The Neuromuscular Junction

A signal to move a muscle travels from the central nervous system along a motor neuron in the form of a depolarization wave until it reaches a neuromuscular junction in the periphery. There, the motor neuron synapses with the targeted muscle cell, forming the neuromuscular junction; that is, a joining of a nervous cell and a muscle cell. The neuromuscluar junction has been studied extensively, and is the most understood type of synapse in the human nervous system. The electrical impulse, or action potential, moving along the motor neuron carries the depolarization wave to terminal buttons of the presynaptic neuron, which depolarize and release acetylcholine.

Acetylcholine (shown to the left) is a common neurotransmitter, and typically has an excitatory affect in the peripheral nervous system. The released acetylcholine binds to nicotinic acetylcholine receptors, found along the postsynaptic skeletal muscle cell membrane. When acetylcholine binds to an acetylcholine receptor it causes associated ion channels to open, allowing sodium ions to flow through the channel into the muscle cell. The influx of sodium locally depolarizes the postsynaptic muscle cell membrane, which triggers the release of calcium ions from the sarcoplastic reticulum within the cell. These calcium ions then cause cell wide depolarizations, and initiate the muscle contraction.

For infomation on the effect that myasthenia gravis has on neuromuscular junctions see Myasthenia gravis and acetylcholine.

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