Anatomy and Physiology, Seventh Edition Rod R. Seeley Idaho State University Trent D. Stephens Idaho State University Philip Tate Phoenix College Chapter 09 Lecture Outline* *See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Muscular System: Histology and Physiology Chapter 9 Muscular System: Histology and Physiology

Muscular System Functions Body movement Maintenance of posture Respiration Production of body heat Communication Constriction of organs and vessels Heart beat

Properties of Muscle Contractility: ability of a muscle to shorten with force Excitability: capacity of muscle to respond to a stimulus Extensibility: muscle can be stretched to its normal resting length and beyond to a limited degree Elasticity: ability of muscle to recoil to original resting length after stretched

Muscle Tissue Types Skeletal Smooth Cardiac Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement Voluntary Smooth Walls of hollow organs, blood vessels, eye, glands, skin Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow In some locations, autorhythmic Controlled involuntarily by endocrine and autonomic nervous systems Cardiac Heart: major source of movement of blood Autorhythmic

Skeletal Muscle Structure Composed of muscle cells (fibers), connective tissue, blood vessels, nerves Fibers are long, cylindrical, multinucleated Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length Develop from myoblasts; numbers remain constant Striated appearance due to light and dark banding

Connective Tissue Layers Fascia: connective tissue sheet External lamina. Delicate, reticular fibers. Surrounds sarcolemma Endomysium. Loose C.T. with reticular fibers. Perimysium. Denser C.T. surrounding a group of muscle fibers. Each group called a fasciculus Epimysium. C.T. that surrounds a whole muscle (many fascicles) Fascia: connective tissue sheet Forms layer under the skin Holds muscles together and separates them into functional groups. Allows free movements of muscles. Carries nerves (motor neurons, sensory neurons), blood vessels, and lymphatics. Continuous with connective tissue of tendons and periosteum. Connective Tissue

Nerve and Blood Vessel Supply Motor neurons: stimulate muscle fibers to contract. Nerve cells with cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves Axons branch so that each muscle fiber is innervated Capillary beds surround muscle fibers

Muscle Fiber Anatomy Nuclei just inside sarcolemma Cell packed with myofibrils within cytoplasm (sarcoplasm) Threadlike Composed of protein threads called myofilaments: thin (actin) and thick (myosin) Sarcomeres: highly ordered repeating units of myofilaments

Parts of a Muscle

Structure of Actin and Myosin

Actin (Thin) Myofilaments Two strands of fibrous (F) actin form a double helix extending the length of the myofilament; attached at either end at sarcomere. Composed of G actin monomers each of which has an active site Actin site can bind myosin during muscle contraction. Tropomyosin: an elongated protein winds along the groove of the F actin double helix. Troponin is composed of three subunits: one that binds to actin, a second that binds to tropomyosin, and a third that binds to calcium ions. Spaced between the ends of the tropomyosin molecules in the groove between the F actin strands. The tropomyosin/troponin complex regulates the interaction between active sites on G actin and myosin. Actin (Thin) Myofilaments

Myosin (Thick) Myofilament Many elongated myosin molecules shaped like golf clubs. Molecule consists of two heavy myosin molecules wound together to form a rod portion lying parallel to the myosin myofilament and two heads that extend laterally. Myosin heads Can bind to active sites on the actin molecules to form cross-bridges. Attached to the rod portion by a hinge region that can bend and straighten during contraction. Have ATPase activity: activity that breaks down adenosine triphosphate (ATP), releasing energy. Part of the energy is used to bend the hinge region of the myosin molecule during contraction

Sarcomeres: Z Disk to Z Disk Z disk: filamentous network of protein. Serves as attachment for actin myofilaments Striated appearance I bands: from Z disks to ends of thick filaments A bands: length of thick filaments H zone: region in A band where actin and myosin do not overlap M line: middle of H zone; delicate filaments holding myosin in place In muscle fibers, A and I bands of parallel myofibrils are aligned. Titin filaments: elastic chains of amino acids; make muscles extensible and elastic Sarcomeres: Z Disk to Z Disk

Sliding Filament Model Actin myofilaments sliding over myosin to shorten sarcomeres Actin and myosin do not change length Shortening sarcomeres responsible for skeletal muscle contraction During relaxation, sarcomeres lengthen because of some external force, like contraction of antagonistic muscles Insert Animation: Sliding Filaments.exe

Sarcomere Shortening

Physiology of Skeletal Muscle Nervous system controls muscle contractions through action potentials Resting membrane potentials Membrane voltage difference across membranes (polarized) Inside cell more negative due to accumulation of large protein molecules. More K+ on inside than outside. K+ leaks out but not completely because negative proteins hold some back. Outside cell more positive and more Na+ on outside than inside. Na/K pump maintains this situation. Must exist for action potential to occur Insert Fig. 9.7; insert Animation Sodium-Potassium Exchange Pump.exe

Ion Channels Types Each is specific for one type of ion Ligand-gated. Ligands are molecules that bind to receptors. Receptor: protein or glycoprotein with a receptor site Example: neurotransmitters Gate is closed until neurotransmitter attaches to receptor molecule. When Ach attaches to receptor on muscle cell, Na gate opens. Na moves into cell due to concentration gradient Voltage-gated Open and close in response to small voltage changes across plasma membrane Each is specific for one type of ion

Action Potentials Phases Depolarization: Inside of plasma membrane becomes less negative. If change reaches threshold, depolarization occurs Repolarization: return of resting membrane potential. Note that during repolarization, the membrane potential drops lower than its original resting potential, then rebounds. This is because Na plus K together are higher, but then Na/K pump restores the resting potential All-or-none principle: like camera flash system Propagate: Spread from one location to another. Action potential does not move along the membrane: new action potential at each successive location. Frequency: number of action potential produced per unit of time

Gated Ion Channels and the Action Potential

Action Potential Propagation

Neuromuscular Junction Synapse: axon terminal resting in an invagination of the sarcolemma Neuromuscular junction (NMJ): Presynaptic terminal: axon terminal with synaptic vesicles Synaptic cleft: space Postsynaptic membrane or motor end-plate

Function of Neuromuscular Junction Synaptic vesicles Neurotransmitter: substance released from a presynaptic membrane that diffuses across the synaptic cleft and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane. Acetylcholine Acetylcholinesterase: A degrading enzyme in synaptic cleft. Prevents accumulation of ACh Insert Fig. 9.12 WITHOUT PROCESS; Insert Animation Neuromuscular Junction.exe

Excitation-Contraction Coupling Mechanism where an action potential causes muscle fiber contraction Involves Sarcolemma Transverse (T) tubules: invaginations of sarcolemma Terminal cisternae Sarcoplasmic reticulum: smooth ER Triad: T tubule, two adjacent terminal cisternae Ca2+ Troponin

Action Potentials and Muscle Contraction Insert Process Figure 9.14 with verbiage; Insert Animation Action Potentials and Muscle Contraction.exe

Cross-Bridge Movement Insert Process Figure 9.15 with verbiage; Insert Animation Breakdown of ATP and Cross Bridge Movement During Muscle Cont.exe

Relaxation Ca2+ moves back into sarcoplasmic reticulum by active transport. Requires energy Ca2+ moves away from troponin-tropomyosin complex Complex re-establishes its position and blocks binding sites.

Muscle Twitch Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers Phases Lag or latent Contraction Relaxation

Stimulus Strength and Muscle Contraction All-or-none law for muscle fibers Contraction of equal force in response to each action potential Sub-threshold stimulus: no action potential; no contraction Threshold stimulus: action potential; contraction Stronger than threshold; action potential; contraction equal to that with threshold stimulus Motor units: a single motor neuron and all muscle fibers innervated by it

Contraction of the Whole Muscle Strength of contraction is graded: ranges from weak to strong depending on stimulus strength Multiple motor unit summation: strength of contraction depends upon recruitment of motor units. A muscle has many motor units Submaximal stimuli Maximal stimulus Supramaximal stimuli

Multiple-Wave Summation As the frequency of action potentials increase, the frequency of contraction increases Incomplete tetanus: muscle fibers partially relax between contraction Complete tetanus: no relaxation between contractions Multiple-wave summation: muscle tension increases as contraction frequencies increase

Treppe Graded response Occurs in muscle rested for prolonged period Each subsequent contraction is stronger than previous until all equal after few stimuli Possibly explanation: more and more Ca2+ remains in sarcoplasm and is not all taken up into the sarcoplasmic reticulum

Types of Muscle Contractions Isometric: no change in length but tension increases Postural muscles of body Isotonic: change in length but tension constant Concentric: overcomes opposing resistance and muscle shortens Eccentric: tension maintained but muscle lengthens Muscle tone: constant tension by muscles for long periods of time

Muscle Length and Tension Active tension: force applied to an object to be lifted when a muscle contracts Stretched muscle- not enough cross-bridging Crumpled muscle- myofilaments crumpled, cross-bridges can't contract Passive tension: tension applied to load when a muscle is stretched but not stimulated Total tension: active plus passive

Fatigue Decreased capacity to work and reduced efficiency of performance Types Psychological: depends on emotional state of individual Muscular: results from ATP depletion Synaptic: occurs in NMJ due to lack of acetylcholine

Physiological Contracture and Rigor Mortis Physiological contracture: state of fatigue where due to lack of ATP neither contraction nor relaxation can occur Rigor mortis: development of rigid muscles several hours after death. Ca2+ leaks into sarcoplasm and attaches to myosin heads and crossbridges form. Rigor ends as tissues start to deteriorate.

Energy Sources ATP provides immediate energy for muscle contractions. Produced from three sources Creatine phosphate During resting conditions stores energy to synthesize ATP Anaerobic respiration Occurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acid Aerobic respiration Requires oxygen and breaks down glucose to produce ATP, carbon dioxide and water More efficient than anaerobic Oxygen debt: oxygen taken in by the body, above that required for resting metabolism after exercise. ATP produced from anaerobic sources contributes

Slow and Fast Fibers Slow-twitch or high-oxidative Contract more slowly, smaller in diameter, better blood supply, more mitochondria, more fatigue-resistant than fast-twitch, large amount of myoglobin. Postural muscles, more in lower than upper limbs. Dark meat of chicken. Fast-twitch or low-oxidative Respond rapidly to nervous stimulation, contain myosin that can break down ATP more rapidly than that in Type I, less blood supply, fewer and smaller mitochondria than slow-twitch Lower limbs in sprinter, upper limbs of most people. White meat in chicken. Distribution of fast-twitch and slow-twitch Most muscles have both but varies for each muscle Effects of exercise: change in size of muscle fibers Hypertrophy: increase in muscle size Increase in myofibrils Increase in nuclei due to fusion of satellite cells Increase in strength due to better coordination of muscles, increase in production of metabolic enzymes, better circulation, less restriction by fat Atrophy: decrease in muscle size Reverse except in severe situations where cells die

Heat production Exercise: metabolic rate and heat production increase. Post-exercise: metabolic rate stays high due to oxygen debt. Excess heat lost because of vasodilation and sweating Shivering: uncoordinated contraction of muscle fibers resulting in shaking and heat production

Not striated, fibers smaller than those in skeletal muscle Spindle-shaped; single, central nucleus More actin than myosin Caveolae: indentations in sarcolemma; may act like T tubules Dense bodies instead of Z disks as in skeletal muscle; have noncontractile intermediate filaments Ca2+ required to initiate contractions; binds to calmodulin which regulates myosin kinase. Cross-bridging occurs Relaxation: caused by enzyme myosin phosphatase Smooth Muscle

Types of Smooth Muscle Visceral or unitary: cells in sheets; function as a unit Numerous gap junctions; waves of contraction Often autorhythmic Multiunit: cells or groups of cells act as independent units Sheets (blood vessels); bundles (arrector pili and iris); single cells (capsule of spleen)

Electrical Properties of Smooth Muscle Slow waves of depolarization and repolarization transferred from cell to cell Depolarization caused by spontaneous diffusion of Na+ and Ca2+ into cell Does not follow all-or-none law May have pacemaker cells Contraction regulated by nervous system and by hormones

Functional Properties of Smooth Muscle Some visceral muscle exhibits autorhythmic contractions Tends to contract in response to sudden stretch but not to slow increase in length Exhibits relatively constant tension: smooth muscle tone Amplitude of contraction remains constant although muscle length varies

Smooth Muscle Regulation Innervated by autonomic nervous system Neurotransmitters are acetylcholine and norepinephrine Hormones important as epinephrine and oxytocin Receptors present on plasma membrane which neurotransmitters or hormones bind determines response

Cardiac Muscle Found only in heart Striated Each cell usually has one nucleus Has intercalated disks and gap junctions Autorhythmic cells Action potentials of longer duration and longer refractory period Ca2+ regulates contraction

Effects of Aging on Skeletal Muscle Reduced muscle mass Increased time for muscle to contract in response to nervous stimuli Reduced stamina Increased recovery time Loss of muscle fibers Decreased density of capillaries in muscle