Mechanism of Rapid Synaptic Transmission

Arrival of an action potential at an axon terminal causes voltage-gated ion channels to open, thereby allowing Ca2+ ions to enter the terminal. Within 100 msec, their presence triggers quantal release of neurotransmitter into the synaptic cleft by exocytosis

This Organelle Cell Synapse

Figure 29 Chemical synapse: an axon terminal typically shows a few neurofilaments, many mitochondria, and numerous synaptic vesicles. These spherical organelles, unique to neurons, are found attached to the actin cytoskeleton in relatively large numbers or in smaller numbers released to positions of readiness or docked for immediate use at the presynaptic membrane. They release their contents into the synaptic cleft (12-20 nm wide) by coalescing with the plasma membrane upon the arrival of nerve impulses in the axon terminal. Following release, transmitter diffuses across the cleft and binds to specific receptors in the postsynaptic membrane. These receptors undergo immediate conformational change that leads to the opening of channels so as to alter the permeability of the membrane to certain ions, thus changing the membrane potential. After release, presynaptic membrane is taken up by endocytosis and synaptic vesicles are reloaded with locally synthesized transmitter. From J. B. Angevine, Jr., Dendrites, axons, and synapses, BNI Quarterly, Vol. 4, No. 2, 1988 (illustration by Steven J. Harrison).

Figure 29 Chemical synapse: an axon terminal typically shows a few neurofilaments, many mitochondria, and numerous synaptic vesicles. These spherical organelles, unique to neurons, are found attached to the actin cytoskeleton in relatively large numbers or in smaller numbers released to positions of readiness or docked for immediate use at the presynaptic membrane. They release their contents into the synaptic cleft (12-20 nm wide) by coalescing with the plasma membrane upon the arrival of nerve impulses in the axon terminal. Following release, transmitter diffuses across the cleft and binds to specific receptors in the postsynaptic membrane. These receptors undergo immediate conformational change that leads to the opening of channels so as to alter the permeability of the membrane to certain ions, thus changing the membrane potential. After release, presynaptic membrane is taken up by endocytosis and synaptic vesicles are reloaded with locally synthesized transmitter. From J. B. Angevine, Jr., Dendrites, axons, and synapses, BNI Quarterly, Vol. 4, No. 2, 1988 (illustration by Steven J. Harrison).

from synaptic vesicles docked in the active zone of the synapse. This exocytosis is far more rapid than anywhere else. It is virtually instantaneous, probably because a subset of vesicles is already docked at the active zone (where the presynaptic membrane appears thickened) and evanescent fusion pores form between the vesicles and the presynaptic membrane. The transmitter diffuses directly across the synaptic cleft and binds to specific receptors in the postsynaptic membrane. The receptors undergo conformational changes that open ion channels, resulting in depolarization and excitation or hyperpolarization and inhibition of the target cell. By this time, all of 200 msec has elapsed.

Within the axon terminal are two pools of synaptic vesicles: a relatively small releasable pool from which vesicles fuse with the presynaptic membrane when the action potential arrives and a larger reserve pool in which vesicles are bound to the actin cytoskeleton and mobilized as required. After transmitter release, membrane is taken up by endocytosis and vesicles are reloaded with transmitter synthesized locally from precursors transported to the terminal by kinesin-mediated axonal transport. As for the transmitter just released, its molecules must be removed quickly after receptor binding to prepare the postsynaptic membrane for new releases. Removal may be performed in one or more ways, acting in concert or in combinations

Figure 30 Synaptic variety: until the late 1950s, chemical synapses of axodendritic, axosomatic, and (later) axoaxonic types dominated the field of neurobiology. These familiar synapses were considered ''conventional.'' But by 1972, many ''unconventional'' junctions had been recognized. That year, David Bodian reviewed these ''unconventional'' coupling paradigms. He included the intriguing glomeruli: many of these are problematic, but some exemplify, in a reductionist manner, Sherrington's key principles of divergence and convergence in the organization of brain circuitry. The desmosomal type junction shown is a punctum adherens (plural: puncta adherentia), a small region of contact and adhesion between neurons corresponding to the zonula adherens found on the lateral surfaces of cells in various nonneural epithelia (e.g., simple columnar epithelium of the small intestine). See also text. From Neuron junctions: A revolutionary decade, D. Bodian, The Anatomical Record, copyright 1972. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. (illustration by Elinor W. Bodian).

Figure 30 Synaptic variety: until the late 1950s, chemical synapses of axodendritic, axosomatic, and (later) axoaxonic types dominated the field of neurobiology. These familiar synapses were considered ''conventional.'' But by 1972, many ''unconventional'' junctions had been recognized. That year, David Bodian reviewed these ''unconventional'' coupling paradigms. He included the intriguing glomeruli: many of these are problematic, but some exemplify, in a reductionist manner, Sherrington's key principles of divergence and convergence in the organization of brain circuitry. The desmosomal type junction shown is a punctum adherens (plural: puncta adherentia), a small region of contact and adhesion between neurons corresponding to the zonula adherens found on the lateral surfaces of cells in various nonneural epithelia (e.g., simple columnar epithelium of the small intestine). See also text. From Neuron junctions: A revolutionary decade, D. Bodian, The Anatomical Record, copyright 1972. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. (illustration by Elinor W. Bodian).

peculiar to a given synapse: enzymatic inactivation, reuptake by the axon terminal, uptake by a nearby astrocytic process, uptake by the postsynaptic region of the target neuron, and simple diffusion away from the synaptic cleft into the extracellular space, which although narrow in the CNS (2-4 nm) would allow some movement of transmitter molecules away from the cleft. It is too slow, however, to be an effective mechanism by itself.

E. Overview of Synaptic Transmission

Synapses vary in their mode of action (electrical or chemical), form and ultrastructure, number and specified laminar organization on postsynaptic neurons, types and rapidly increasing numbers of chemical messengers (now including gases and growth hormones), functional effects, and affiliations with the neuroglia.

We have noted puncta adherentia: sticky spots that hold neurons together and make fast their synapses. They are widely distributed in the nervous system, prominent between astrocytes, and evident everywhere: between dendrites, cell bodies, dendrites, and axons, dendrites and cell bodies, axon terminals and axon initial segments (at axoaxonic synapses), axon membranes and their sheaths, and neuronal processes and astrocytes. The larger zonula adherens is seen between the lateral surfaces of ependymal cells, cells of the choroid plexus, and the subventricular tanycytes. Although these surface specializations play purely adhesive roles, they are often seen close beside synapses, chemical or electrical, and at nonspecific sites. Some see the active zone of a synapse, the site of transmitter release, as a modified zonula or punctum adherens. Unlike these symmetrical adhesive junctions, a chemical synapse has become structurally and functionally asymmetrical and, hence, polarized. If this is true, the site of transmission and permanence of place of neuronal communication evolved from simple adhesive junctions found universally in epithelial tissues.

Three key points on neuroactive substances: First, multiple transmitters and the corelease of transmitters seem now to be the rule for neurons. Second, a given transmitter may elicit different functional effects in different situations. Third, the effect of a neuroactive substance on its target cell depends exclusively on the postsynaptic receptors to which it binds.

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