Transcript Document
PowerPoint® Lecture Slides prepared by Janice Meeking, Mount Royal College CHAPTER 9 Muscles and Muscle Tissue: Part A Copyright © 2010 Pearson Education, Inc. Three Muscle Types All muscle tissue exhibit: Responsiveness - The ability to receive and respond to a stimulus Conductivity – the ability of the impulse to travel along the plasma membrane of the muscle cell. Contractility - The ability to shorten Elasticity - The ability to recoil and resume original length Copyright © 2010 Pearson Education, Inc. Skeletal Muscle Functions 1.Movement of bones or fluids (e.g., blood) 2.Maintaining posture and body position 3.Stabilizing joints 4.Heat generation (especially skeletal muscle) Each muscle is served by one artery, one nerve, and one or more veins Copyright © 2010 Pearson Education, Inc. Skeletal Muscle • Connective tissue sheaths of skeletal muscle: • Epimysium: dense regular connective tissue surrounding entire muscle • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) • Endomysium: fine areolar surrounding each muscle fiber Copyright © 2010 Pearson Education, Inc. connective tissue Skeletal Muscle organization • In order of decreasing size… • Myofiber - entire cell. • Myofibrils - bundles of myofilaments inside myofiber. • Myofilaments - actin and myosin proteins. Copyright © 2010 Pearson Education, Inc. Copyright © 2010 Pearson Education, Inc. Muscle terminology • Muscle fiber – muscle cell • Sarcolema – cell membrane • Sarcoplasm – cytoplasm • Sarcoplasic reticulum – endoplasmic reticulum Copyright © 2010 Pearson Education, Inc. 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 • Sarcoplasm has numerous glycosomes (granules that store glycogen) and a unique oxygen-binding protein called myoglobin (similar to hemoglobin) • Fibers contain the usual organelles, sarcoplasmic reticulum, and T tubules Copyright © 2010 Pearson Education, Inc. myofibrils, Myofibrils • Myofibrils are densely packed contractile elements • They make up most of the muscle volume Sarcolemma Mitochondrion Myofibril Dark A band Light I band Nucleus (b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril is extended afrom the cut end of the fiber. Copyright © 2010 Pearson Education, Inc. Ultrastructure of Myofilaments: Thick Filaments • Thick filaments are composed of about 300 molecules of a protein myosin • Each myosin molecule has a rod-like tail and two globular heads • Tails – two interwoven, heavy polypeptide chains • Heads – two smaller, light polypeptide chains • connects the thick and thin filaments forming the cross bridges • contain ATPase that split ATP to release ATP for muscle contraction Copyright © 2010 Pearson Education, Inc. Ultrastructure of Myofilaments: Thin Filaments • Thin filaments are composed mainly of the protein actin • Each actin molecule is a helical polymer of globular subunits called G (globular) actin • The subunits contain the active sites to which myosin heads attach during contraction • Tropomyosin filaments regulate the interaction of actin and myosin. • Troponin are regulatory subunits bound to actin G actin Copyright © 2010 Pearson Education, Inc. Sarcomeres (within myofibril) • 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 – actin and myosin Copyright © 2010 Pearson Education, Inc. Structure of Skeletal Muscle: Microstructure • Sarcoplasmic reticulum (SR) • forms a tubular network around each individual myofibril • Most run longitudinally along myofibrils • Some form cross channels at the A band and I band junction (terminal cisterna) • Regulate intracellular levels of calcium – stores Ca and release it when needed Copyright © 2010 Pearson Education, Inc. Structure of Skeletal Muscle: Microstructure • Transverse tubules • all regions of simultaneously. • the signal to contract must be distributed quickly. • This signal is conducted through the transverse tubules (T- tubules) that are narrow tubes that are continuous with the sarcolemma and extend into the sarcoplasm • Because T-tubules are continuous of the sarcolemma they conduct impulses to the deepest regions of the muscle cell Copyright © 2010 Pearson Education, Inc. the cell must contract Triad Relationships • T tubules conduct impulses deep into muscle fiber • Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes • T tubule proteins: voltage sensors • SR foot proteins: gated channels that regulate Ca2+ release from the SR cisternae Copyright © 2010 Pearson Education, Inc. Sarcomere: contractile unit inside myofiber Further divisions of myofibrils: • I-band – actin only (light) • Z-line union of 2 actin heads (in the middle of I band) • A-band – actin and myosin overlap (dark) • H band – only myosin (in middle of A band) Copyright © 2010 Pearson Education, Inc. Sliding Filament Model 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 Copyright © 2010 Pearson Education, Inc. During contraction: Z discs move towards each other I and H bands – almost disappear A band length – remain unchanged Copyright © 2010 Pearson Education, Inc. Requirements for Skeletal Muscle Contraction 1. Activation: neural neuromuscular junction stimulation at a 2. Excitation-contraction coupling: • Generation and propagation of an action potential along the sarcolemma • Final trigger: a brief rise in intracellular Ca2+ levels Copyright © 2010 Pearson Education, Inc. Events at the Neuromuscular Junction • Skeletal muscles are stimulated by somatic motor neurons • Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles • Each axon forms several branches as it enters a muscle • Each axon ending forms a neuromuscular junction with a single muscle fiber Copyright © 2010 Pearson Education, Inc. Neuromuscular Junction • Situated midway along the length of a muscle fiber • Axon terminal and muscle fiber are separated by a synaptic cleft • Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh) • the sarcolemma contain ACh receptors • Nerve impulse arrives at axon terminal • ACh is released and binds with receptors on the sarcolemma • Electrical events lead to the generation of an action potential PLAY A&P Flix™: Events at the Neuromuscular Junction Copyright © 2010 Pearson Education, Inc. Events in Generation of an Action Potential 1. Local depolarization (end plate potential): • ACh binding opens chemically (ligand) gated ion channels • Simultaneous diffusion of Na+ (inward) and K+ (outward) • More Na+ diffuses, so the interior of the sarcolemma becomes less negative • Local depolarization – end plate potential Copyright © 2010 Pearson Education, Inc. Events in Generation of an Action Potential 2. Generation and propagation of an action potential: • End plate potential spreads to adjacent membrane areas • Voltage-gated Na+ channels open • Na+ influx decreases the membrane voltage toward a critical threshold • If threshold is reached, an action potential is generated Copyright © 2010 Pearson Education, Inc. Events in Generation of an Action Potential • Local depolarization wave continues to spread, changing the permeability of the sarcolemma • Voltage-regulated Na+ channels open in the adjacent patch, causing it to depolarize to threshold Copyright © 2010 Pearson Education, Inc. Events in Generation of an Action Potential 3. Repolarization: • Na+ channels close and voltage-gated K+ channels open • K+ efflux rapidly restores the resting polarity • Fiber cannot be stimulated and is in a refractory period until repolarization is complete • Ionic conditions of the resting state are restored by the Na+-K+ pump Copyright © 2010 Pearson Education, Inc. Excitation-Contraction (E-C) Coupling • Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments • Latent period: • Time when E-C coupling events occur • Time between AP initiation and the beginning of contraction • AP is propagated along sarcomere to T tubules • Voltage-sensitive proteins stimulate Ca2+ release from SR • Ca2+ is necessary for contraction Copyright © 2010 Pearson Education, Inc. Role of Calcium (Ca2+) in Contraction • At low intracellular Ca2+ concentration: • Tropomyosin blocks the active sites on actin • Myosin heads cannot attach to actin • Muscle fiber relaxes • At higher intracellular Ca2+ concentrations: • Ca2+ binds to troponin • Troponin changes shape and moves tropomyosin away from active sites • Events of the cross bridge cycle occur • When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends Copyright © 2010 Pearson Education, Inc.