SYNAPTIC AND EPHAPTIC TRANSMISSION 



■49 



absence of transneuronal degeneration) constituted 

 the evidence brought forward by Ramon y Cajal and 

 others in support of the neuron theory. 



When the neuron theory became accepted, the 

 electrical theory of transmission, essentially as formu- 

 lated by Kiihne for the neuromuscular synapse, was 

 also generally adopted (cf. 45, 57, 140). Nevertheless, 

 Sherrington's life-long study of the central nervous 

 system emphasized that the physiological actions of 

 the latter were dominated by the properties of syn- 

 aptic transmission. These, he thought (183), were in 

 many respects fundamentally different from the 

 properties of conductile activity of nerve or muscle 

 fibers, in which all-or-none impulses, spikes, are 

 propagated by electrical local circuit excitation 

 within the confines of a single cell, even though the 

 latter may be very long in extent. A Russian school of 

 physiology headed by Ukhtom.sky (cf i 76) ahso main- 

 tained that central nervous phenomena could not be 

 explained solely in terms of all-or-none activity. 



The neuron theory incorporates and gives physio- 

 logical meaning to the doctrine of polarized conduc- 

 tion which is embodied in the Bell-Magendie Law. 

 The presynaptic terminals impinge upon the synaptic, 

 or subsynaptic (cf. 60) membrane of the postjunctional 

 cell with various types of contacts. These are located 

 chiefly, but not exclusively, at the dendrites and soma 

 of neurons, and Ramon y Cajal distinguished the 

 different sites of contact as axodendritic and axo- 

 somatic synapses (cf. 1^9). Contacts between the 

 nerve fibers and the effector cells, muscle or gland, 

 are also made at specialized regions, tho.se of mus- 

 cle fibers being termed endplates, as noted above. 

 Impulses afferent in a prefiber evoke activity in the 

 postjunctional cell. If the cell is a neuron, its junc- 

 tional activity may result in a spike which propa- 

 gates along the latter's axon. At the terminals of this 

 axon, a new transfer may then take place to another 

 neuron or to an effector cell. In some instances 

 unidirectional progression is apparently invalidated, 

 but the general mechanism of these cases is probably 

 b\ ephaptic transmission (10). This appears to be 

 fundamentally different from synaptic transmission 

 and will be discussed in the last section of this chap- 

 ter. One recently discovered case of unidirectional 

 conduction (83) produced by an electrical local cir- 

 cuit mechanism will also be discussed at that time. 



The concept of unidirectional synaptic transmission 

 permitted Ramon y Cajal to deduce many functional 

 properties of the central nervous system from anatom- 

 ical data (168). Changes that occur in gross and fine 

 structure, in histochemical properties and in physio- 



logical behavior after extirpation or damage of 

 specific elements also give clues to function. The 

 information obtained by these methods relates chiefly, 

 however, to the study of integrative activity which is 

 the subject of later chapters. 



While it, too, bears largely on integrative functions, 

 the analysis of reflexes as exemplified in Sherrington's 

 work (cf 44, 182) nevertheless also provides data on 

 the synaptic processes them.selves and discloses 

 phenomena such as cumulative, long-lasting excita- 

 tory and inhibitory slates. These two synaptic prop- 

 erties endow the central nervous system with its 

 remarkable flexibility and variety of responsiveness. 

 Both characteristics may also be present in simpler 

 peripheral synaptic organizations and are commonly 

 found in the peripheral synaptic structures of inverte- 

 brates. Sherrington's basic method, stimulation of 

 selected pathways and study of their effects and inter- 

 actions, has been refined by application of modern 

 electrophysiological techniques. The combination 

 has given information on the effects of different syn- 

 aptic inflows, their relative potencies, the temporal 

 and spatial distribution of excitatory and inhibitory 

 actions, particularly in the spinal cord (cf. 140; and 

 later chapters in this volume). 



The electrophysiological study of single unit path- 

 ways such as nerve-muscle or neuron-neuron provides 

 still more detailed and intimate information on synap- 

 tic mechanisms (cf. 62). Microelectrode recording, 

 either from the vicinity of single cells or from their 

 interior, is a recent extension of the technique which 

 can provide the most definitive information (52, 59, 

 60, 95, 97). In all cases, transmissional activity is 

 found to be associated with a special type of electrical 

 response, the postsynaptic potential or p.s.p. The 

 transmissional electrogenesis at the endplates of 

 skeletal muscle fibers is known as the endplate po- 

 tential (e.p.p.). Basically, however, the properties of 

 e.p.p.'s arc identical with those of p.s.p.'s. A presyn- 

 aptic potential, occurring at the terminals of dorsal 

 root fibers, has also been described ijut from indirect 

 evidence only (140). 



Pharmacological data provide much of the oldest 

 evidence that synaptic transmis.sion is different from 

 the conductile process. Claude Bernard (18) found 

 that curare, the Indian arrow poison, blocked excita- 

 tion of a muscle by its nerve. The muscle and nerve 

 individually retain their conductile properties, and 

 the primary effect of the drug is on the transmission 

 process. Attempts to account for the synaptic blockade 

 in terms of electrical transmission were not successful 

 (cf. 98). A host of other chemicals exert actions chiefly 



