Key Points

• Actin and myosin, arranged in thin and thick filaments respectively, are the main contractile proteins in muscle.

• Myosin crossbridges interact with actin during contraction to effect sliding of thick and thin filaments over one another, thus effecting force development and muscle cell shortening.

• Contractions are initiated by increases in calcium that occur in response to action potentials.

• Skeletal muscle action potentials are initiated by sudden increases in sodium permeability of the sarcolemma.

• Cardiac muscle action potentials are initiated by sudden increases in sodium permeability followed by slower and longer lasting increases in calcium permeability of the sarcolemma.

• The force of muscle contraction in vitro is a function of muscle length before stimulation. The velocity of shortening is a function of the after-load against which the muscle is contracting.

• The force and velocity of contraction of skeletal muscle in vivo are regulated by recruitment of muscle fibers and by alterations in the frequency of stimulation of those fibers.

• The force and velocity of cardiac muscle contraction in vivo depend on length; in addition, force and velocity at any given length and afterload can vary due to changes in contractility.

• ATP is used to fuel crossbridge cycling, calcium re-uptake, and the maintenance of ionic gradients across the sarcolemma. The rate of ATP use is highest in fast-twitch skeletal muscle, lower in slow-twitch skeletal and cardiac muscle, and lowest in smooth muscle.

• ATP is generated by both glycolysis (in most fast-twitch muscle cells) and oxidative phosphorylation (in most slow-twitch and cardiac muscle cells).

Essential Medical Physiology, Third Edition

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