THE EVOKED POTENTIALS 



301 



In the visual system, however, the radiation potential 

 is discernible in the cortical response to stimulation of 

 the optic nerve for two reasons: first, distinct groups 

 of fibers according to size are present throughout the 

 pathway from the optic nerve to the optic radiation; 

 and, second, there are no intercalated neurons in the 

 lateral geniculate body which, if present, would 

 destroy the synchrony of conduction of optic impulses 

 and thus obliterate the characteristic triple-spike 

 radiation potential. In the auditory system as well as 

 in the somesthetic system the presynaptic component 

 of the evoked potential can also be demonstrated, 

 though less prominently than in the visual system, if 

 the corresponding thalamic nucleus is directly 

 stimulated with a brief electric shock. 



In spite of the composite nature of the primary 

 response, the two components can be readily dif- 

 ferentiated from each other by various experimental 

 procedures as described below. 



The Latent Period 



The presynaptic component of the evoked po- 

 tential, being the initial sign of activity, has the 

 shortest latency with a value contingent upon the 

 fiber diameter and the conduction distance of a 

 given system. It is readily identifiable in a system 

 composed of fibers of uniform size. The temporal 

 dispersion of the presynaptic impulses passing along 

 a bundle of fibers of different sizes may make the time 

 of arrival at the point of recording vary over a wide 

 range so that the last impulses may overlap with the 

 postsynaptic discharge set up by the fast fibers. 

 Under such circumstances it is impossible to dis- 

 tinguish presynaptic from postsynaptic activity merely 

 by the latency; only the activity of the fastest fibers of 

 the group can be ascertained. In fact, such is always 

 the case in the cortical and subcortical potentials 

 evoked by adequate stimulation of peripheral sense 

 organs. 



Another factor which may seriously limit the ap- 

 plicability and the value of latency measurement is 

 the possible reduction in conduction velocity of 

 impulses at nerve terminals resulting from the diminu- 

 tion of fiber diameter. It is not known whether the 

 impulses vanish instantly at the specialized pre- 

 synaptic endings. According to Barron & Matthews 

 (4), and Lloyd & Mclntyre (46) a prolonged nega- 

 tivity persists at the afferent terminals of spinal dorsal 

 root fibers and is detectable at a considerable distance 

 from the active fibers as the dorsal root potential 

 DR-IV in Lloyd's terminology. The prolonged 



depolarization of a dorsal root fiber following a 

 single shock stimulation can be recorded with an 



intracellular microelectrode (41). 



Efect oj Repetitive Stimulation 



It has long Iseen known that synaptic transmission 

 can be blocked by stimuli delivered in quick suc- 

 cession. At certain rates of stimulation the amplitude 

 of the electrical response involving synapses decreases 

 with each response, becoming succes.sively smaller 

 than the preceding one until finally the response 

 disappears entirely. The repolarization process of 

 the membrane of the neuron .soma which receives the 

 presynaptic excitation apparently requires a longer 

 time than the axon. The postsynaptic neuron is not 

 al)le to respond to successively arriving impulses 

 until the recovery of its excitability becomes complete. 

 The effect of synaptic block by repetitive stiinulation 

 is especially pronounced in subjects under deep 

 anesthesia by barbiturates. The method has been 

 successfully used in differentiation of the presynaptic 

 froin the postsynaptic components of the evoked 

 potential in the lateral geniculate body by Bishop & 

 McLeod (8). As another example to illustrate the 

 differential effect of repetitive stimulation on the 

 pre- and the postsynaptic potentials, the electrical 

 response of the pyramidal tract to electrical stimula- 

 tion of the motor cortex may be taken. According to 

 the study by Patton & Amassian (57) the pyramidal 

 response to cortical stimulation consists of an early- 

 wave resulting from direct stimulation of the motor 

 neurons and a later wave resulting from the activity 

 of cortical internuncial neurons, which is elicited 

 indirectly or synaptically. The former can follow 

 repetitive stimulation at frequencies as high as 340 per 

 sec. with only slight reduction in amplitude, whereas 

 the latter disappears from the response when stimulus 

 frequency is increased from 44.7 per sec. to 127 per 

 sec. 



The presynaptic response or the response of an axon 

 to direct electrical stimulation is usually able to 

 follow faithfully the stimuli at a high rate limited only 

 l)y the refractory period. The degree of the blocking 

 effect of repetitive stimulation seems to increase with 

 the increase in number of synapses involved. For 

 instance, in the first relay station of the dorsal root 

 fiijers, i.e. the cuneate nucleus, the postsynaptic 

 discharge of a single neuron to repetitive stimulation 

 of peripheral sense organs or of its nerve can follow 

 the rate of stimulation as high as 100 per sec. without 

 substantial modification of either the response ampli- 



