Overview

The human brain contains at least 100 billion neurons, each with the ability to influence many other cells. Clearly, highly sophisticated and efficient mechanisms are needed to enable communication among this astronomical number of elements. Such communication is made possible by synapses, the functional contacts between neurons. Although there are many kinds of synapses within the brain, they can be divided into two general classes: electrical synapses and chemical synapses. Electrical synapses permit direct, passive flow of electrical current from one neuron to another. The current flows through gap junctions, which are specialized membrane channels that connect the two cells. In contrast, chemical synapses enable cell-to-cell communication via the secretion of neurotransmitters; the chemical agents released by the presynaptic neurons produce secondary current flow in postsynaptic neurons by activating specific receptor molecules. The secretion of neurotransmitters is triggered by the influx of Ca²⁺ through voltage-gated channels, which gives rise to a transient increase in Ca²⁺ concentration within the presynaptic terminal. The rise in Ca²⁺ concentration causes synaptic vesicles—the presynaptic organelles that store neurotransmitters—to fuse with the presynaptic plasma membrane and release their contents into the space between the pre- and postsynaptic cells. Although it is not yet understood exactly how Ca²⁺ triggers exocytosis, specific proteins on the surface of the synaptic vesicle and elsewhere in the presynaptic terminal evidently mediate this process.

Contents

Electrical Synapses

Chemical Synapses

Quantal Transmission at Neuromuscular Synapses

Release of Transmitters from Synaptic Vesicles

Local Recycling of Synaptic Vesicles

The Role of Calcium in Transmitter Secretion

Molecular Mechanisms of Transmitter Secretion

Summary

Additional Reading