Synaptic Transmission At The Neuromuscular Junction

The chemical synapse from which most of our information about synaptic transmission has been derived is the synaptic contact made by a spinal motor neuron with a skeletal muscle cell. Figure 2 is a schematic diagram of some of the general features of this synapse. The myelinated axon of a motor neuron whose cell body is located in the ventral horn of the spinal cord innervates a number of individual muscle fibers (Fig. 2A). At each muscle fiber, the axon branches further and forms a series of contacts with the muscle cell. An expanded view of one such synaptic contact, which illustrates some of the characteristic morphologic features of the chemical synapse, is shown in Fig. 2B. There are (1) a large concentration of small vesicles and

FIGURE 2 Sketch of the skeletal neuromuscular junction. (Modified from Lester HA. Sci Am 1977; 236:106; Hille B. Ionic channels of excitable membranes. Sunderland, MA: Sinauer Associates, 1984.)

mitochondria in the presynaptic terminal, and (2) a distinct synaptic cleft that separates the presynaptic terminal from the postsynaptic cell (in this case, the skeletal muscle cell). This particular synapse, because of its characteristic shape, is known as the motor end plate. It is also known as the neuromuscular junction because it is the junction made by a motor axon with a muscle cell. Additional morphologic features of the synapse at the neuromuscular junction are illustrated by the expanded view in Fig. 2C. Here, it is seen that: (1) the synaptic vesicles are clustered in distinct regions of the terminal known as the active zone; (2) the active zone is opposite an invagination of the muscle membrane known as the junctional fold; and (3) there is a characteristic thickening of the postsynaptic membrane, perhaps due to the high density of postsynaptic receptors for acetylcholine (ACh). In addition, the synaptic cleft is filled with a fibrous matrix and the enzyme acetylcholinesterase (AChE). The functions of ACh and AChE are described below. Figure 3 is an electron micrograph of the synaptic junction illustrating the high concentration of vesicles and mitochondria in the presynaptic terminal, the fibrous matrix in the synaptic cleft, and the characteristic thickening of the postjunctional membrane.

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