EXCITATORY AND INHIBITORY SYNAPTIC ACTION

Electron microscopy has fully confirmed the neuron theory (Cajal, 1934) in that this technique has revealed the continuity of the surface membrane over the nerve cell, each nerve cell being structurally an independent unit. It has further shown that the functional connections (synapses) between nerve cells and between nerve and muscle occur across regions of very intimate contact, there being a gap (the synaptic cleft) of 150 to 400 A between the respective presynaptic and postsynaptic membranes (De Robertis, 1956; Palay, 1956, 1958; Robertson, 1956, and personal communication). The existence of two types of synaptic action, excitatory and inhibitory, was recognized long before intracellular recording gave information relative to the ways in which excitatory synaptic action evoked the discharge of impulses and in which inhibitory synaptic action was able to prevent this discharge. By degeneration experiments Szentagothai (1958) has recently produced the first evidence distinguishing between the actual synaptic structures responsible for these two opposed functional actions. When, by degeneration procedures, motoneurons were left with only the inhibitory connections from Renshaw cells, they were devoid of all synaptic knobs, but there still remained fine fibrils lacing over their surface. Complementarily, destruction of the inhibitory pathway to oculomotor neurons caused a degeneration only of a similar fine fibrillar structure. Hence it seems possible that inhibitory synapses are formed by the functional contacts made by these fine fibers, and that all of the conventional synaptic knobs are excitatory. Synaptic excitatory action was first studied intracellularly by recording activity a t the neuromuscular junction (Fatt and Katz, 1951), but this transmission lies outside the scope of the present treatment, which is especially concerned with those synaptic transmissions where there is antagonism between excitatory and inhibitory actions. Since the first intracellular studies on the motoneuron (Brock et al., 1952), this dual synaptic action has been investigated in several other types of nerve-cell and nerve-muscle junction. It has therefore been thought appropriate to adopt a comparative approach to the problems of synaptic excitation and inhibition. Because of the much more extensive investigations on motoneurons, their responses will necessarily be treated more fully, but a t each stage the relevant data for other synaptic mechanisms will be discussed in relation to the motoneuronal responses. There have been two rival hypotheses, the electric and the chemical, for the manner in which an excitatory influence is transmitted across the synaptic contact between one nerve cell and the next. With the great majority of excitatory synapses it may now be taken as established that a nerve impulse causes a specific chemical transmitter to be released from the presynaptic terminals and thence to traverse the synaptic cleft and exert a depolarizing influence on the postsynaptic membrane; thus far, most of these transmitters in the central nervous system have not been identified. However, Furshpan and