 Electrical synapses: Common in invertebrates, these synapses contain gap junctions that permit movement of ions between cells and consequently permit the direct spread of electrical current from one cell to another. These synapses do not require neurotransmitters for their function. Mammalian equivalents of electrical synapses include gap junctions in smooth muscle and cardiac muscle cells.
A typical chemical synapse contains a presynaptic element, synaptic cleft, and postsynaptic membrane.
Components of a typical chemical synapse include the following.
 A presynaptic element (presynaptic knob, presynaptic component, or synaptic bouton) is the end of the neuron process from which neurotransmitters are released. The presynaptic element is characterized by the presence of synaptic vesicles, membrane-bound structures that range from 30 to 100 nm in diameter and contain neurotransmitters (Fig. 12.7). The binding and fusion of synaptic vesicles to the presynaptic plasma membrane is mediated by a family of transmembrane proteins called SNAREs (which stands for “soluble NSF attachment receptors”), (see page 34). The specific SNARE proteins involved in this activity are known as v-SNARE (vesicle–bound) proteins and t-SNARE (targetmembrane–bound found in specialized areas of the presynaptic membrane). Another vesicle-bound protein called synaptotagmin 1 then replaces the SNARE complex, which is subsequently dismantled and recycled by
FIGURE 12.7 • Diagram of a chemical axodendritic synapse. This diagram illustrates three components of a typical synapse. The presynaptic knob is located at the distal end of the axon from which neurotransmitters are released. The presynaptic element of the axon is characterized by the presence of numerous neurotransmitter-containing synaptic vesicles. The plasma membrane of the presynaptic knob is recycled by the formation of clathrin-coated endocytotic vesicles. The synaptic cleft separates the presynaptic knob of the axon from the postsynaptic membrane of the dendrite. The postsynaptic membrane of the dendrite is frequently characterized by a postsynaptic density and contains receptors with an affinity for the neurotransmitters. Note two types of receptors: Green-colored molecules represent transmitter-gated channels, and the purple-colored structure represents a G-protein–coupled receptor that, when bound to a neurotransmitter, may act on G-protein–gated ion channels or on enzymes producing a second messenger. a. Diagram showing the current view of neurotransmitter release from a presynaptic knob by a fusion of the synaptic vesicles with presynaptic membrane. b. Diagram showing a new proposed model of the neurotransmitter release via porocytosis. In this model, the synaptic vesicle is anchored and juxtaposed to calcium-selective channels in the presynaptic membrane. In the presence of Ca2, the bilayers of the vesicle and presynaptic membranes are reorganized to create a 1-nm transient pore connecting the lumen of the vesicle, with the synaptic cleft allowing the release of a neurotransmitter. Note the presence of the SNARE complex and the synaptotagmin that anchor the vesicle to the active zones within plasma membrane of the presynaptic element.
presynaptic element of axon postsynaptic membrane of dendrite