The neuromuscular junction

Each motor axon branches so as to supply an appreciable number of muscle fibres. Fig. 7.1 shows the arrangement in most of the muscle fibres in the frog. Each axon branch loses its myelin sheath where it contacts the muscle cell and splits up into a number of fine terminals which run for a short distance along its surface. The region of the muscle fibre with which the terminals make contact is known as the end-plate. Structures and events occurring in the axon are called presynaptic whereas those occurring in the muscle cell are called post-synaptic.

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Fig. 7.1. Diagrammatic picture of a vertebrate muscle motor nerve terminal. In most cases a single motor axon innervates many more muscle fibres than the three shown here.

Further details of the structure in the junctional region can be determined by electron microscopy of thin sections, with the results shown in Fig. 7.2. The nerve cell is separated from the muscle cell by a gap about 50 nm wide between the two apposing cell membranes. This gap is called the synaptic cleft and is in contact at its edges with the other extracellular spaces of the body. The axon terminal contains a large number of small membrane-bound spheres, the synaptic vesicles, and also a considerable number of mitochondria. We shall see that the synaptic vesicles and the synaptic cleft play a vital role in the transmission process at this and other synapses.

The muscle cell membrane immediately under the axon terminal is thrown into a series of folds. The outside of the axon terminal is covered with a Schwann cell and the whole is held in position by some collagen fibres.

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