Muscle Contraction

Neuromuscular pathway from the spinal cord to the muscle fibers via motor neurons.

Muscle contraction is a process that begins at the neuromuscular junction. A motor neuron releases acetylcholine into the synaptic cleft, which then binds to the cell membrane of the muscle cell. 

This triggers a change of the membrane potential of the muscle cell, which depolarizes. Depolarization quickly leads to generating an action potential, which opens the cellular depots of calcium ions from the sarcoplasmic reticulum. 

Calcium is important for inducing the interaction between the actin and myosin filaments in the muscle cell. This interaction is sustained by the molecule of adenosine triphosphate (ATP), which is the main source of energy in the muscle cell. 

As long as there is enough calcium and ATP, the interaction of actin and myosin continues, hence, the shortening (contractions) of the muscle fibers continues.

Muscle contraction usually ends when the neuron stops releasing acetylcholine into the synaptic cleft. Alternatively, if the neuronal signal continues, the contraction will stop when the energy is spent.

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Action potential transported through the axons down to the muscle plate causing muscular contraction.
Skeletal muscle contraction physiology

In skeletal muscle fibers, electrochemical activity triggers myofilament movement. These linked events are referred to as excitation-contraction coupling.
Action potential transported through the axons down to the muscle plate causing muscular contraction
Excitation-Contraction Coupling

As the action potentials travel down the T tubules, calcium channels in the nearby terminal cisternae of SR open, and Ca2+ ions diffuse into the surrounding sarcoplasm. The Ca2+ ions bind to troponin proteins embedded along the thin filaments and the contraction process begins.