscletension </p><p>Time of • Myogram—a chart of the stimulation timing and strength of a Time muscle’s contraction Figure 11.13 </p><p>11-57 Threshold, Latent Period, and Twitch </p><p>• Weak, subthreshold </p><p> electrical stimulus causes no Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p> contraction Relaxation Contraction phase </p><p> phase </p><p>• Threshold—minimum Latent </p><p> voltage necessary to period Muscletension generate an action potential </p><p> in the muscle fiber and Time of stimulation </p><p> produce a contraction Time – Twitch—a quick cycle of Figure 11.13 contraction when stimulus is at threshold or higher </p><p>11-58 Threshold, Latent Period, and Twitch </p><p>• Latent period—2 ms delay between the onset of stimulus and the onset of twitch response – Time required for excitation, excitation–contraction coupling, and tensing of elastic components of the muscle – Internal tension: force generated during latent period and no shortening of the muscle occurs </p><p>• Contraction phase—phase in which filaments slide and the muscle shortens – Once elastic components are taut, muscle begins to produce external tension in muscle that moves a load – Short-lived phase </p><p>11-59 Threshold, Latent Period, and Twitch </p><p>• Relaxation phase—SR quickly reabsorbs Ca2+, myosin releases the thin filaments, and tension declines – Muscle returns to resting length – Entire twitch lasts from 7 to 100 ms </p><p>11-60 Contraction Strength of Twitches </p><p>• At subthreshold stimulus—no contraction at all </p><p>• At threshold intensity and above—a twitch is produced – Twitches caused by increased voltage are no stronger than those at threshold </p><p>11-61 Contraction Strength of Twitches </p><p>• Not exactly true that muscle fiber obeys an all-or- none law—contracting to its maximum or not at all – Electrical excitation of a muscle follows all-or-none law – Not true that muscle fibers follow the all-or-none law – Twitches vary in strength depending upon: • Stimulus frequency—stimuli arriving closer together produce stronger twitches • Concentration of Ca+2 in sarcoplasm can vary the frequency </p><p>11-62 Contraction Strength of Twitches </p><p>Cont. • How stretched muscle was before it was stimulated • Temperature of the muscles—warmed-up muscle contracts more strongly; enzymes work more quickly • Lower than normal pH of sarcoplasm weakens contraction— fatigue • State of hydration of muscle affects overlap of thick and thin filaments </p><p>• Muscles need to be able to contract with variable strengths for different tasks </p><p>11-63 Contraction Strength of Twitches </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Threshold Stimulus voltage Stimulus</p><p>1 2 3 4 5 6 7 8 9 Stimuli to nerve </p><p>Proportion of nerve fibers excited </p><p>Maximum contraction </p><p>Tension</p><p>1 2 3 4 5 6 7 8 9 Figure 11.14 Responses of muscle </p><p>• Stimulating the nerve with higher and higher voltages produces stronger contractions – Higher voltages excite more and more nerve fibers in the motor nerve which stimulates more and more motor units to contract • Recruitment or multiple motor unit (MMU) summation—the process of bringing more motor units into play 11-64 Contraction Strength of Twitches </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Twitch Treppe </p><p>Muscle twitches </p><p>Stimuli (a) Figure 11.15a,b (b) </p><p>• When stimulus intensity (voltage) remains constant twitch strength can vary with the stimulus frequency • Up to 10 stimuli per second – Each stimulus produces identical twitches and full recovery between twitches </p><p>11-65 Contraction Strength of Twitches </p><p>• 10–20 stimuli per second produces treppe (staircase) phenomenon – Muscle still recovers fully between twitches, but each twitch develops more tension than the one before – Stimuli arrive so rapidly that the SR does not have time between stimuli to completely reabsorb all of the Ca2+ it released – Ca2+ concentration in the cytosol rises higher and higher with each stimulus causing subsequent twitches to be stronger – Heat released by each twitch causes muscle enzymes such as myosin ATPase to work more efficiently and produce stronger twitches as muscle warms up 11-66 </p><p>Contraction Strength of Twitches </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Incomplete tetanus Complete tetanus </p><p>Fatigue </p><p>(c) (d) Figure 11.15c,d • 20–40 stimuli per second produces incomplete tetanus – Each new stimulus arrives before the previous twitch is over – New twitch ―rides piggy-back‖ on the previous one generating higher tension – Temporal summation: results from two stimuli arriving close together </p><p>11-67 Contraction Strength of Twitches </p><p>Cont. – Wave summation: results from one wave of contraction added to another – Each twitch reaches a higher level of tension than the one before – Muscle relaxes only partially between stimuli – Produces a state of sustained fluttering contraction called incomplete tetanus </p><p>11-68 Contraction Strength of Twitches </p><p>• 40–50 stimuli per second produces complete tetanus – Muscle has no time to relax between stimuli – Twitches fuse to a smooth, prolonged contraction called complete tetanus – A muscle in complete tetanus produces about four times the tension as a single twitch – Rarely occurs in the body, which rarely exceeds 25 stimuli per second – Smoothness of muscle contractions is because motor units function asynchronously • When one motor unit relaxes, another contracts and takes over so the muscle does not lose tension </p><p>11-69 Isometric and Isotonic Contraction </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Figure 11.16 </p><p>Muscle develops Muscle shortens, Muscle lengthens tension but does tension remains while maintaining not shorten constant tension Movement No movement Movement </p><p>(a) Isometric contraction (b) Isotonic concentric contraction (c) Isotonic eccentric contraction • Isometric muscle contraction – Muscle is producing internal tension while an external resistance causes it to stay the same length or become longer – Can be a prelude to movement when tension is absorbed by elastic component of muscle – Important in postural muscle function and antagonistic muscle joint stabilization 11-70 Isometric and Isotonic Contraction </p><p>• Isotonic muscle contraction – Muscle changes in length with no change in tension – Concentric contraction: muscle shortens as it maintains tension – Eccentric contraction: muscle lengthens as it maintains tension </p><p>11-71 Isometric and Isotonic Contraction </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Muscle Muscle </p><p> tension length Length or Tension or Length</p><p>Isometric Isotonic Figure 11.17 phase phase </p><p>Time • At the beginning of contraction—isometric phase – Muscle tension rises but muscle does not shorten • When tension overcomes resistance of the load – Tension levels off • Muscle begins to shorten and move the load—isotonic phase 11-72 Muscle Metabolism </p><p>• Expected Learning Outcomes – Explain how skeletal muscle meets its energy demands during rest and exercise. – Explain the basis of muscle fatigue and soreness. – Define oxygen debt and explain why extra oxygen is needed even after an exercise has ended. – Distinguish between two physiological types of muscle fibers, and explain their functional roles. – Discuss the factors that affect muscular strength. – Discuss the effects of resistance and endurance exercises on muscles. </p><p>11-73 ATP Sources </p><p>• All muscle contraction depends on ATP </p><p>• ATP supply depends on availability of: – Oxygen – Organic energy sources such as glucose and fatty acids </p><p>11-74 ATP Sources </p><p>• Two main pathways of ATP synthesis – Anaerobic fermentation • Enables cells to produce ATP in the absence of oxygen • Yields little ATP and toxic lactic acid, a major factor in muscle fatigue </p><p>– Aerobic respiration • Produces far more ATP </p><p>• Less toxic end products (CO2 and water) • Requires a continual supply of oxygen </p><p>11-75 ATP Sources </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>0 10 seconds 40 seconds Repayment of Duration of exercise oxygen debt </p><p>Mode of ATP synthesis </p><p>Aerobic respiration Phosphagen Glycogen– Aerobic using oxygen from system lactic acid respiration myoglobin system supported by (anaerobic cardiopulmonary fermentation) function </p><p>Figure 11.18 </p><p>11-76 Immediate Energy </p><p>• Short, intense exercise (100 m dash) – Oxygen need is briefly supplied by myoglobin for a limited amount of aerobic respiration at onset—rapidly depleted – Muscles meet most of ATP demand by borrowing phosphate </p><p> groups (Pi) from other molecules and transferring them to ADP </p><p>• Two enzyme systems control these phosphate transfers </p><p>– Myokinase: transfers Pi from one ADP to another, converting the latter to ATP </p><p>– Creatine kinase: obtains Pi from a phosphate-storage molecule creatine phosphate (CP) • Fast-acting system that helps maintain the ATP level while </p><p> other ATP-generating mechanisms are being activated 11-77 </p><p>Immediate Energy </p><p>• Phosphagen system—ATP and CP collectively – Provides nearly all energy used for short bursts of intense activity • 1 minute of brisk walking • 6 seconds of sprinting or fast swimming • Important in activities requiring brief but maximum effort – Football, baseball, and weightlifting </p><p>11-78 Immediate Energy </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. ADP ADP </p><p>Pi </p><p>Myokinase </p><p>AMP ATP </p><p>Creatine ADP phosphate </p><p>Pi </p><p>Figure 11.19 </p><p>Creatine 11-79 Creatine kinase ATP Short-Term Energy </p><p>• As the phosphagen system is exhausted muscles shift to anaerobic fermentation – Muscles obtain glucose from blood and their own stored glycogen – In the absence of oxygen, glycolysis can generate a net gain of 2 ATP for every glucose molecule consumed – Converts glucose to lactic acid </p><p>• Glycogen–lactic acid system—the pathway from glycogen to lactic acid </p><p>• Produces enough ATP for 30 to 40 seconds of maximum activity </p><p>11-80 Long-Term Energy </p><p>• After 40 seconds or so, the respiratory and cardiovascular systems ―catch up‖ and deliver oxygen to the muscles fast enough for aerobic respiration to meet most of the ATP demands </p><p>11-81 Long-Term Energy </p><p>• Aerobic respiration produces 36 ATP per glucose – Efficient means of meeting the ATP demands of prolonged exercise – One’s rate of oxygen consumption rises for 3 to 4 minutes and levels off to a steady state in which aerobic ATP production keeps pace with demand </p><p>11-82 Long-Term Energy </p><p>Cont. – Little lactic acid accumulates under steady-state conditions – Depletion of glycogen and blood glucose, together with the loss of fluid and electrolytes through sweating, set limits on endurance and performance even when lactic acid does not </p><p>11-83 Fatigue and Endurance </p><p>• Muscle fatigue—progressive <a href="/tags/Weakness/" rel="tag">weakness</a> and loss of contractility from prolonged use of the muscles – Repeated squeezing of rubber ball – Holding textbook out level to the floor </p><p>• Fatigue is thought to result from: – ATP synthesis declines as glycogen is consumed – ATP shortage slows down the Na+–K+ pumps • Compromises their ability to maintain the resting membrane potential and excitability of the muscle fibers – Lactic acid lowers pH of sarcoplasm • Inhibits enzymes involved in contraction, ATP synthesis, and other aspects of muscle function 11-84 Fatigue and Endurance </p><p>• Fatigue is thought to result from (cont.): – Release of K+ with each action potential causes the accumulation of extracellular K+ • Hyperpolarizes the cell and makes the muscle fiber less excitable – Motor nerve fibers use up their ACh • Less capable of stimulating muscle fibers—junctional fatigue – Central nervous system, where all motor commands originate, fatigues by unknown processes, so there is less signal output to the skeletal muscles </p><p>11-85 Fatigue and Endurance </p><p>• Endurance—the ability to maintain high-intensity exercise for more than 4 to 5 minutes – Determined in large part by one’s maximum oxygen </p><p> uptake (VO2max) – Maximum oxygen uptake: the point at which the rate of oxygen consumption reaches a plateau and does not increase further with an added workload • Proportional to body size • Peaks at around age 20 • Usually greater in males than females • Can be twice as great in trained endurance athletes as in untrained persons – Results in twice the ATP production 11-86 </p><p>Beating Fatigue </p><p>• Taking oral creatine increases level of creatine phosphate in <a href="/tags/Muscle_tissue/" rel="tag">muscle tissue</a> and increases speed of ATP regeneration – Useful in burst-type exercises: weightlifting – Risks are not well known • Muscle cramping, electrolyte imbalances, dehydration, water retention, stroke • Kidney disease from overloading kidney with metabolite creatinine </p><p>• Carbohydrate loading—dietary regimen – Packs extra glycogen into muscle cells – Extra glycogen is hydrophilic and adds 2.7 g water per gram of glycogen • Athletes feel sense of heaviness outweighs benefits of extra available glycogen 11-87 Oxygen Debt </p><p>• Heavy breathing continues after strenuous exercise – Excess postexercise oxygen consumption (EPOC): the difference between the resting rate of oxygen consumption and the elevated rate following exercise – Typically about 11 L extra is needed after strenuous exercise </p><p>• Needed for the following purposes: – Replace oxygen reserves depleted in the first minute of exercise • Oxygen bound to myoglobin and blood hemoglobin, oxygen dissolved in blood plasma and other extracellular fluid, and oxygen in the air in the lungs </p><p>11-88 Oxygen Debt </p><p>• Needed for the following purposes (cont.): – Replenishing the phosphagen system • Synthesizing ATP and using some of it to donate the phosphate groups back to creatine until resting levels of ATP and CP are restored </p><p>– Oxidizing lactic acid • 80% of lactic acid produced by muscles enter bloodstream </p><p>11-89 Oxygen Debt </p><p>Cont. • Reconverted to pyruvic acid in the kidneys, cardiac muscle, and especially the liver • Liver converts most of the pyruvic acid back to glucose to replenish the glycogen stores of the muscle </p><p>– Serving the elevated metabolic rate • Occurs while the body temperature remains elevated by exercise and consumes more oxygen </p><p>11-90 Physiological Classes of Muscle Fibers </p><p>• Slow oxidative (SO), slow-twitch, red, or type I fibers – Abundant mitochondria, myoglobin, capillaries: deep red color • Adapted for aerobic respiration and fatigue resistance – Relative long twitch lasting about 100 ms – Soleus of calf and postural muscles of the back </p><p>11-91 Physiological Classes of Muscle Fibers </p><p>• Fast glycolytic (FG), fast-twitch, white, or type II fibers – Fibers are well adapted for quick responses, but not for fatigue resistance – Rich in enzymes of phosphagen and glycogen–lactic acid systems generate lactic acid, causing fatigue – Poor in mitochondria, myoglobin, and blood capillaries which gives pale appearance • SR releases and reabsorbs Ca2+ quickly so contractions are quicker (7.5 ms/twitch) – Extrinsic eye muscles, gastrocnemius, and biceps brachii </p><p>11-92 Physiological Classes of Muscle Fibers </p><p>• Ratio of different fiber types have genetic predisposition— Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. born sprinter – Muscles differ in fiber types: gastrocnemius is FG predominantly FG for quick movements SO (jumping) – Soleus is predominantly Dr. Gladden Willis/Visuals Unlimited, Inc. SO used for endurance (jogging) Figure 11.20 </p><p>11-93 Muscular Strength and Conditioning </p><p>• Muscles can generate more tension than the bones and tendons can withstand </p><p>• Muscular strength depends on: – Primarily muscle size • A muscle can exert a tension of 3 or 4 kg/cm2 of cross- sectional area – Fascicle arrangement • Pennate are stronger than parallel, and parallel stronger than circular – Size of motor units • The larger the motor unit the stronger the contraction </p><p>11-94 Muscular Strength and Conditioning </p><p>• Muscular strength depends on (cont.) – Multiple motor unit summation: recruitment • When stronger contraction is required, the nervous system activates more motor units </p><p>– Temporal summation • Nerve impulses usually arrive at a muscle in a series of closely spaced action potentials • The greater the frequency of stimulation, the more strongly a muscle contracts </p><p>11-95 Muscular Strength and Conditioning </p><p>Cont. – Length–tension relationship • A muscle resting at optimal length is prepared to contract more forcefully than a muscle that is excessively contracted or stretched </p><p>– Fatigue • Fatigued muscles contract more weakly than rested muscles </p><p>11-96 Muscular Strength and Conditioning </p><p>• Resistance training (weightlifting) – Contraction of a muscle against a load that resists movement – A few minutes of resistance exercise a few times a week is enough to stimulate muscle growth – Growth is from cellular enlargement – Muscle fibers synthesize more myofilaments and myofibrils and grow thicker </p><p>11-97 Muscular Strength and Conditioning </p><p>• Endurance training (aerobic exercise) – Improves fatigue-resistant muscles – Slow twitch fibers produce more mitochondria, glycogen, and acquire a greater density of blood capillaries – Improves skeletal strength – Increases the red blood cell count and oxygen transport capacity of the blood – Enhances the function of the cardiovascular, respiratory, and nervous systems </p><p>11-98 Cardiac and <a href="/tags/Smooth_muscle/" rel="tag">Smooth Muscle</a> </p><p>• Expected Learning Outcomes – Describe the structural and physiological differences between cardiac muscle and skeletal muscle. – Explain why these differences are important to cardiac function. – Describe the structural and physiological differences between smooth muscle and skeletal muscle. – Relate the unique properties of smooth muscle to its locations and functions. </p><p>11-99 Cardiac Muscle </p><p>• Limited to the heart where it functions to pump blood • Properties of cardiac muscle – Contraction with regular rhythm – Muscle cells of each chamber must contract in unison – Contractions must last long enough to expel blood – Must work in sleep or wakefulness, without fail, and without conscious attention – Must be highly resistant to fatigue </p><p>11-100 Cardiac Muscle </p><p>• Characteristics of cardiac muscle cells – Striated like skeletal muscle, but myocytes (cardiocytes) are shorter and thicker – Each myocyte is joined to several others at the uneven, notched linkages—intercalated discs • Appear as thick, dark lines in stained tissue sections • Electrical gap junctions allow each myocyte to directly stimulate its neighbors • Mechanical junctions that keep the myocytes from pulling apart </p><p>11-101 Cardiac Muscle </p><p>• Sarcoplasmic reticulum less developed, but T tubules are larger and admit supplemental Ca2+ from the extracellular fluid • Damaged cardiac muscle cells repair by fibrosis – A little mitosis observed following heart attacks – Not in significant amounts to regenerate functional muscle </p><p>11-102 Cardiac Muscle </p><p>• Can contract without need for nervous stimulation – Contains a built-in pacemaker that rhythmically sets off a wave of electrical excitation – Wave travels through the muscle and triggers contraction of heart chambers – Autorhythmic: able to contract rhythmically and independently </p><p>11-103 Cardiac Muscle </p><p>– Autonomic nervous system does send nerve fibers to the heart • Can increase or decrease heart rate and contraction strength </p><p>– Very slow twitches; does not exhibit quick twitches like skeletal muscle • Maintains tension for about 200 to 250 ms • Gives the heart time to expel blood </p><p>– Uses aerobic respiration almost exclusively • Rich in myoglobin and glycogen • Has especially large mitochondria – 25% of volume of cardiac muscle cell </p><p>– 2% of skeletal muscle cell with smaller mitochondria 11-104 Smooth Muscle </p><p>• Sarcoplasmic reticulum is scanty and there are no T tubules • Ca2+ needed for muscle contraction comes from the ECF by way of Ca2+ channels in the sarcolemma • Some smooth muscles lack nerve supply, while others receive autonomic fibers, not somatic motor fibers as in skeletal muscle • Capable of mitosis and hyperplasia • Injured smooth muscle regenerates well </p><p>11-105 Myocyte Structure </p><p>• Myocytes have a fusiform shape – There is only one nucleus, located near the middle of the cell – No visible striations – Reason for the name ―smooth muscle‖ – Thick and thin filaments are present, but not aligned with each other </p><p>• Z discs are absent and replaced by dense bodies – Well-ordered array of protein masses in cytoplasm – Protein plaques on the inner face of the plasma membrane </p><p>11-106 Myocyte Structure </p><p>• Cytoplasm contains extensive cytoskeleton of intermediate filament – Attach to the membrane plaques and dense bodies – Provide mechanical linkages between the thin myofilaments and the plasma membrane </p><p>11-107 Types of Smooth Muscle </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Autonomic • Multiunit smooth muscle nerve fibers </p><p>– Occurs in some of the largest arteries and pulmonary air passages, in piloerector muscles of hair follicle, and in the iris of the eye Synapses – Autonomic innervation similar to skeletal muscle • Terminal branches of a nerve fiber synapse with individual myocytes and form a motor unit • Each motor unit contracts (a) Multiunit independently of the others smooth muscle </p><p>Figure 11.23a 11-108 Types of Smooth Muscle • Single-unit smooth muscle Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. – More widespread Autonomic – Occurs in most blood nerve fibers vessels, in the digestive, respiratory, urinary, and reproductive tracts Varicosities – Also called visceral muscle • Often in two layers: inner circular and outer longitudinal – Myocytes of this cell type are </p><p> electrically coupled to each Gap junctions other by gap junctions – They directly stimulate each other and a large number of cells contract as a single (b) Single-unit unit smooth muscle Figure 11.23b 11-109 Types of Smooth Muscle </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Autonomic nerve fiber </p><p>Varicosities </p><p>Mitochondrion </p><p>Synaptic vesicle </p><p>Single-unit smooth muscle Figure 11.21 11-110 Types of Smooth Muscle </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Figure 11.22 </p><p>Mucosa: Epithelium Lamina propria Muscularis mucosae </p><p>Muscularis externa: Circular layer Longitudinal layer 11-111 Excitation of Smooth Muscle </p><p>• Smooth muscle is involuntary and can contract without nervous stimulation – Can contract in response to chemical stimuli • Hormones, carbon dioxide, low pH, and oxygen deficiency • In response to stretch • Single-unit smooth muscle in stomach and intestines has pacemaker cells that set off waves of contraction throughout the entire layer of muscle </p><p>11-112 Excitation of Smooth Muscle </p><p>• Most smooth muscle is innervated by autonomic nerve fibers – Can trigger and modify contractions – Stimulate smooth muscle with either acetylcholine or norepinephrine – Can have contrasting effects • Relax the smooth muscle of arteries • Contract smooth muscles of the bronchioles </p><p>11-113 Excitation of Smooth Muscle </p><p>• In single-unit smooth, each autonomic nerve fiber has up to 20,000 beadlike swellings called varicosities – Each contains synaptic vesicles and a few mitochondria – Nerve fiber passes amid several myocytes and stimulates all of them at once when it releases its neurotransmitter • No motor end plates, but receptors scattered throughout the surface—diffuse junctions—no one-to-one relationship between nerve fiber and myocyte </p><p>11-114 Contraction and Relaxation </p><p>• Contraction is triggered by Ca2+, energized by ATP, and achieved by sliding thin past thick filaments </p><p>• Contraction begins in response to Ca2+ that enters the cell from ECF, a little internally from sarcoplasmic reticulum – Voltage, ligand, and mechanically gated (stretching) – Ca2+ channels open to allow Ca2+ to enter cell </p><p>11-115 Contraction and Relaxation </p><p>• Calcium binds to <a href="/tags/Calmodulin/" rel="tag">calmodulin</a> on thick filaments – Activates myosin light-chain kinase; adds phosphate to regulatory protein on myosin head – Myosin ATPase, hydrolyzing ATP • Enables myosin similar power and recovery strokes like skeletal muscle </p><p>11-116 Contraction and Relaxation </p><p>Cont. – Thick filaments pull on thin ones, thin ones pull on dense bodies and membrane plaques – Force is transferred to plasma membrane and entire cell shortens – Puckers and twists like someone wringing out a wet towel </p><p>11-117 Contraction and Relaxation </p><p>• Contraction and relaxation very slow in comparison to skeletal muscle – Latent period in skeletal 2 ms, smooth muscle 50 to 100 ms – Tension peaks at about 500 ms (0.5 sec) – Declines over a period of 1 to 2 seconds – Slows myosin ATPase enzyme and pumps that remove Ca2+ – Ca2+ binds to calmodulin instead of troponin • Activates kinases and ATPases that hydrolyze ATP </p><p>11-118 Contraction and Relaxation </p><p>• Latch-bridge mechanism is resistant to fatigue – Heads of myosin molecules do not detach from actin immediately – Do not consume any more ATP – Maintains tetanus tonic contraction (smooth muscle tone) • Arteries—vasomotor tone; intestinal tone – Makes most of its ATP aerobically </p><p>11-119 Smooth Muscle Contraction </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Plaque </p><p>Intermediate filaments of cytoskeleton </p><p>Actin filaments </p><p>Dense body </p><p>Myosin </p><p>(b) Contracted smooth muscle cells </p><p>Figure 11.24a,b </p><p>(a) Relaxed smooth muscle cells 11-120 Response to Stretch </p><p>• Stretch can open mechanically gated calcium channels in the sarcolemma causing contraction – Peristalsis: waves of contraction brought about by food distending the esophagus or feces distending the colon • Propels contents along the organ </p><p>• Stress–relaxation response (receptive relaxation)— helps hollow organs gradually fill (urinary bladder) – When stretched, tissue briefly contracts then relaxes; helps prevent emptying while filling </p><p>11-121 Response to Stretch </p><p>• Skeletal muscle cannot contract forcefully if overstretched </p><p>• Smooth muscle contracts forcefully even when greatly stretched – Allows hollow organs such as the stomach and bladder to fill and then expel their contents efficiently </p><p>11-122 Response to Stretch </p><p>• Smooth muscle can be anywhere from half to twice its resting length and still contract powerfully </p><p>• Three reasons – There are no Z discs, so thick filaments cannot butt against them and stop contraction – Since the thick and thin filaments are not arranged in orderly sarcomeres, stretching does not cause a situation where there is too little overlap for cross- bridges to form – The thick filaments of smooth muscle have myosin heads along their entire length, so cross-bridges can form anywhere </p><p>11-123 Response to Stretch </p><p>• Plasticity—the ability to adjust its tension to the degree of stretch – A hollow organ such as the bladder can be greatly stretched yet not become flabby when empty </p><p>11-124 Muscular Dystrophy </p><p>• Muscular dystrophy―group of hereditary diseases in which skeletal muscles degenerate and weaken, and are replaced with fat and fibrous scar tissue </p><p>• Duchenne muscular dystrophy is caused by a sex- linked recessive trait (1 of 3,500 live-born boys) – Most common form – Disease of males; diagnosed between 2 and 10 years of age – Mutation in gene for muscle protein dystrophin • Actin not linked to sarcolemma and cell membranes damaged during contraction; necrosis and scar tissue result – Rarely live past 20 years of age due to effects on respiratory and cardiac muscle; incurable </p><p>11-125 Muscular Dystrophy </p><p>• Facioscapulohumeral MD―autosomal dominant trait affecting both sexes equally – Facial and shoulder muscles more than pelvic muscles </p><p>• Limb-girdle dystrophy – Combination of several diseases of intermediate severity – Affects shoulder, arm, and pelvic muscles </p><p>11-126 Myasthenia Gravis • Autoimmune disease in which antibodies attack neuromuscular junctions and bind ACh receptors together in clusters – Disease of women between 20 and 40 – Muscle fibers then remove the clusters of receptors from the sarcolemma by endocytosis – Fiber becomes less and less sensitive to Ach – Effects usually first appear in facial muscles • Drooping eyelids and double vision, difficulty swallowing, and weakness of the limbs – Strabismus: inability to fixate on the same point with both eyes </p><p>11-127 Myasthenia Gravis </p><p>Cont. • Treatments – Cholinesterase inhibitors retard breakdown of ACh allowing it to stimulate the muscle longer – Immunosuppressive agents suppress the production of antibodies that destroy ACh receptors – Thymus removal (thymectomy) helps to dampen the overactive immune response that causes myasthenia gravis – Plasmapheresis: technique to remove harmful antibodies from blood plasma </p><p>11-128 Myasthenia Gravis </p><p>Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. </p><p>Figure 11.25 </p><p>• Drooping eyelids and weakness of muscles of eye movement upon upward gaze </p><p>11-129 </p>
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