Despite the fact that there are billions of synapses, they share many common features. The synapse for which most is known is the one formed between a spinal motor neuron and a skeletal muscle cell. Historically, it has been studied extensively because it is relatively easy to analyze. But the basic properties of synaptic transmission at the skeletal neuromuscular junction are very similar to the process of synaptic transmission in the central nervous system. Consequently, an understanding of this synapse leads to an understanding of the others. Therefore, we will first discuss the process of synaptic transmission at the skeletal neuromuscular junction. The features of the synaptic junction at the neuromuscular junction are shown in the figure at left. Skeletal muscle fibers are innervated by motor neurons whose cell bodies are located in the ventral horn of the spinal cord. The terminal region of the axon gives rise to very fine processes that run along skeletal muscle cells. Along these processes are specialized structures known as synapses. The particular synapse made between a spinal motor neuron and skeletal muscle cell is called the motor endplate, because of its specific structure. The synapse at the neuromuscular junction has three characteristic features of chemical synapses in the nervous system. First, there is a distinct separation between the presynaptic and the postsynaptic membrane. The space between the two is known as the synaptic cleft. The space tells us there must be some intermediary signaling mechanism between the presynaptic neuron and the postsynaptic neuron in order to have information flow across the synaptic cleft. Second, there is a characteristic high density of small spherical vesicles. These synaptic vesicles contain neurotransmitter substances. Synapses are also associated with a high density of mitochondria. Third, in most cases, there is a characteristic thickening of the postsynaptic membrane, which is due, at least in part, to the fact that the postsynaptic membrane has a high density of specialized receptors that bind the chemical transmitter substances released from the presynaptic neuron. |
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