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Chapter 11 Muscular Tissue ehow.com Objectives • Know the microanatomy of skeletal muscle and what a sarcomere is • Understand the relationship between the somatic nervous system and muscle cells • Understand how the sliding filament mechanism works • Know how to interpret a myogram • Know some characteristics of cardiac and smooth muscles Structure • Macro to micro – Epimysium – Fascicles • Wrapped by perimysium training.seer.cancer.gov – Muscle fibers (cells) • Wrapped by endomysium – Myofibrils • Contractile element faculty.etsu.edu Microanatomy of Myofibril • Dark band – A band • Light band – I band • H zone • Z disc – Sarcomere • Titin – Largest protein in body – Stabilizes thick fillaments – Gives muscle elasticity http://legacy.owensboro.kctcs.edu/gcaplan/anat/images/Image286.gif The Sarcomere • Functional unit • Microfilaments – Actin • Troponin • Tropomyosin scioly.org – Myosin • Heads contain two binding sites • Actin and ATPase people.eku.edu Somatic Nervous System • Motor neurons supply skeletal muscle • Single neuron with cell bodies in ventral horn of spinal cord • Release acetylcholine – Only excitatory Neuromuscular Junction • Junction of motor axon with a muscle cell – Only one junction per muscle cell • Motor end plate – Specialized area on muscle cell for synapse – High concentration of cholinergic receptors – Open nonspecific ion channels • Na+ rushes in producing an end-plate potential • Action potential on muscle cell – EPP opens ion channels in adjacent membrane – Threshold is reached and an action potential propagates away from the motor end plate adinstruments.com Acetylcholinesterase • Enzyme that inactivate Ach – Ensures that muscles only contract when needed • Please look at chemicals and disease associated with the neuromuscular junction Muscular Contraction • What needs to happen in order for a muscular contraction to begin? Sliding Filament Theory • Proposed by Hugh Huxley in 1954 • Mechanism by which sarcomeres contract • Involves formation of cross-bridges between actin and myosin • Thin filaments slide over thick filaments – Greater overlap between filaments Sliding Filament Mechanism • Calcium released from sarcoplasmic reticulum – Foot proteins stimulated by dihydropyridine receptors of T tubules • Calcium binds to troponin • Troponin–tropomysin complex shifts position • Myosin binding site exposed • Myosin heads form cross-bridges with actin • Actin filaments pulled toward center of sarcomere Calcium Initiates the Sliding Filament Mechanism (cont.) Figure 6.7 Role of ATP octc.kctcs.edu Sliding Filament Mechanism • http://youtu.be/WRxsOMenNQM Muscle Relaxation • Nerve activation ends, contraction ends • Calcium pumped back into sarcoplasmic reticulum – Ca2+ - ATPase pump • Calcium removed from troponin • Myosin-binding site covered • No calcium = no cross-bridges Energy Required for Muscle Activity • Principle source of energy: ATP • ATP replenished by variety of means: – Creatine phosphate – Stored glycogen – Aerobic metabolism of glucose, fatty acids, and other high-energy molecules Activity of Muscles Can Vary • Isotonic contractions: muscle tension remains constant as muscle changes length – Concentric – muscle shortens – Eccentric – muscle stretched, but still contracting • Isometric contractions: muscle prevented from shortening, tension develops at constant muscle length • Degree of nerve activation influences force • Terms to know: – Motor unit – Muscle tension – All-or-none principle Muscle Contraction: Myogram • Latent period • Contraction • Relaxation • Summation vs. tetanus Types of Muscle Fibers Slow-twitch • Slow oxidative (type I) – Red fibers • Low myosin ATPase activity Fast-twitch • Fast-oxidative (type IIa) – Red fibers – High myosin ATPase activity – Intermediate • Fast-glycolytic (typeIIb) – White fibers • Slow twitch: endurance, long duration contraction : – Jogging, swimming, biking • Fast twitch: strength, white muscle, short duration contraction: – Sprinting, weight lifting Features of Cardiac and Smooth Muscles • Activation of cardiac and smooth muscles: – Involuntary – Specialized adaptations in each • Cardiac pacemaker cells • Dense bodies • Calcium binds to calmodulin – Activates myosin light-chain kinase • Speed and sustainability of contractions • Arrangement of myosin and actin filaments
