Muscle Fiber Contraction and Relaxation

Muscle Fiber Contraction and Relaxation

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The sequence of events that result in the contraction of an individual muscle fiber begins with a signal—the neurotransmitter, ACh—from the motor neuron innervating that fiber.

The local membrane of the fiber will depolarize as positively charged sodium ions (Na+) enter, triggering an action potential that spreads to the rest of the membrane will depolarize, including the T-tubules.

This triggers the release of calcium ions (Ca++) from storage in the sarcoplasmic reticulum (SR).

The Ca++ then initiates contraction, which is sustained by ATP (Figure 1). As long as Ca++ ions remain in the sarcoplasm to bind to troponin.

Which keeps the actin-binding sites “unshielded,” and as long as ATP is available to drive the
cross-bridge cycling and the pulling of actin strands by myosin, the muscle fiber will continue to shorten to an anatomical limit.

 

 

 

Figure 1.

Contraction of a Muscle Fiber.

A cross-bridge forms between actin and the myosin heads triggering contraction.

As long as Ca++ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten.

Muscle contraction usually stops when signaling from the motor neuron ends, which repolarizes the sarcolemma and T-tubules.

Closes the voltage-gated calcium channels in the SR. Ca++ ions are then pumped back into the SR.
Which causes the tropomyosin to shield (or recover) the binding sites on the actin strands.

A muscle also can stop contracting when it runs out of ATP and becomes fatigued (Figure 2).

 

 

Figure 2.

Relaxation of a Muscle Fiber.

Ca++ ions are pumped back into the SR, which causes the tropomyosin to shield the binding sites on the actin strands.

A muscle may also stop contracting when it runs out of ATP and becomes fatigued.

Striated muscle is composed of multinucleated muscle fiber cells surrounded by an electrically excitable plasma membrane, the sarcolemma.

 

An individual muscle fiber cell, which may extend the entire length of the muscle, contains a bundle of many myofibrils arranged in parallel, embedded in intracellular fluid termed sarcoplasm.

Within this fluid is contained glycogen, the high-energy compounds ATP and phosphocreatine, and the enzymes of glycolysis.

 

 

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