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1H06 Anatomy & Physiology 1H06: Muscular System.docx

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Health Sciences
Alexander Ball

Muscles  Myology – study of muscles  Elongated cells called muscle fibres  Produce body movements, maintain posture, generate heat, provide protection  Classified as skeletal, cardiac, smooth  Make up 40-50% of total adult body weight Skeletal muscle tissue  Striated  Voluntary  Most controlled subconsciously to some extent (diaphragm) Cardiac muscle tissue  Striated  Involuntary  Autorhythmicity – built in heart rhythm Smooth muscle tissue  Walls of hollow internal structures (blood vessels, airways)  Non-striated  Usually involuntary, has autorhythmicity Functions 1. Producing body movements a. Walking, running, grasping pencil, keyboarding, nodding head 2. Stabilizing body position a. Stabilize joints and maintain body position (sitting, standing) b. Postural muscles contrat continuously when awake 3. Storing and moving substances within the body a. Storing accomplished by sustained contractions of ringlike bands of smooth muscle called sphincters, preventing outflow of contents of a hollow organ b. Smooth muscle contractions also move food and substances through gastrointestinal tract 4. Generating heat a. Thermogenesis – muscles produce heat as they contract Properties of Muscular Tissue 1. Electrical excitability a. Ability to respond to certain stimuli by producing electrical signals called action potentials b. Two types: electrical signals (heart’s pacemaker) and chemical stimuli (neurotransmitters released by neruons, hormones distributed by blood) 2. Contractility a. Ability of musclar tissue to contract forcefully when stimulated by action potential (lifting book off a table) 3. Extensibility a. Ability of muscular tissue to stretch, within limits, without being damaged b. Smooth muscle usually most stretchy 4. Elasticity a. Ability to muscular tissue to return its original length and shape after contraction or extension Connective Tissue Components  Surround and protect muscular tissue  Subcutaneous layer or hypodermis (separating muscle from skin) composed of areolar connective tissue and adipose tissue  Provides pathway for nerves, blood vessels, and lymphatic vessels to enter and exit muscles  Fascia – dense sheet or broad band of irregular connective tissue that lines body wall and limbs and supports and surrounds muscles and other organs of the body Layers of connective tissue  Epimysium – outermost layer of dense tissue  Perimysium – surrounds groups of 10 to 100 or more muscle fibers, separating them into bundles called fascicles  Endomysium – penetrates interior of each fascile and separates individual muscle fibers from one another  When the connective tissue elements extend as broad, flat sheet, it is an aponeurosis. Nerve and Blood Supply  Artery and one or two veins accompany each nerve penetrating skeletal muscle  Neurons that stimulate skeletal muscle to contract are somatic motor neurons  Each contraction, muscle fiber synthesizes and uses considerable ATP Skeletal Muscle Fiber  Typical length of mature skeletal muscle fiber is about 10cm  Myoblasts – fusion of hundred or more small mesodermal cells  Number of skeletal muscle fibers set before born, lasting a lifetime because once fusion occurs, muscle fiber loses ability to undergo cell division. Sarcolemma – plasma membrane of muscle cell, multiple nuclei of skeletal muscle fiber located beneath Transverse tubules – tiny invaginations of sarcolemma Sarcoplasm – cytoplasm of muscle fiber Myoglobin – red-colored protein in sarcoplasm, which binds oxygen molecules that diffuse into muscle fibers from interstitial fluid Myofibrils – little threads that make muscle appear striated Sarcoplasmic reticulum – fluid-filled system of membranous sacs Filaments and Sarcomere Thin filaments – 8nm in diameter, composed mostly of the protein actin Thick filaments – 16nm in diameter, composed mostly of protein myosin  Both thin and thick directly involved in contractile process, arranged in compartments called sarcomeres Z discs – separate one sarcomere from the next A band – darker middle part of sarcomere I band – lighter, less dense area that contains rest of thin filaments but no thick H zone – center of each A band contains thick and not thin filaments M line – supporting proteins form the line, in middle of sarcomere Muscle Proteins Myofibrils built from: 1. Contractile proteins (generate force during contraction) 2. Regulatory proteins (help switch contraction process on and off) 3. Structural proteins (keep the thick and thin filaments in proper alignment, give myofibril elasticity and extensibility, and link myofibrils to sarcolemma and extracellular matrix) Myosin – main component of thick filament, consists of tail and two myosin heads, bind to myosin binding sites on actin molecules
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