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How a muscle contracts and the sliding filament theory.

The Neuromuscular Junction

When we think to move a muscle, a nerve impulse travels from the brain down our nerves and reaches the axon terminal of a motor neuron (see above). This stimulates the release of the neurotransmitter ACh (acetylcholine). The ACh attaches to receptors on the sarcolemma. This causes the sarcolemma to allow positively charged sodium ions to pass through the membrane, causing it to become positively charged and causing an action potential. The action potential travels along the sarcolemma like fire rushing along a dry twig.  This releases calcium, in the form of calcium ions (Ca+2).

Definition: The junction between the nerve fibre (AKA axon) terminal and sarcolemma is called the neuromuscular junction.

Action Potential: An electric signal that travels along a nerve or muscle cell membrane (sarcolemma) depolarizing and repolarizing it, to initiate something, eg a muscle contraction.




Sliding Filament Theory

The contraction of a muscle cell occurs at the thin filaments slide pass the thick filaments. During contraction, the sarcomere shortens and the thin and thick filaments overlap to a greater degree.

Terminal cisternae: Sac like regions of the sarcoplasmic reticulum containing calcium ions.
Binding site: A receptor on a protein to which another molecule attaches.

Definition: Cytosol is the intracellular fluid in which the organelles are suspended and molecules and ions are dissolved.

See also:

For further reading, see the following recommended texts:

Essentials of Anatomy and Physiology, Elaine Marieb, 8th Edition
Anatomy and Physiology, R. Seeley et al