9 Muscles and Muscle Physiology

Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4) 6/24/2012 MDufilho

Special Characteristics of Muscle Tissue Excitability: Contractility: Extensibility: Elasticity: 6/24/2012 MDufilho

Four important functions Muscle Functions Four important functions Movement of bones or fluids (e.g., blood) Maintaining posture and body position Stabilizing joints Heat generation (especially skeletal muscle) Additional functions Protects organs, forms valves, controls pupil size, causes "goosebumps" 6/24/2012 MDufilho

Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Bone Perimysium Tendon Endomysium Muscle fiber in middle of a fascicle Blood vessel Perimysium wrapping a fascicle Endomysium (between individual muscle fibers) Muscle fiber Fascicle Perimysium 6/24/2012 MDufilho

Skeletal Muscle: Attachments Attach in at least two places Insertion – movable bone Origin – immovable (less movable) bone Attachments direct or indirect Direct—epimysium fused to periosteum of bone or perichondrium of cartilage Indirect—connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis 6/24/2012 MDufilho

Table 9.1 Structure and Organizational Levels of Skeletal Muscle (1 of 3) 6/24/2012 MDufilho

Table 9.1 Structure and Organizational Levels of Skeletal Muscle (2 of 3) 6/24/2012 MDufilho 8

Sarcolemma Mitochondrion Dark A band Light I band Nucleus Figure 9.2b Microscopic anatomy of a skeletal muscle fiber. Diagram of part of a muscle fiber showing the myofibrils. One myofibril extends from the cut end of the fiber. Sarcolemma Mitochondrion Myofibril Dark A band Light I band Nucleus 6/24/2012 MDufilho

Densely packed, rodlike elements ~80% of cell volume Myofibrils Densely packed, rodlike elements ~80% of cell volume Contain sarcomeres - contractile units Sarcomeres contain myofilaments Exhibit striations - perfectly aligned repeating series of dark A bands and light I bands 6/24/2012 MDufilho

Z disc H zone Z disc I band A band I band M line Sarcomere Figure 9.2c Microscopic anatomy of a skeletal muscle fiber. Thin (actin) filament Z disc H zone Z disc Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Thick (myosin) filament I band A band I band M line Sarcomere 6/24/2012 MDufilho

Striations H zone: M line: Z disc (line): Thick filaments: Thin filaments: Sarcomere: 6/24/2012 MDufilho

Z disc H zone Z disc I band A band I band M line Sarcomere Figure 9.2c Microscopic anatomy of a skeletal muscle fiber. Thin (actin) filament Z disc H zone Z disc Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next. Thick (myosin) filament I band A band I band M line Sarcomere 6/24/2012 MDufilho

one sarcomere (sectioned length- wise). Notice the myosin heads on Figure 9.2d Microscopic anatomy of a skeletal muscle fiber. Sarcomere Thin (actin) filament Z disc M line Z disc Enlargement of one sarcomere (sectioned length- wise). Notice the myosin heads on the thick filaments. Elastic (titin) filaments Thick (myosin) filament 6/24/2012 MDufilho 14

Portion of a thick filament Portion of a thin filament Figure 9.3 Composition of thick and thin filaments. Longitudinal section of filaments within one sarcomere of a myofibril Thick filament Thin filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Thick filament. Thin filament Each thick filament consists of many myosin molecules whose heads protrude at oppositeends of the filament. A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thick filament Portion of a thin filament Myosin head Tropomyosin Troponin Actin Actin-binding sites Heads Tail ATP- binding site Active sites for myosin attachment Actin subunits Flexible hinge region Myosin molecule Actin subunits 6/24/2012 MDufilho

Part of a skeletal muscle fiber (cell) I band A band I band Z disc Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle. Part of a skeletal muscle fiber (cell) I band A band I band Z disc H zone Z disc M line Sarcolemma Myofibril Triad: • T tubule • Terminal cisterns of the SR (2) Sarcolemma Tubules of the SR Myofibrils Mitochondria 6/24/2012 MDufilho

Sliding Filament Model of Contraction Generation of force Does not necessarily cause shortening of fiber Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening 6/24/2012 MDufilho

Sliding Filament Model of Contraction In relaxed state, thin and thick filaments overlap only at ends of A band Sliding filament model of contraction During contraction, thin filaments slide past thick filaments  actin and myosin overlap more Occurs when myosin heads bind to actin  cross bridges 6/24/2012 MDufilho

Fully relaxed sarcomere of a muscle fiber Figure 9.6 Sliding filament model of contraction. Slide 1 Slide 1 1 Fully relaxed sarcomere of a muscle fiber Z H Z I A I 2 Fully contracted sarcomere of a muscle fiber Z Z 6/24/2012 I A I MDufilho

Sliding Filament Model of Contraction Myosin heads bind to actin; sliding begins Cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere Causes shortening of muscle fiber Pulls Z discs toward M line I bands shorten; Z discs closer; H zones disappear; A bands move closer (length stays same) Review Sliding Filament Theory on IP 6/24/2012 MDufilho

Fully relaxed sarcomere of a muscle fiber Figure 9.6 Sliding filament model of contraction. Slide 4 1 Fully relaxed sarcomere of a muscle fiber Z H Z I A I 2 Fully contracted sarcomere of a muscle fiber Z Z 6/24/2012 I A I MDufilho 21

Physiology of Skeletal Muscle Fibers For skeletal muscle to contract Activation (at neuromuscular junction) Must be nervous system stimulation Must generate action potential in sarcolemma Excitation-contraction coupling Action potential propagated along sarcolemma Intracellular Ca2+ levels must rise briefly 6/24/2012

Figure 9.8 When a nerve impulse reaches a neuromuscular junction, acetylcholine (ACh) is released. Slide 1 Myelinated axon of motor neuron Action potential (AP) Axon terminal of neuromuscular junction Sarcolemma of the muscle fiber Action potential arrives at axon terminal of motor neuron. 1 Voltage-gated Ca2+ channels open. Ca2+ enters the axon terminal moving down its electochemical gradient. Synaptic vesicle containing ACh 2 Axon terminal of motor neuron Synaptic cleft Fusing synaptic vesicles Ca2+ entry causes ACh (a neurotransmitter) to be released by exocytosis. 3 ACh Junctional folds of sarcolemma ACh diffuses across the synaptic cleft and binds to its receptors on the sarcolemma. 4 Sarcoplasm of muscle fiber ACh binding opens ion channels in the receptors that allow simultaneous passage of Na+ into the muscle fiber and K+ out of the muscle fiber. More Na+ ions enter than K+ ions exit, which produces a local change in the membrane potential called the end plate potential. 5 Postsynaptic membrane ion channel opens; ions pass. Degraded ACh ACh Ion channel closes; ions cannot pass. ACh effects are terminated by its breakdown in the synaptic cleft by acetylcholinesterase and diffusion away from the junction. 6 6/24/2012 Acetylcho- linesterase MDufilho

Action potential (AP) arrives at axon Figure 9.7 The phases leading to muscle fiber contraction. Action potential (AP) arrives at axon terminal at neuromuscular junction ACh released; binds to receptors on sarcolemma Phase 1 Motor neuron stimulates muscle fiber (see Figure 9.8). Ion permeability of sarcolemma changes Local change in membrane voltage (depolarization) occurs Local depolarization (end plate potential) ignites AP in sarcolemma AP travels across the entire sarcolemma AP travels along T tubules Phase 2: Excitation-contraction coupling occurs (see Figures 9.9 and 9.11). SR releases Ca2+; Ca2+ binds to troponin; myosin-binding sites (active sites) on actin exposed Myosin heads bind to actin; contraction begins 6/24/2012 MDufilho

Figure 9.9 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber. Slide 1 Open Na+ channel Closed K+ channel Na+ − − − − − − − − − − − − − − − − − − − + + + + ACh-containing synaptic vesicle + + + + + + + + + + + + + + + + − − − − K+ Axon terminal of neuromuscular junction Action potential Ca2+ Ca2+ Synaptic cleft Depolarization: Generating and propagating an action potential (AP). The local depolarization current spreads to adjacent areas of the sarcolemma. This opens voltage-gated sodium channels there, so Na+ enters following its electrochemical gradient and initiates the AP. The AP is propagated as its local depolarization wave spreads to adjacent areas of the sarcolemma, opening voltage-gated channels there. Again Na+ diffuses into the cell following its electrochemical gradient. 2 Wave of depolarization Closed Na+ channel Open K+ channel An end plate potential is generated at the neuromuscular junction (see Figure 9.8). 1 Na+ + + + ++ + + + + + + + + + + + + + + + + + − − − − − − − − − − − − − − − −− − − − K+ Repolarization: Restoring the sarcolemma to its initial polarized state (negative inside, positive outside). Repolarization occurs as Na+ channels close (inactivate) and voltage-gated K+ channels open. Because K+ concentration is substantially higher inside the cell than in the extracellular fluid, K+ diffuses rapidly out of the muscle fiber. 3 6/24/2012 MDufilho

Membrane potential (mV) Figure 9.10 Action potential tracing indicates changes in Na+ and K+ ion channels. +30 Na+ channels close, K+ channels open Depolarization due to Na+ entry Membrane potential (mV) Repolarization due to K+ exit Na+ channels open K+ channels closed –95 5 10 15 20 6/24/2012 Time (ms) MDufilho

Excitation-Contraction (E-C) Coupling Events that transmit AP along sarcolemma lead to sliding of myofilaments AP brief; ends before contraction Causes rise in intracellular Ca2+ which  contraction Latent period Time when E-C coupling events occur Time between AP initiation and beginning of contraction 6/24/2012 MDufilho

The events at the neuromuscular junction (NMJ) Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments. Slide 2 Setting the stage The events at the neuromuscular junction (NMJ) set the stage for E-C coupling by providing excitation. Released acetylcholine binds to receptor proteins on the sarcolemma and triggers an action potential in a muscle fiber. Synaptic cleft Axon terminal of motor neuron at NMJ Action poten- tial is generated ACh Sarcolemma T tubule Terminal cistern of SR Muscle fiber Triad One sarcomere One myofibril 6/24/2012 MDufilho

A&P Flix™: Excitation-contraction coupling. Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments. Slide 9 Steps in E-C Coupling: Sarcolemma The action potential (AP) propagates along the sarcolemma and down the T tubules. 1 Voltage-sensitive tubule protein T tubule PLAY Ca2+ release channel Calcium ions are released. Transmission of the AP along the T tubules of the triads causes the voltage-sensitive tubule proteins to change shape. This shape change opens the Ca2+ release channels in the terminal cisterns of the sarcoplasmic reticulum (SR), allowing Ca2+ to flow into the cytosol. 2 Terminal cistern of SR A&P Flix™: Excitation-contraction coupling. Actin Troponin Tropomyosin blocking active sites Myosin Calcium binds to troponin and removes the blocking action of tropomyosin. When Ca2+ binds, troponin changes shape, exposing binding sites for myosin (active sites) on the thin filaments. 3 Active sites exposed and ready for myosin binding Contraction begins: Myosin binding to actin forms cross bridges and contraction (cross bridge cycling) begins. At this point, E-C coupling is over. 4 Myosin cross bridge The aftermath When the muscle AP ceases, the voltage-sensitive tubule proteins return to their original shape, closing the Ca2+ release channels of the SR. Ca2+ levels in the sarcoplasm fall as Ca2+ is continually pumped back into the SR by active transport. Without Ca2+, the blocking action of tropomyosin is restored, myosin-actin interaction is inhibited, and relaxation occurs. Each time an AP arrives at the neuromuscular junction, the sequence of E-C coupling is repeated. 6/24/2012 MDufilho

Figure 9.11 Excitation-contraction (E-C) coupling is the sequence of events by which transmission of an action potential along the sarcolemma leads to the sliding of myofilaments. Steps in E-C Coupling: Sarcolemma Voltage-sensitive tubule protein T tubule The action potential (AP) propagates along the sarcolemma and down the T tubules. 1 Setting the stage The events at the neuromuscular junction (NMJ) set the stage for E-C coupling by providing excitation. Released acetylcholine binds to receptor proteins on the sarcolemma and triggers an action potential in a muscle fiber. Ca2+ release channel Calcium ions are released. Transmission of the AP along the T tubules of the triads causes the voltage-sensitive tubule proteins to change shape. This shape change opens the Ca2+ release channels in the terminal cisterns of the sarcoplasmic reticulum (SR), allowing Ca2+ to flow into the cytosol. 2 Terminal cistern of SR Synaptic cleft Axon terminal of motor neuron at NMJ Action potential is generated ACh Sarcolemma Actin T tubule Troponin Tropomyosin blocking active sites Terminal cistern of SR Myosin Calcium binds to troponin and removes the blocking action of tropomyosin. When Ca2+ binds, troponin changes shape, exposing binding sites for myosin (active sites) on the thin filaments. 3 Muscle fiber Triad Active sites exposed and ready for myosin binding One sarcomere Contraction begins: Myosin binding to actin forms cross bridges and contraction (cross bridge cycling) begins. At this point, E-C coupling is over. 4 Myosin cross bridge One myofibril The aftermath When the muscle AP ceases, the voltage-sensitive tubule proteins return to their original shape, closing the Ca2+ release channels of the SR. Ca2+ levels in the sarcoplasm fall as Ca2+ is continually pumped back into the SR by active transport. Without Ca2+, the blocking action of tropomyosin is restored, myosin-actin interaction is inhibited, and relaxation occurs. Each time an AP arrives at the neuromuscular junction, the sequence of E-C coupling is repeated. 6/24/2012 MDufilho

Continues as long as Ca2+ signal and adequate ATP present Cross Bridge Cycle Continues as long as Ca2+ signal and adequate ATP present Cross bridge formation—high-energy myosin head attaches to thin filament Working (power) stroke—myosin head pivots and pulls thin filament toward M line 6/24/2012 MDufilho

Cross Bridge Cycle Cross bridge detachment—ATP attaches to myosin head and cross bridge detaches "Cocking" of myosin head—energy from hydrolysis of ATP cocks myosin head into high-energy state 6/24/2012 MDufilho

A&P Flix™: The Cross Bridge Cycle Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Slide 6 Actin Ca2+ Thin filament PLAY Myosin cross bridge Thick filament A&P Flix™: The Cross Bridge Cycle Myosin Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge. 1 ATP hydrolysis Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. * 4 The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state. As a result it pulls the actin filament toward the M line. 2 In the absence of ATP, myosin heads will not detach, causing rigor mortis. *This cycle will continue as long as ATP is available and Ca2+ is bound to troponin. Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”). 3 6/24/2012 MDufilho

Role of Calcium (Ca2+) in Contraction At low intracellular Ca2+ concentration? - At high intracellular Ca2+ concentration? 6/24/2012 MDufilho

To re-establish RMP at sarcolemma and synaptic knob ATP is needed …… To re-establish RMP at sarcolemma and synaptic knob For detachment and “re-cocking” of myosin heads For sarcoplasmic reticulum to reabsorb Ca++ ( by ATP dependant calcium pump) 6/24/2012 MDufilho

Review Principles of Muscle Mechanics Contraction may/may not shorten muscle Isometric contraction: no shortening; muscle tension increases but does not exceed load Isotonic contraction: muscle shortens because muscle tension exceeds load Force and duration of contraction vary in response to stimuli of different frequencies and intensities 6/24/2012 MDufilho

Ach were not removed from synaptic cleft. What if?????? Ach were not removed from synaptic cleft. Little or no ATP could be produced The CNS sends volleys of high frequency impulses to various muscles 6/24/2012 MDufilho