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Anatomy Chapter 8.docx

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Biological Sciences
Ted Petit

Chapter 8 Muscle Physiology • Basics - 3 types of muscle 1. skeletal (striated, voluntary, multinucleate) a. movement of whole body or parts 2. cardiac (striated, involuntary, one nucleus) a. pump blood 3. smooth (unstriated, involuntary, one nucleus) a. movement of substances through hollow organs • Skeletal muscle - functional anatomy 1. whole muscle a. made up of long cells bundled with connective tissue b. connective tissue extends to form tendons attaching muscle to bone 2. muscle cell (fiber) a. contains myofibrils (specialized organelles) which are made up of protein filaments (myofilaments) b. thick filaments (1) made of myosin molecules, each of which has a head and tail end (2) heads form cross bridges, have an actin binding site and ATPase site c. thin filaments (1) made mostly of actin molecules, each of which has a myosin binding site (cross bridge binding site) (2) tropomyosin blocks binding sites when cell at rest (3) troponin holds tropomyosin in place, has Ca 2+binding site d. thick and thin filaments arranged into sarcomeres (functional units that contract) this arrangement results in striations e. plasma membrane also called sarcolemma (1) forms T tubules, which project into cell (continuous with surface membrane, allowing electrical activity at cell surface to be transmitted throughout cell) f. sarcoplasmic reticulum is modified smooth ER (1) network of tubules surrounding myofibrils (2) ends of each portion expand into sacs called terminal cisternae 2+ (a.k.a. lateral sacs, stores Ca ) - Contraction (excitation-contraction coupling and the sliding filament theory) 1. EPP at motor end plate results in AP a. spreads throughout cell surface and T tubules by local current flow 2. triggers release of Ca 2+ from terminal cisternae 2+ 3. Ca binds to troponin, changing its shape so that it moves tropomyosin out of the way of the binding sites 4. cross bridge attachment - myosin heads bind to actin 5. power stroke - myosin had uses energy of ATP to pivot and pull thin filaments toward the center of sarcomere 6. cross bridge detachment - a new ATP molecule binds to myosin head, cross bridge releases 7. "cocking" of the myosin head - ATP  ADP + P by ATiase, myosin head returns to original position (high energy conformation) 8. steps 3-7 repeat until full contraction reached and Ca 2+used up (called cross bridge cycling) a. once AP is over (1-2 msec) Ca 2+ no longer released 2+ 2+ b. Ca -ATPase pump transports Ca back into SR c. troponin and tropomyosin again cover binding sites d. muscle cell is relaxed 9. other important points a. during contraction, at any given time, only some cross bridges are attached - as they release others attach so thin filaments don't slide backward b. latent period is time between AP and contraction c. contraction and relaxation last about 100 msec d. contraction of an individual cell is an all-or-none response • Skeletal muscle mechanics - gradation of whole muscle tension 1. a single AP to a muscle fiber results in a weak contraction (twitch) - muscle cells work together to produce more force 2. recruiting more motor units  more tension (more force, stronger contraction) a. a motor unit is a motor neuron plus all the fibers it innervates (fibers spread throughout muscle) b. smaller motor units in muscles needing precise control (eyes, fingers), larger motor units in muscles designed for power (legs) c. asynchronous recruitment of motor units prevents fatigue - alternate motor units (e.g., postural muscles, holding a heavy object) 3. influencing tension in each fiber a. increased frequency of stimulation  increased tension (1) twitch summation occurs when fiber does not relax completely between APs, 2+ greater cross bridge cycling from prolonged availability of Ca (2) if there is no relaxation between APs, tetanus occurs (a smooth, sustained contraction of maximal strength) b. length-tension relationship (optimal length  increased tension) (1) maximal force possible at optimal length - myosin cross bridges have maximal access to actin binding sites c. less fatigue  increased tension d. thicker fibers  increased tension (1) more myofilaments in cell - types of contraction 1. isotonic a. tension constant, muscle changes length (1) concentric contraction - muscle shortens (lifting a load) (2) eccentric contraction - muscle lengthens (lowering load) 2. isometric a. tension develops, length stays the same (trying to lift too heavy a load, pushing against a wall) - muscles accomplish work (force x distance), but most of the energy muscles use (about 75%) converted to heat - lever systems 1. muscles provide force to move bones (levers) around joints (fulcrum) 2. depending on construction of system, allows a given effort to move a heavier load, or to move it farther and faster • Skeletal muscle metabolism - muscle cells have enough ATP reserves to last 4-6 seconds of strenuous activity - 3 ways to form ATP 1. creatine phosphate
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