THE EVOKED POTENTIALS 



305 



mechanism of impulse initiation in a neuron (30-32). 

 Eyzaguirre & Kuffler found that suljliminal excita- 

 tion of the dendrite by mechanical stretch of the 

 muscle in which the dendrites are imbedded produces 

 a reduction of the membrane potential of the cell body 

 through the electrotonic effect. A propagated discharge 

 takes place only when the membrane potential is 

 reduced to a certain level. From this it appears that 

 the prepotential of the evoked response may represent 

 nothing but a partial depolarization of the resting 

 membrane potential resulting from subliminal excita- 

 tion of dendrites by bombardments of presynaptic 

 nerve endings. Antidromic discharge of a neuron 

 apparently calls for no such build-up of the ex- 

 citability level as a prerequisite and is therefore devoid 

 of the characteristic small prepotential often seen in 

 responses produced by synaptic action. 



.\FTER-DISCH.^RGES 



The term ' after-discharge' has been conventionally 

 employed to describe the epileptiform discharges of 

 neurons following strong tetanic stimulation which 

 persist long after the cessation of stimulation. Since 

 accumulated experimental data show that repetitive 

 bursts occur not only after tetanic stimulation but also 

 after a single shock applied to the system, we will 

 designate all the discharges which outlast the duration 

 of stimulation, tetanic or single shock, as after- 

 discharges, in the very loose sense of the terminology. 



For the sake of convenience in discussion, after- 

 discharges can be classified into three types: /) repeti- 

 tive firings of single elements which are self-main- 

 tained without the participation of other elements in 

 their production; 2) persistent local after-discharges 

 involving the activity of closely situated intrinsic 

 neurons which form short neuronal circuits; and 

 j) periodic discharges involving re\erberating activi- 

 ties of a closed neuronal circuit formed by long chains 

 of neurons connecting remotely separated structures. 



Repetitive Firing of Individual Neurons 



Microelectrode recordings from the thalamic and 

 the cortical neurons show that a .single neuron fires 

 several times in response to an afferent volley. To a 

 stimulus at threshold strength a neuron generally 

 responds by giving rise to a single spike. As the stim- 

 ulus increases in strength, the number of the spikes 

 increases correspondingly. It has been reported that a 

 thalamic unit may fire seven times in quick succession 



in response to optimal stimulation of the skin re- 

 ceptors (60). A single neuron in the reticular forma- 

 tion, for instance, may give rise to a train of as many as 

 20 spikes in response to a single stimulus (3). In so far 

 as the length of the train is concerned, natural ade- 

 quate stimulation of the sense organs seems to be more 

 effective than electrical stimulation of the nerve. In 

 electrical stimulation, once the threshold is reached 

 further increase in intensity seems to be rather in- 

 effective in inducing any greater responses. On the 

 contrary, strong stimulation may inhibit the suc- 

 cessive spikes. This is true for somesthetic, auditorv, 

 visual and olfactory systems. 



In a long train of repetiti\e discharges at high 

 frequency, the first spike is usually the largest in 

 amplitude and the second the smallest. The rest of the 

 spikes following the second gradually increase in size 

 until they approach, but rarely become as large as, 

 the first one. The deficit of the .successive spikes is 

 probably caused either by the incomplete repolariza- 

 tion of the neuron membrane following a forceful dis- 

 charge, or by the postexcitatory depression associated 

 with the process of hyperpolarization of the mem- 

 brane. 



The repetitive discharge of a single neuron is 

 believed to be a self-sustained process which is 

 initiated only by the afferent volley but is not the 

 result of repeated bombardments by presynaptic 

 impulses. This belief is based on the observation that a 

 small deflection immediately preceding the first spike 

 of a train, which can be interpreted as a presynaptic 

 potential, is present only once at the beginning of the 

 train. No similar potential has ever been observed 

 preceding the successive spikes. The individual spikes 

 in a train do not correspond to the successive arrivals 

 of presynaptic volleys of impulses. Therefore, the 

 repetiti\e firing cannot be regarded as resulting from 

 the repetitise arrival of presynaptic impulses. Single 

 nerve elements are endowed with the capacity to 

 discharge repetitively in response to a stimulus. It has 

 been repeatedly demonstrated that following a single 

 shock applied to the dorsal root a burst of four or five 

 unit spikes can be recorded in the dorsal column 

 where, due to the absence of intercalated neurons, 

 synapses are not in\'olved. 



The intimate nature of the .self-generating mechan- 

 ism of after-discharge inside the neuron is not known. 

 Burns (13) suggests that the repetitive firing of a 

 cortical neiu-on following stimulation is due to the 

 difference in recovery rates of resting membrane 

 potentials at the two ends of a neuron such that one 

 end is repolarized more slowly than the other. By 



