A mechanosensory neuron (center, green) sends its axon to form excitatory synaptic connections with two motor neurons (red, orange) in cell culture, recapitulating the connections in the living animal. The neurons were isolated from the marine snail Aplysia californica. (Reproduced, with permission, from Harshad Vishwasrao and Eric R. Kandel.)
IN PART II, WE EXAMINED HOW ELECTRICAL signals are initiated and propagated within an individual neuron. We now turn to synaptic transmission, the process by which one nerve cell communicates with another.
With some exceptions, the synapse consists of three components: (1) the terminals of the presynaptic axon, (2) a target on the postsynaptic cell, and (3) a zone of apposition. Based on the structure of the apposition, synapses are categorized into two major groups: electrical and chemical. At electrical synapses, the presynaptic terminal and the postsynaptic cell are in very close apposition at regions termed gap junctions. The current generated by an action potential in the presynaptic neuron directly enters the postsynaptic cell through specialized bridging channels called gap junction channels, which physically connect the cytoplasm of the presynaptic and postsynaptic cells. At chemical synapses, a cleft separates the two cells, and the cells do not communicate through bridging channels. Rather, an action potential in the presynaptic cell leads to the release of a chemical transmitter from the nerve terminal. The transmitter diffuses across the synaptic cleft and binds to receptor molecules on the postsynaptic membrane, which regulates the opening and closing of ion channels in the postsynaptic cell. This leads to changes in the membrane potential of the postsynaptic neuron that can either excite or inhibit the firing of an action potential.
Receptors for transmitters can be classified into two major groups depending on how they control ion channels in the postsynaptic cell. One type, the ionotropic receptor, is an ion channel that opens when the transmitter binds. The second type, the metabotropic receptor, acts indirectly on ion channels by activating a biochemical second-messenger cascade within the postsynaptic cell. Both types of receptors can result in excitation or inhibition. The sign of the signal depends not on the identity of the transmitter but on the properties of the receptor with which the transmitter interacts. Most transmitters are low-molecular-weight molecules, but certain peptides also can act as messengers at synapses. The methods of electrophysiology, biochemistry, and molecular biology have been used to characterize the receptors in postsynaptic cells that respond to these various chemical messengers. These methods have also clarified how second-messenger pathways transduce signals within cells.
In this part of the book, we consider synaptic transmission in its most elementary forms. We first compare and contrast the two major classes of synapses, chemical and electrical (see Chapter 11). We then focus on a model chemical synapse in the peripheral nervous system, the neuromuscular ...