Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity.

Skeletal muscles contract and relax to mechanically move the body.

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Messages from the nervous system cause these muscle contractions.

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(1) A message travels from the nervous system to the muscular system, triggering chemical reactions.

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(2) The chemical reactions lead to the muscle fibers reorganizing themselves in a way that shortens the muscle--that’s the contraction.

• Open channels allow an influx of sodium ions into the cytoplasm of the muscle fiber.

• The sodium influx also sends a message within the muscle fiber to trigger the release of stored calcium ions.

• The calcium ions diffuse into the muscle fiber.

• The relationship between the chains of proteins within the muscle cells changes, leading to the contraction.​

(3) When the nervous system signal is no longer present, the chemical process reverses, and the muscle fibers rearrange again and the muscle relaxes.

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​Label the parts

Thick Myosin Filament

Thin Actin Filament

Elastic Thin Filament

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I Band

A Band

Z Disc

H Zone

M Line​

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Sarcomere:

a structural unit of a myofibril in striated muscle​

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