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Comparative Vertebrate Physiology Skeletal muscle Motor unit Neurons plus muscles they innervate Innervate 5 to 100 muscles End plate potential Depolarization at the neuromuscular junction Depolarization depends upon: Size Number of vesicles Muscle tension Active process of generating force Types Isotonic: change in joint angle Isometric: no change in joint angle. Tension without sliding Twitch Brief threshold stimulus Latent period (few msec.) Contraction period (10 - 100 msec.) Relaxation period (10 - 100 msec.) Summation Caused by stimuli in rapid succession Tetanus Refractory period is always honored Contraction delay Cross bridges attach before muscle tension is developed Elastic properties of muscle Twitch does not lead to full contraction Summation leads to full contraction Tension development Factors effecting: Number of fibers stimulated (large vs. small muscles) Frequency of AP’s Tetanus causes fatigue Neuromuscular fatigue Physiological fatigue Depletion of ATP Ach depletion (more likely) Versus psychological fatigue Tension development Factors effecting: Degree of muscle stretch Types of skeletal muscles Tonic fibers slow, isometric contraction (e.g. postural muscles) Phasic (twitch) fibers slow twitch (contract and fatigue slowly) (e.g. postural muscles) fast twitch oxidative (rapid, repetitive movement) (e.g. flight muscle) fast twitch glycolytic (few, rapid contractions) (e.g. breast of domestic fowl; amphibians, reptiles) Rigor mortis Muscle stiffness 3 - 4 hr < 12 hours > 48 to 60 hr Why? Ca++ influx no ATP = no detachment of myosin from actin protein degradation