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Department of Physiology

The Neuromuscular Junction is not a Typical Synapse

Because the neuromuscular junction is better understood than any other synapse, it is used as a model for synaptic transmission generally. The essential features of the release and action of transmitter seen at the neuromuscular junction apply to other synapses, but there are a number of ways in which the NMJ is atypical.
Single pre-synaptic cell
In an adult skeletal muscle, each muscle cell is innervated by one and only one motor neuron. In the central nervous system, most neurons are innervated by large numbers of neurons, some of which are excitatory and others inhibitory. This allows for integration of information, whereas the neuromuscular junction is a relatively straightforward relay.
No overlap of post-synaptic cells
In an adult skeletal muscle, each neuron has exclusive control over the muscle cells it innervates. In the central nervous system, the post-synaptic cells of most neurons are also innervated by other neurons which may augment or compete with their action.
The synaptic potential
The synaptic potential, specifically known as the End Plate Potential (EPP) of the neuromuscular junction is normally large enough to produce a post-synaptic action potential. At most synapses in the CNS, a single synaptic potential is subthreshold.
No inhibitory transmission
Synaptic transmission at the neuromuscular junction is only excitatory. Many synapses in the CNS are inhibitory. The innervation of many other effector types, by the autonomic nervous system, involves both excitatory and inhibitory transmission.
Single post-synaptic receptor type
The neuromuscular junction has only acetylcholine receptors. Most neurons in the CNS and most autonomic effectors have at least two receptor populations, one of which will subserve excitation and the other inhibition.

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©D.F. Davey, Department of Physiology, University of Sydney
Last updated 10 April 2002