Rqf

Presynaptic

Postsynaptic

Mitochondria _

Synaptic Cleft Synaptic Vesicles

FIGURE 1 Schematic diagram of two types of synaptic junctions. (A) Electrical synapse; transmission takes place through gap junctions that provide an ionic pathway between the pre- and postsynaptic junctions. (B) Chemical synapse; neurotransmitter is released from the presynaptic terminal, which diffuses across the synaptic cleft and interacts with receptor sites on the postsynaptic membrane.

Electrical junctions are found not only in the nervous system but also between other excitable membranes, such as smooth muscle and cardiac muscle cells. In these muscle cells, they provide an important pathway for the propagation of action potentials from one muscle cell to another.

For chemical synapses, there is a distinct cytoplasmic discontinuity that separates the presynaptic and post-synaptic membranes. This discontinuity is known as the synaptic cleft. The presynaptic terminal of chemical synapses contains a high concentration of mitochondria and synaptic vesicles, and there is a characteristic thickening of the postsynaptic membrane. As a result of a depolarization or an action potential in the presynaptic terminal, chemical transmitters are released from the presynaptic terminal, which diffuse across the synaptic cleft and bind to receptor sites on the postsynaptic membrane. This leads to a permeability change that produces the postsynaptic potential. For chemical synapses, there is a delay (usually, approximately 0.5-1 msec in duration) between the initiation of an action potential in the presynaptic terminal and a potential change in the postsynaptic cell. The synaptic delay is due to the time necessary for transmitter to be released, diffuse across the cleft, and bind with receptors on the postsynaptic membrane. Chemical synaptic transmission is generally unidirectional. A potential change in the presynaptic cell releases a transmitter that produces a postsynaptic potential, but a depolarization in the postsynaptic cell does not produce any effects in the presynaptic cell because no transmitter is released from the postsynaptic cell at the synaptic region. The most predominant type of synapse is the chemical synapse, and for that reason this chapter will focus on chemical synaptic transmission.

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