Figure 116

Schematic diagram of different types of synapses, a. Axodendritic or axosomatic. b. Axodendritic, in which an axon terminal synapses with a dendritic spine, c. Axoaxonic. The axoaxonic

synaptic cleft

synaptic vesicles postsynaptic membrane

presynaptic membrane

synaptic vesicles postsynaptic membrane a axon ending (presynaptic knob)

synaptic cleft

• Electrical synapses, which are common in invertebrates, 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 knob, synaptic cleft, and postsynaptic membrane

Components of a typical chemical synapse include

• Presynaptic knob (presynaptic component), the end of the neuron process from which neurotransmitters are released. The presynaptic component, i.e., the bouton terminal, is characterized by the presence of synaptic vesicles, membrane-limited structures that range from 30 to 100 nra in diameter and contain neurotransmitters (Fig. 11.8). A specific ATP-binding protein called N-etbyl-maleimide-sensitive factor (NSF) found on the membrane of synaptic vesicles is required for formation, targeting, and fusion of these vesicles with the presynaptic membrane. Numerous small mitochondria and a layer of dense material, the presynaptic density, are present on the cytoplasmic side of the plasma membrane.

• Synaptic cleft, the 20- to 30-nm space that separates the presynaptic neuron from the postsynaptic neuron or target cell, which the neurotransmitter must cross.

• Postsynaptic membrane (postsynaptic component), which contains receptor sites with which the neurotransmitter interacts. This component is formed from a portion of the plasma membrane of the postsynaptic neuron (Fig. 11.9) and is characterized by a layer of dense material, the postsynaptic density, on the cytoplasmic side of the membrane.

axon synaptic vesicle

0 0

Post a comment