Release of Neurotransmitters

The synapse is the point of functional contact between one neuron and another. It is the primary place at which information is transmitted from neuron to neuron in the central nervous system or from neuron to target (gland or muscle) in the periphery. The simplest way for one cell to inform another of its activity is by direct electrical interaction, in which the current generated extracellularly from the action potential in the first cell passes through neighboring cells. Owing to the shunting of current by the highly conductive extracellular fluid, a 100 mV action potential may generate only 10–100 μV in a neighboring neuron. This coupling can be improved if neighboring cells are joined by a specialized conductive pathway through gap junctions (see Chapter 15); even then, a presynaptic spike is not likely to generate more than about 1 mV postsynaptically, unless the presynaptic process is nearly as large or larger than the postsynaptic process. This biophysical constraint limits the number of presynaptic cells that can converge on and influence a postsynaptic cell, and such electrical connections normally can be only excitatory and short lasting, are bidirectional in transmission, and show little plasticity or modifiability. They have limited potential for complex computation but can be useful when a postsynaptic neuron must be activated with high reliability and speed or when concurrent activity in a large number of presynaptic afferents must be signaled. ORGANIZATION OF THE CHEMICAL SYNAPSE