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Encyclopedia > Neuromuscular junction

A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface.


Mechanism of Action

Upon the arrival of an action potential at the axon terminal, calcium ions flow from the extracellular fluid into the motoneuron's cytosol and bind to particular intracellular proteins. These calcium-bound proteins cause neurotransmitter-containing vesicles to attach to the motoneuron's cell membrane and thus release acetylcholine into the synaptic cleft. Acetylcholine then binds to the nicotinic acetycholine receptors that dot the motor end plate. The receptors are ion channels, and when bound by acetylcholine, they open, allowing sodium and potassium ions to flow in and out of the muscle's cytosol, respectively. Because of the differences in electrochemical gradients across the plasma membrane, more sodium moves in than potassium out, producing a local depolarization of the motor end plate known as an end plate potential (EPP). This depolarization spreads across the surface of the muscle fiber into transverse tubules, eliciting the release of calcium from the sarcoplasmic reticulum, thus initiating muscle contraction.


  Results from FactBites:
 
Precision of Reinnervation and Synaptic Remodeling Observed in Neuromuscular Junctions of Living Frogs -- Astrow et al. ... (9308 words)
We concluded that the structure of reinnervated neuromuscular junctions was not altered by in vivo observation.
In many junctions such as this, the terminal formed stable synaptic contacts on patches of sarcolemma that were not previously seen to be innervated (distal tip of lower branch on the left) while abandoning former synaptic gutters in the vicinity (lower branch on the right).
Junctions that were most distant from the point where the sartorius nerve entered seemed to form a separate group in which regeneration more faithfully reproduced the pattern of the former terminal.
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