MUSCLE PHYSIOLOGY

Structure and actions of skeletal muscles

· Skeletal muscles- attached by tendons to bones

· Muscle in vitro- twitch, summation, and tetanus

· Muscle in vivo- stimulated by somatic motor neurons

Mechanisms of contraction

· Muscle cells (fibers)- myofibrils

Dark A band

Light I band

Z - in the middle of each I band

· Myosin cross bridges- extend out from thick to thin filaments

· Activity of the cross bridges causes the filaments to slide towards the centers of the sarcomeres

· Muscle at rest- Ca++ concentration in sarcoplasm- low

Cross bridges- prevented from attaching to actin

· Action potentials- conducted by transverse tubules into the muscle fibers

Neural control of skeletal muscles

· Lower motor neurons (LMN)- somatic motor neurons that innervate the muscle

1. Alpha motorneurons- innervate the ordinary, extrafusal muscle fibers

2. Gamma motorneurons- innervate the intrafusal fibers of the

muscle spindles- length detectors in the muscle-

Monosynaptic reflex- see text

3. Golgi tendon organs- monitor the tension that the muscle exerts on its tendon.

As tension increases, sensory neurons from

Golgi tendon organs inhibit the activity of

alpha motorneurons- disynaptic reflex

Upper motor neurons

Neurons in the brain that affect the lower motor neuron

Motor cortex-

Energy requirements of skeletal muscle

· Aerobic respiration- production of ATP

· ADP + P (from phosphocreatine) = ATP

· Three types of muscle fibers-

1. Slow twitch red fibers- adapted for aerobic respiration and resistant to fatigue

2. Fast twitch white fibers- anaerobic respiration

3. Intermediate- fast twitch but adapted for aerobic respiration

· All muscle fibers of a given motor neuron- same type.

· Muscle fiber type- determined by the motor neuron

that innervates it

· Smaller motor neurons- easily depolarized, activated

slow twitch muscle fibers ; stimulate more frequently than larger motor neurons( fast twitch muscle fibers)

· Physical training- affects the characteristics of muscle fibers

Endurance training- increases aerobic capacity

Hypertrophy- increase in the size and number of myofibrils

· Isometric vs isotonic

1. Isotonic- contraction that results in muscle shortening

Force of contraction- remains constant throughout

shortening process

2. Isometric- motor units stimulated, contraction strength may not be strong enough to shorten muscle.

Cardiac and smooth muscles

Smooth muscles- contain myosin and actin, no sarcomere

· Myosin - long; can contract even when greatly stretched

· During contraction- Ca++ enters smooth muscle cell,

combines with calmodulin; this activates an enzyme

that phosphorylates myosin bridges

· Can be classify as single unit or multiunit.

Cardiac muscles-

· Striated

· Gap junctions- intercalated discs

· Potentials originated in myocardial cells, stimulation

by neuron is not required

· Unlike skeletal muscles, action potentials can cross

from one nyocardial cell to another