Histology and Physiology of Muscles Chapter 8 Histology and Physiology of Muscles Skeletal Muscle Fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Functions of the Muscular System Body movement (Skeletal) Maintenance of posture (Skeletal) Respiration (Skeletal) Production of body heat (Skeletal) Communication (Skeletal) Constriction of organs and vessels (Smooth) Heartbeat (Cardiac)

Functional Characteristics of Muscle Contractility: ability to shorten forcibly Excitability: ability to receive and respond to stimuli Extensibility: ability to be stretched or extended Elasticity: ability to recoil and resume original resting length

3 Types of Muscle Tissue skeletal, smooth, cardiac These types differ in Structure Location Function Means of activation Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

Types of Muscle Tissue Skeletal muscles Smooth muscle Cardiac muscle responsible for most body movements Maintain posture, stabilize joints, and generate heat Smooth muscle found in walls of hollow organs and tubes; moves substances through them Helps maintain blood pressure Squeezes or propels substances (i.e., food, feces) through organs Cardiac muscle found in heart; pumps blood throughout body

Tab. 8.1

Skeletal Muscle Structure Cells elongated, often called skeletal muscle fibers Each cell contains several nuclei located around periphery of fiber near plasma membrane Fibers appear striated due to actin and myosin myofilaments A single fiber can extend from one end of a muscle to the other!!! Contracts rapidly but tires easily controlled voluntarily (i.e., by conscious control)

Skeletal Muscle Structure Fascia: general term for connective tissue sheets 3 muscular fascia separate and compartmentalize individual muscles or groups of muscles Epimysium: overcoat of dense collagenous connective tissue that surrounds entire muscle Perimysium: fibrous connective tissue that surrounds groups of muscle fibers called fascicles (bundles) Endomysium: fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber

Skeletal Muscle Structure connective tissue provides a pathway for blood vessels and nerves to reach muscle fibers Fig. 8.1

Skeletal Muscle Structure Muscle connective tissue blends with other connective tissue based structures, such as tendons connect muscle to bone Fig. 8.2

Skeletal Muscle Structure Muscle Fibers Sarcolemma: muscle cell plasma membrane Sarcoplasm: cytoplasm of a muscle cell Myo, mys, and sarco: prefixes used to refer to muscle Muscle contraction depends on 2 kinds of myofilaments: actin and myosin Myofibrils - densely packed, rod-like contractile elements make up most of muscle volume

Fig. 8.2

Skeletal Muscle Structure: ACTIN (thin) myofilaments 2 helical polymer strands of F actin (composed of G actin), tropomyosin, and troponin G actin contains active sites to which myosin heads attach during contraction Tropomyosin and troponin are regulatory subunits bound to actin Fig. 8.2

Skeletal Muscle Structure: MYOSIN (thick) myofilaments consist of myosin molecules Each myosin molecule has A head with an ATPase, which breaks down ATP A hinge region, which enables head to move A rod A cross-bridge is formed when a myosin head binds to the active site on G actin Fig. 8.2

Skeletal Muscle Structure Sarcomeres smallest contractile unit of a muscle bound by Z disks that hold actin myofilaments 6 actin myofilaments surround a myosin myofilament Myofibrils appear striated because of A bands and I bands Fig. 8.2

Skeletal Muscle Structure Thick filaments: extend the entire length of an A band Thin filaments: extend across the I band and partway into A band Z-disc: coin-shaped sheet of proteins (connectins) that anchors thin filaments and connects myofibrils to one another Thin filaments do not overlap thick filaments in lighter H zone M lines appear darker due to presence of protein desmin arrangement of myofibrils within a fiber is so organized a perfectly aligned repeating series of dark A bands and light I bands is evident

Fig. 8.3bc

Sliding Filament Model Summary Actin and myosin myofilaments do not change in length during contraction Thin filaments slide past thick ones so actin and myosin filaments overlap to a greater degree Upon stimulation, myosin heads bind to actin and sliding begins Each myosin head binds and detaches several times during contraction (acting like a ratchet to generate tension and propel the thin filaments to the center of sarcomere) In relaxed state, thin and thick filaments overlap only slightly As this event occurs throughout sarcomeres, muscle shortens I band and H zones become narrower during contraction, and A band remains constant in length

Fig. 8.4

Sliding Filament Model Actin and myosin myofilaments in a relaxed muscle (below) and a contracted muscle are the same length. Myofilaments do not change length during muscle contraction! Fig. 8.4

Sliding Filament Model During contraction, actin myofilaments at each end of the sarcomere slide past the myosin myofilaments toward each other. As a result, the Z disks are brought closer together, and the sarcomere shortens Fig. 8.4

Sliding Filament Model As the actin myofilaments slide over the myosin myofilaments, H zones (yellow) and I bands (blue) narrow. A bands, equal to length of myosin myofilaments, do not narrow because length of myosin myofilaments does not change Fig. 8.4

Sliding Filament Model In a fully contracted muscle, ends of actin myofilaments overlap at center of the sarcomere and H zone disappears Fig. 8.4

Physiology of Skeletal Muscle Fibers Membrane Potentials!!! nervous system stimulates muscles to contract through electric signals called action potentials Plasma membranes are polarized, which means there is a charge difference (resting membrane potential) across plasma membrane inside of plasma membrane is negative as compared to the outside in a resting cell action potential: reversal of resting membrane potential so that inside of plasma membrane becomes positive

Physiology Ion Channels Assist with production of action potentials Ligand-gated channels Voltage-gated channels Fig. 8.5

Fig. 8.6

Physiology of Skeletal Muscle Fibers Action Potentials Depolarization results from an increase in the permeability of the plasma membrane to Na+ If depolarization reaches threshold, an action potential is produced The depolarization phase of the action potential results from the opening of many Na+ channels Fig. 8.6

Physiology of Skeletal Muscle Fibers Action Potentials The repolarization phase of the action potential occurs when the Na+ channels close and K+ channels open briefly Fig. 8.6

Fig. 8.6

Physiology of Skeletal Muscle Fibers Action Potentials Occur in an all-or-none fashion stimulus below threshold produces no action potential stimulus at threshold or stronger will produce an action potential Propagate (travel) across plasma membranes

Physiology of Skeletal Muscle Fibers Nerve Stimulus of Skeletal Muscle Skeletal muscles are stimulated by motor neurons of the somatic nervous system Axons of these neurons travel in nerves to muscle cells Axons of motor neurons branch profusely as they enter muscles Each axonal branch forms a neuromuscular junction with a single muscle fiber

Physiology of Skeletal Muscle Fibers neuromuscular junction is formed from: Axonal endings small membranous sacs (synaptic vesicles) Contain neurotransmitter acetylcholine (ACh) Motor end plate of a muscle Specific part of the sarcolemma Contains ACh receptors Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

Fig. 8.7

Neuromuscular Junction Physiology action potential (orange arrow) arrives at presynaptic terminal and causes voltage-gated Ca2+ channels in the presynaptic membrane to open Calcium ions enter the presynaptic terminal and initiate the release of the neurotransmitter acetylcholine (ACh) from synaptic vesicles ACh is released into the synaptic cleft by exocytosis Fig. 8.8

Neuromuscular Junction Physiology ACh diffuses across the synaptic cleft and binds to ligand-gated Na+ channels on the postsynaptic membrane Ligand-gated Na+ channels open and Na+ enters the postsynaptic cell, causing the postsynaptic membrane to depolarize. If depolarization passes threshold, an action potential is generated along the postsynaptic membrane ACh is removed from the ligand-gated Na+ channels, which then close Fig. 8.8

Neuromuscular Junction Physiology enzyme acetylcholinesterase, which is attached to postsynaptic membrane, removes acetylcholine from synaptic cleft by breaking it down into acetic acid and choline Choline is symported with Na+ into presynaptic terminal, where it can be recycled to make ACh. Acetic acid diffuses away from synaptic cleft ACh is reformed within presynaptic terminal using acetic acid generated from metabolism and choline recycled from synaptic cleft. ACh is then taken up by synaptic vesicles Fig. 8.8

Fig. 8.8

Excitation-Contraction Coupling In order to contract, a skeletal muscle must: Be stimulated by a nerve ending Propagate an electrical current, or action potential, along its sarcolemma Have a rise in intracellular Ca2+ levels, the final trigger for contraction Linking the electrical signal to the contraction is excitation-contraction coupling

Excitation-Contraction Coupling Invaginations of sarcolemma form T tubules, which wrap around sarcomeres and penetrate into cell’s interior at each A band –I band junction Sarcoplasmic reticulum (SR) is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinal and surrounds each myofibril Paired terminal cisternae form perpendicular cross channels Functions in regulation of intracellular calcium levels A triad is a T tubule and two terminal cisternae Fig. 8.9

Excitation-Contraction Coupling An action potential produced at presynaptic terminal in the neuromuscular junction is propagated along sarcolemma of the skeletal muscle. depolarization also spreads along membrane of T tubules depolarization of T tubule causes gated Ca2+ channels in SR to open, resulting in an increase in permeability of SR to Ca2+, especially in terminal cisternae. Calcium ions then diffuse from SR into sarcoplasm Calcium ions released from SR bind to troponin molecules. troponin molecules bound to G actin molecules are released, causing tropomyosin to move, exposing active sites on G actin Once active sites on G actin molecules are exposed, heads of myosin myofilaments bind to them to form cross-bridges

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