Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
The Muscular System © 2007 McGraw-Hill Higher Education. All rights reserved. Introduction © 2007 McGraw-Hill Higher Education. All rights reserved. Skeletal muscle anatomy • Muscle – Made up of fascicles (a bundle of muscle fibers) – Fiber (muscle cell or myocyte) • Made up of myofibrils – Contain sarcomeres (functional unit) » Myofilaments (actin and myosin) – Connective tissue • Epimysium – Surrounds entire muscle • Perimysium – Surrounds fascicles • Sarcolemma – Surrounds fibers • Satellite cells – Allow muscle to adapt to activity © 2007 McGraw-Hill Higher Education. All rights reserved. Skeletal muscle anatomy • Skeletal muscle cells are multi-nucleated – Unusual • Satellite cells – Myoblasts • Essential for growth of fibers • Chemotaxic – Can migrate around cell © 2007 McGraw-Hill Higher Education. All rights reserved. Myotendinous junction • Connection of muscle to tendon – Allow transmission of force to bones – Tendon • connects to bone – Myotendinous junction • Common spot for sports injuries © 2007 McGraw-Hill Higher Education. All rights reserved. Sarcomeres • Striated (striped) appearance • A band – Anisotropic • I band – Isotropic • Means having the same optical properties in all directions • H zone – Light • Z line – Separates sarcomeres © 2007 McGraw-Hill Higher Education. All rights reserved. Contractile and regulatory proteins • Muscle – ~75% water – 20% protein – 5% salts, pigments and substrates • Protein – 12% • myofibrillar – 8% • Enzymes, membrane proteins, transport channels © 2007 McGraw-Hill Higher Education. All rights reserved. Contractile proteins • Major myofibrillar proteins – Actin and myosin • Major contractile proteins • Myosin – ATPase – “head” attaches to actin – Ratchet like action shortens sarcomere © 2007 McGraw-Hill Higher Education. All rights reserved. – Troponin and tropomyosin • On actin • Ca2+ binds to troponin • Causes conformational change in tropomyosin – Titin and nebulin • Titin – Connects myosin to Z disc • Nebulin – Actin binding protein; may limit length – M-line proteins • Keep actin and myosin in correct spatial arrangement – Desmin • Links Z-disks together – α-actinin • Attaches actin filaments ar Z disc – Spectrin and dystrophin • Dystrophin involved in muscular dystrophy © 2007 McGraw-Hill Higher Education. All rights reserved. Contractile proteins • Thick filament – Hexameric protein • One pair of heavy chains (MHC) • Two pairs of light chains (LC) • Muscle often characterized by MHC – Type I – Type II » a, dx, b • Light chains – Modulate the contractile response © 2007 McGraw-Hill Higher Education. All rights reserved. Muscle organization • Myosin – Light meromyosin • Links all myosin molecules together – Heavy meromyosin • Hinge and head • Head interacts with actin when ATP is present – ATPase • Close to actin binding site in head region © 2007 McGraw-Hill Higher Education. All rights reserved. Muscle organization • Actin – Made up of G (globular) actin sub-units – (F) Filamentous actin • String of G actin – Tropomyosin (Tm) • Filament that lies in groove of actin molecule • Blocks myosin binding site – Troponin • On G actin • Ca2+ binding allows myosin and actin to interact © 2007 McGraw-Hill Higher Education. All rights reserved. Sliding filament theory • Huxley and Huxley 1954 • Determined that the following steps occur – Myosin and actin interact – ATP hydrolyzed – Cross bridge stroke – Detachment of myosin and actin © 2007 McGraw-Hill Higher Education. All rights reserved. Sliding filament theory • A) no interaction – ATP is hydrolyzed to “energize” myosin – Ca2+ release – Myosin attaches to actin • B) crossbridge movement – ADP and Pi released • C) ATP binds – Breaks bond betw actin and myosin • D) ATP hydrolyzed to “re-energize” myosin © 2007 McGraw-Hill Higher Education. All rights reserved. Capillaries and the microvascular unit • Capillarity – Several measures • No. per fiber – ~2-4 caps/fiber – Problems? • # Caps/fiber area – ~500-1000 caps/mm2 – Problems? • Cap length/fiber vol – Best because » Capillaries are not straight » Matches the length of the capillary to the fiber volume © 2007 McGraw-Hill Higher Education. All rights reserved. © 2007 McGraw-Hill Higher Education. All rights reserved. Microvascular unit • Notice how tortuous or “non-straight” the capillaries are in these corrosion casts © 2007 McGraw-Hill Higher Education. All rights reserved. Length-tension relationships • The force or tension that a muscle can exert is dependent upon it’s length – Actually, the sarcomere length – Sarcomer has an “optimal length” • This is where maximal actinmyosin overlap occurs © 2007 McGraw-Hill Higher Education. All rights reserved. Types of contraction • Isometric – Generates force, but no movement • Isotonic – Generates force and shortening occurs • Note: distance shortened is less as load increases • However, note how training changes the force-velocity relationship – Specifically, a trained muscle can • Move a given load faster • Move a greater load at the same speed compared to untrained © 2007 McGraw-Hill Higher Education. All rights reserved. Excitability • Muscles exhibit the property of excitability – They can be activated by a nervous impulse – They are polarized (i.e. charged) • Resting membrane potential – Inside negative relative to outside © 2007 McGraw-Hill Higher Education. All rights reserved. Resting membrane potential • Greater # of negatively charged particles (ions) inside cell – Most are proteins – Also Na+, K+ and Cl– Greater extracellular Na+ and intracellular K+ – Resting membrane potential is ~ -70 mV • Na+/K+ ion pump • Membrane is more permeable + than Na+ to K © 2007 McGraw-Hill Higher Education. All rights reserved. Action potential • When cell is stimulated – Stimulus is subthreshold if there is no response – If threshold is reached, action potential • Action potential – Rapid (1 ms) reversal of polarity © 2007 McGraw-Hill Higher Education. All rights reserved. Action potential • Note that each ion has it’s own equilibrium potential (where resting pot would be if only that ion existed) • Note how action potential must be strong enough to reach threshold • Also, Na+ influx is responsible for AP • K+ efflux is responsible for repolarization • Following AP, membrane is transiently more negative, hyperpolarization © 2007 McGraw-Hill Higher Education. All rights reserved. Neuroanatomy made ridiculously simple • Motor neuron – Cell body – Dendrites – Axon • Myelin sheaths –Insulation –Increases the rate of nerve transmission • Nodes of Ranvier • Motor end plate –ACh © 2007 McGraw-Hill Higher Education. All rights reserved. Neuromuscular junction © 2007 McGraw-Hill Higher Education. All rights reserved. NMJ • Connection between nervous system and muscular system – Branch from a motor neuron and the fiber it innervates – Small space (synapse) • Ach is released from motor terminal or Motor end plate –Ach is contained in vesicles –When action potential is reached »Ca2+ enters the Motor end plate »Causes Ach to be released © 2007 McGraw-Hill Higher Education. All rights reserved. AP transmission to muscle cell • AP then transmitted through the T-tubules and sarcoplasmic reticulum – AP travels through Tsystem – Calcium released from Terminal citernae of SR – This caused contraction – Excitation-contraction coupling © 2007 McGraw-Hill Higher Education. All rights reserved. Excitation contraction coupling • AP propagated across NMJ – Down T-tubules – Stimulates Ca2+ release from SR • Ryanodine receptor – Ca2+ release channel – Ca2+ binds to troponin – Causes shift in tropomyosin – Allows actin-myosin interaction – Contraction • Cessation of AP – Ca2+ resequestered © 2007 McGraw-Hill Higher Education. All rights reserved. Muscle fiber typing • Stain for ATPase – Different ATPases in • Slow twitch • Fast twitch – Incubate muscle samples at different pH • Alkaline pH (10.3) – Dark-fast twitch – Light- slow twitch • Acid pH (4.3) – Dark-slow twitch – Light-fast twitch • Succinate dehydrogenase – Krebs cycle enzyme » Darkest type I » Intermediate type IIa » Lightest type IIb © 2007 McGraw-Hill Higher Education. All rights reserved. Muscle fiber typing © 2007 McGraw-Hill Higher Education. All rights reserved. Classification schemes • How fast the respond to a stimulus – FT, ST • How fast they fatigue – Fatigue resistant, fatiguable • Oxidative capacity • Color • Function • ATPase activity © 2007 McGraw-Hill Higher Education. All rights reserved. Size principle • Recruitment of motor units is based upon – The size of the motor unit • Smaller motor units more easily recruited – Thus, they are recruited first • Larger motor units are recruited when work needs are (and thus neural output) higher © 2007 McGraw-Hill Higher Education. All rights reserved.