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muscles and muscle tissues.docx

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Department
Biological Sciences
Course
BIOB32H3
Professor
Kenneth Welch
Semester
Fall

Description
Muscles and Muscle Tissue Muscle Overview • The three types of muscle tissue are skeletal, cardiac, and smooth • These types differ in structure, location, function, and means of activation Muscle Similarities • Skeletal and smooth muscle cells are elongated and are called muscle fibers • Muscle contraction depends on two kinds of myofilaments – actin and myosin • Muscle terminology is similar • Sarcolemma – muscle plasma membrane • Sarcoplasm – cytoplasm of a muscle cell • Prefixes – myo, mys, and sarco all refer to muscle Skeletal Muscle Tissues • Packaged in skeletal muscles that attach to and cover the bony skeleton • Has obvious stripes called striations • Is controlled voluntarily (i.e., by conscious control) • Contracts rapidly but tires easily • Is responsible for overall body motility • Is extremely adaptable and can exert forces over a range from a fraction of an ounce to over 70 pounds Cardiac Muscle Tissue • Occurs only in the heart • Is striated like skeletal muscle but is not voluntary • Contracts at a fairly steady rate set by the heart’s pacemaker • Neural controls allow the heart to respond to changes in bodily needs Smooth Muscle Tissue • Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages • Forces food and other substances through internal body channels • It is not striated and is involuntary Muscle Function • Skeletal muscles are responsible for all locomotion • Cardiac muscle is responsible for coursing the blood through the body • Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs • Muscles also maintain posture, stabilize joints, and generate heat Functional Characteristics of Muscles • Excitability, or irritability – the ability to receive and respond to stimuli • Contractility – the ability to shorten forcibly • Extensibility – the ability to be stretched or extended • Elasticity – the ability to recoil and resume the original resting length Skeletal Muscle • Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue • The three connective tissue wrappings are: • Epimysium – an overcoat of dense regular CT that surrounds the entire muscle • Perimysium – fibrous CT that surrounds groups of muscle fibers called fascicles • Endomysium – fine sheath of CT composed of reticular fibers surrounding each muscle fiber Skeletal Muscle: Nerve and Blood Supply • Each muscle is served by one nerve, an artery, and one or more veins • Each skeletal muscle fiber is supplied with a nerve ending that controls contraction • Contracting fibers require continuous delivery of oxygen and nutrients via arteries • Wastes must be removed via veins Skeletal Muscle: Attachments • Muscles span joints and are attached to bone in at least two places • When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone – the muscle’s origin • Muscles attach: • Directly – epimysium of the muscle is fused to the periosteum of a bone • Indirectly – CT wrappings extend beyond the muscle as a tendon or aponeurosis Microscopic Anatomy of a Skeletal Muscle Fiber • Each fiber is a long, cylindrical cell with multiple nuclei just beneath the sarcolemma • Fibers are 10 to 100 m in diameter, and up to hundreds of centimeters long • Each cell is a syncytium produced by fusion of embryonic cells • Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin • Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules Myofibrils • Myofibrils are densely packed, rodlike contractile elements • They make up most of the muscle volume • The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident Sarcomeres • The smallest contractile unit of a muscle • The region of a myofibril between two successive Z discs • Composed of myofilaments made up of contractile proteins • Myofilaments are of two types – thick and thin Myofilaments: Banding Pattern • Thick filaments – extend the entire length of an A band • Thin filaments – extend across the I band and partway into the A band • Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another • Thin filaments do not overlap thick filaments in the lighter H zone • M lines appear darker due to the presence of the protein desmin Ultrastructure of Myofilaments: Thick Filaments • Thick filaments are composed of the protein myosin • Each myosin molecule has a rodlike tail and two globular heads • Tails – two interwoven, heavy polypeptide chains • Heads – two smaller, light polypeptide chains called cross bridges Ultrastructure of Myofilaments: Thin Filaments • Thin filaments are chiefly composed of the protein actin • Each actin molecule is a helical polymer of globular subunits called G actin • The subunits contain the active sites to which myosin heads attach during contraction • Tropomyosin and troponin are regulatory subunits bound to actin Arrangement of the Filaments in a Sarcomere • Longitudinal section within one sarcomere Sarcoplasmic Reticulum (SR) • SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril • Paired terminal cisternae form perpendicular cross channels • Functions in the regulation of intracellular calcium levels • Elongated tubes called T tubules penetrate into the cell’s interior at each A band–I band junction • T tubules associate with the paired terminal cisternae to form triads T Tubules • T tubules are continuous with the sarcolemma • They conduct impulses to the deepest regions o2+the muscle • These impulses signal for the release of Ca from adjacent terminal cisternae Contraction of Skeletal Muscle Fibers • Contraction – refers to the activation of myosin’s cross bridges (force generating sites) • Shortening occurs when the tension generated by the cross bridge exceeds forces opposing shortening • Contraction ends when cross bridges become inactive, the tension generated declines, and relaxation is induced Sliding Filament Mechanism of Contraction • Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree • In the relaxed state, thin and thick filaments overlap only slightly • 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 the sarcomere • As this event occurs throughout the sarcomeres, the muscle shortens Role of Ionic Calcium (Ca ) in the Contraction Mechanism • At low intracellular Ca concentration: • Tropomyosin blocks the binding sites on actin • Myosin cross bridges cannot attach to binding sites on actin • The relaxed state of the muscle is enforced • At higher intracellular Ca concentrations: • Additional calcium binds to troponin (inactive troponin binds two Ca ) 2+ • Calcium-activated troponin binds an additional two Ca at a separate regulatory site • Calcium-activated troponin undergoes a conformational change • This change moves tropomyosin away from actin’s binding sites • Myosin head can now bind and cycle • This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin Sequential Events of Contraction • Cross bridge attachment – myosin cross bridge attaches to actin filament • Working (power) stroke – myosin head pivots and pulls actin filament toward M line • Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches • “Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high energy state Regulation of Contraction • 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 Ca levels, the final trigger for contraction • Linking the electrical signal to the contraction is excitation-contraction coupling 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 Neuromuscular Junction • The neuromuscular junction is formed from: • Axonal endings, which have small membranous sacs (synaptic vesicles) that contain the neurotransmitteracetylcholine (ACh) • The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors that helps form the neuromuscular junction • Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft • When a nerve impulse reaches the end of an axon at the neuromuscular junction: • Voltage-regulated calcium channels open and allow Ca to enter the axon 2+ • Ca inside the axon terminal causes axonal vesicles to fuse with the axonal membrane • This fusion releases ACh into the synaptic cleft via exocytosis • ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma • Binding of ACh to its receptors initiates an action potential in the muscle Action Potential • A transient depolarization event that includes polarity reversal of a sarcolemma (or nerve cell membrane) and the propagation of an action potential along the membrane Action Potential: Electrical Conditions of a Polarized Sarcolemma • The outside (extracellular) face is positive, while the inside face is negative • This difference in charge is the resting membrane potential • The predominant extracellular ion is Na + + • The predominant intracellular ion is K • The sarcolemma is relatively impermeable to both ions Action Potential: Depolarization and Generation of the Action Potential • An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na (sodium channels open) + • Na enters the cell, and the resting potential is decreased (depolarization occurs) • If the stimulus is strong enough, an action potential is initiated Action Potential: Propagation of the Action Potential • Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch • Voltage-regulated Na channels now open in the adjacent patch causing it to depolarize • Thus, the action potential travels rapidly along the sarcolemma • Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle Action Potential: Repolarization • Immediately after the depolarization wave passes, the sarcolemma permeability changes • Na channels close and K channels open • K diffuses from the cell, restoring the electrical polarity of the sarcolemma • Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period) + + • The ionic concentration of the resting state is restored by the Na -K pump Destruction of Acetylcholine • ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase (AChE) • AChE activity prevents continued muscle fiber contraction in the absence of additional stimuli Excitation-Contraction Coupling • Once generated, the action potential: • Is propagated along the sarcolemma • Travels down the T tubules 2+ • Tr2+gers Ca release from terminal cisternae • Ca binds to troponin and causes: • The blocking action of tropomyosin to cease • Actin active binding sites to be exposed • Myosin cross bridges alternately attach and detach • Thin filaments move toward the center of the sarcomere • Hyd2+lysis of ATP powers this cycling process • Ca is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes Contraction of Skeletal Muscle (Organ Level) • Contraction of muscle fibers (cells) and muscles (organs) is similar • The two types of muscle contractions are: • Isometric contraction – increasing muscle tension (muscle does not shorten) • Isotonic contraction – decreasing muscle length (muscle shortens during contraction) Motor Unit: The Nerve-Muscle Functional Unit • A motor unit is a motor neuron and all the muscle fibers it supplies • The number of muscle fibers per motor unit can vary from four to several hundred • Muscles that control fine movements (fingers, eyes) have small motor units • Large weight-bearing muscles (thighs, hips)
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