9-1 Muscular System: Histology and Physiology Chapter 9

9-2 Muscular System Functions Body movement Posture Respiration Production of body heat Communication Constriction of organs and vessels Heart beat

9-3 Properties of Muscle Contractility Excitability Extensibility Elasticity

9-4 Muscle Tissue Types Skeletal Smooth Cardiac

9-5 Skeletal Muscle Structure Composed of muscle fibers (cells), CT, blood vessels, nerves Fibers are long, multinucleated Develop from myoblasts; numbers remain constant Striated

9-6 CT Layers –External lamina. Reticular fibers. Surrounds sarcolemma –Endomysium. Loose C.T. with reticular fibers; surrounds muscle fiber –Perimysium. Denser C.T. surrounding a group of muscle fibers called a fasciculus –Epimysium. C.T. that surrounds a whole muscle (many fascicles); aka fascia

9-7 Fascia (epimysium): CT sheet –Forms layer under the skin –Holds muscles together; separates them into groups –Allows free movement –Carries nerves (motor neurons, sensory neurons), blood vessels, and lymphatics –Continuous with tendons and periosteum.

9-8 Nerves Motor neurons: stimulate muscle fibers to contract. Cell bodies in brain or spinal cord; axons extend to skeletal muscle fibers through nerves Axons branch so that each muscle fiber is innervated

9-9 Blood Supply Capillary beds surround muscle fibers

9-10 Muscle Fiber Anatomy Cell packed with myofibrils in sarcoplasm –Composed of protein myofilaments Actin (thin) Myosin (thick) –Sarcomeres: repeating units of actin & myosin

9-11 Sarcomeres: Z Disk to Z Disk Z disk: network of protein; attachment for actin myofilaments Striations –I bands –A bands –H zone –M line

9-12 Parts of a Muscle

Other Muscle Fiber Structures: Sarcolemma – cell membrane Sarcoplasm – cytoplasm Sarcoplasmic reticulum – sER that acts as holding tank for Ca 2 ; uses active transport to pump ions into lumen T-tubules – tubes from sarcolemma that project into muscle fiber; brings sarcomeres in contact with extracellular fluid 9-13

9-14 Structure of Actin and Myosin

9-15 Actin Myofilaments Proteins involved: –F (fibrous) actin: forms attachment site for myosin filament –Tropomyosin: block F actin binding sites with troponin –Troponin

9-16 Myosin Myofilaments Myosin heads –Attached to the rod portion by a hinge region that can bend and straighten during contraction –Bind to active sites on the actin molecules to form cross-bridges –Have ATPase activity

9-17 Sliding Filament Model Actin myofilaments sliding over myosin to shorten sarcomeres

9-18 Sarcomere Shortening

9-19 Physiology of Skeletal Muscle Nervous system controls muscle contractions through action potentials Resting membrane potentials (RMP) –Voltage difference across membranes (polarized) Protein molecules More K + on inside than outside Na + /K + pump maintains RMP

9-20 Neuromuscular Junction NMJ: –Presynaptic terminal –Synaptic cleft NT Acetylcholinesterase –Motor end-plate (postsynaptic membrane)

9-21 Con’t. Synaptic vesicles –Neurotransmitter: substance released from a presynaptic membrane, diffuses across the synaptic cleft, and stimulates (or inhibits) the production of an action potential in the postsynaptic membrane Acetylcholine (ACh)

9-22 Events of Contraction 1.Action potential in motor neuron causes Ca2+ channels to open 2.Ca2+ diffuses into axon terminal and causes release of Ach from synaptic vesicles 3.Ach diffuses across synaptic cleft and binds to receptors on sarcolemma at motor end plate 4.Ligand-gated Na+ channels open and Na+ diffuses into muscle fiber causing depolarization 5.Depolarization triggers an a.p. in the muscle fiber

9-23 Con.’t 6. T-tubules depolarize and cause Ca2+ voltage- gated channels to open in sarcoplasmic reticulum 7. Ca2+ ruches out of SR into sarcoplasm and binds to troponin on actin filament 8. Binding sites on F actin are exposed and crossbridges form between actin and myosin 9. ATP is used to move myosin head and then release crossbridges

9-24

9-25 Excitation-Contraction Coupling Mechanism where an action potential causes muscle fiber contraction

9-26 Action Potentials and Muscle Contraction

9-27 Cross-Bridge Movement

9-28 Events of Relaxation 1.Acetylcholinesterase removes excess Ach from synaptic cleft 2.Sarcoplasmic reticulum pumps Ca 2+ back into its lumen (active transport) 3.Troponin releases Ca 2+ and actin binding sites are blocked

9-29 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

9-30 Stimulus Strength All-or-none law –Contraction of equal force in response to each action potential Sub-threshold stimulus: no action potential; no contraction Threshold stimulus: action potential; contraction Motor units: a single motor neuron and all muscle fibers innervated by it

9-31 Contraction of Entire Muscle Strength of contraction is graded Multiple motor unit summation: strength of contraction depends upon recruitment of motor units

9-32 Multiple-Wave Summation As the frequency of action potentials increase, the frequency of contraction increases –Incomplete tetanus – Complete tetanus –Multiple-wave summation

9-33 Treppe Graded response Occurs in muscle rested for prolonged period Each subsequent contraction is stronger than previous until all equal after few stimuli

9-34 Types of Contractions Isometric Isotonic Muscle tone

9-35 Fatigue Types –Psychological –Muscular –Synaptic

9-36 Rigor Mortis

9-37 Energy Sources ATP; 3 sources: –Creatine phosphate During resting conditions stores energy to synthesize ATP –Anaerobic respiration –Aerobic respiration Oxygen debt: oxygen taken in by the body above that required for resting metabolism after exercise

9-38 Muscle Fiber Types Slow-twitch or high-oxidative (red) Fast-twitch or low-oxidative Effects of exercise: change in size of muscle fibers –Hypertrophy –Atrophy

9-39 Heat production Exercise Excess heat lost due to vasodilation and sweating Shivering

9-40 Smooth Muscle Not striated, smaller fibers Single nucleus More actin than myosin Caveolae Ca 2+ required to initiate contractions; Cross-bridging occurs but takes longer Relaxation caused by enzyme myosin phosphatase

9-41

9-42 Types of Smooth Muscle Visceral (unitary): cells in sheets; function as a unit –GI, urinary, reproductive tracts –Numerous gap junctions; waves of contraction –Often autorhythmic Multiunit: cells act as independent units –Sheets (blood vessels); bundles (arrector pili and iris); single cells (capsule of spleen) –Only contract when stimulated by neuron or hormone

9-43 Electrical Properties (of Smooth Muscle) Slow waves of depolarization and repolarization transferred from cell to cell Does not follow all-or-none law May have pacemaker cells Contraction regulated by nervous system and by hormones

9-44 Functional Properties Some visceral muscle exhibits autorhythmic contractions Contracts in response to sudden stretch Constant tension: smooth muscle tone Intensity of contraction constant Aerobic respiration

9-45 Smooth Muscle Regulation Innervated by ANS Neurotransmitters – acetylcholine, norepinephrine Hormones – epinephrine Cell membranes have receptors

9-46 Cardiac Muscle Only in heart Striated One nucleus Has intercalated disks and gap junctions Autorhythmic; pacemaker cells (SA node) Action potentials of longer duration and longer refractory period Ca 2+ regulates contraction

9-47 Effects of Aging Increased time Loss of muscle fibers; more slow than fast Decreased density of capillaries in muscle