Comparative Vertebrate Physiology Skeletal muscle

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:

SizeNumber 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