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Muscle Physiology Chapter 11 Connective Tissue Components • Muscle cell = muscle fiber • Endomysium – covers muscle fiber • Perimysium – binds groups of muscle fibers (fasicles) • Epimysium – covers the entire muscle • Tendon – fibrous tissue that connects muscle to bone • Aponeurosis – broad, flat sheet of connective tissue • Fascia – fibrous CT surrounding muscle and tendon General Function of Muscles • Movement – Excitability – Contractility – extensibility • Heat Production • Posture Overview of Muscle Cell • • • • Muscle cell = muscle fiber Sarcolemma = plasma membrane Sarcoplasm = cytoplasm Sarcoplasmic reticulum (SR) = network of tubules and sacs • Multi-nucleated, multiple mitochondrion • Bundles of myofibrils extend lengthwise & fill sarcoplasm – Composed of thick and thin myofilaments Sarcomere • Contractile unit of a muscle fiber • each myofibril consists of many sarcomeres • Z line – Anchors thin filaments – Boundary of sarcomere • • • • M line – anchors thick filaments A band: segment of thick & thin filaments I band: segment of thin filaments H zone: where thin and thick filaments will not overlap (only thick) Sarcomere cont… • Elastic filaments – connect thick filaments to Z line • T (transverse) tubules – allows impulses traveling along sarcolemma to move deeper within the cell • Triad – t tubule sandwiched between sacs of the SR – Allows impulses traveling along a t tubule to stimulate sacs of the SR Myofilaments • Myofibrils – made up of 1000s of thin and thick myofilaments • Thin filaments – Actin – Tropomyosin – Troponin • Thick filaments – myosin Muscle Excitation • Nerve impulse reaches the end of a motor neuron releases acetylcholine (Ach) • Ach diffuses across the neuromuscular junction and binds with the receptors on the motor endplate Muscle Contraction • Impulses travel along the sarcolemma t tubules sacs of SR • Ca2+ is released into the sarcoplasm binds with troponin on thin myofilaments • Tropomyosin shift to expose actin’s active site • Energized myosin heads bind with actin’s active site and pulls thin filament towards center of sarcomere – Requires ATP Muscle Relaxation • Nerve impulse is complete Ca2+ is pumped back into the sacs of the SR • Ca2+ is stripped from the troponin tropomyosin covers the actin’s active site • Myosin heads can no longer bind with actin muscle fiber returns to its resting length http://www.sci.sdsu.edu/movies/actin_myosin_gif.html Rigor Mortis • “stiffness of death” • SR releases excess Ca2+ myosin heads bind with actin’s active sites contraction of myofilaments • Lack of ATP after death causes cross bridges to “stick” http://highered.mcgrawhill.com/sites/0072507470/student_view0/chapter9/ Sliding Filament Theory In fully contracted muscle: • H zone disappears • I band narrows • A band remains the same Energy for Contractions • Hydrolysis (breakdown) of ATP – ATP ADP (breaking high energy bond btwn 2nd and 3rd phosphate groups) • ATP binds myosin head moves to resting position (11-7A) • Breakdown of ATP allow myosin head to bind with actin and perform “power stroke” (11-7B-D) • ATP binds to return myosin head back to resting position Alternate Source of Energy • ATP must be continually resynthesized • Breakdown of creatinephosphate (CP) provides energy for ATP resynthesis • Catabolism of food provides energy for ATP and CP synthesis Oxygen & Glucose • O2 and glucose are the starting materials for cellular respiration (process that makes ATP) • During rest oxygen is stored in myoglobin – – – Supplies muscle fibers with oxygen during period of exercise High amounts of myoglobin = red fibers = slow twitch fibers Low levels of myoglobin = white fibers = fast twitch fibers Aerobic vs. Anaerobic Respiration • Aerobic Respiration – Oxygen-requiring process – Produces maximum amount of ATP from one glucose molecule • Anaerobic Respiration – Does not require oxygen – Short-term, rapid process to re-synthesize ATP – Produces lactic acid – Burning/soreness in muscles Heat Production • Some energy from catabolic processes is lost as heat • Muscle release massive amts of heat • Thermoreceptors sense decrease in body temp hypothalamus integrates information signal sent to skeletal muscle to contract shivering homeostatic balance is maintained Motor Unit • Motor unit = motor neuron + muscle fibers it attaches to • Motor neurons can innervate few to 100s of muscle fibers • A lower number of muscle fibers within a motor unit = more precise movement – Ex: hand vs abdomen http://natchem.files.wordpress.com/2009/11/motor-unit-lg.jpg Isotonic vs Isometric Contractions • Isotonic – tension remains the same; length of the muscle changes – Concentric contraction: muscle shortens (contracts) – Eccentric contraction : muscle lengthens • Isometric – tension changes; length of the muscle remains the same – Myosin heads unable to move thin filaments – Static tension Smooth Muscle Contractions • Small tapered cell w/ single nuclei • No t-tubules; loosely organized SR • No sarcomeres – Contract to shorter lengths – Myofilaments crisscross (balled up appearance when contracted) • Calcium binds to calmodulin http://www.cytochemistry.net/microanatomy/muscle/smooth1.jpg Smooth Muscle Tissue Types 1. Visceral – Gap junctions connect smooth muscle fibers into sheets – Forms inner muscular layer of hollow structures – Exhibits autorhythmicity • Peristalsis, excretion of urine, childbirth, mixing of stomach contents 2. Multiunit – Composed of many single-cell units – Ex: arrector pili muscles, lines blood vessels