3o8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



tenance of the periodic \\a\es in the cerei)ral cortex, 

 although considered by him to be fundamentally a 

 manifestation of autorhythmicity of neurons, neces- 

 sitate a minimal influx of corticipetal impulses. The 

 requirement of this minimal number of afferent 

 impulses for the production of periodic cortical waves 

 is in some respects close to the essential concept under- 

 lying the proposed mechanism of corticothalamic 

 reverberation. The dependence of the so-called 

 spontaneous activity of cortical neurons on activation 

 by afferent impuLses has been con\incingly pro\'ed by 

 Burns (12). He demonstrated that the spontaneous 

 activity of isolated slabs of the cerebral cortex did not 

 ensue unless a bridge was left connected with the 

 rest of the brain. However, such slabs may exhibit 

 periodic activity if kept normalK- oxygenated (42). 



excit.IlBIlity ch.i,nges .accompanying and 

 following the evoked potential 



Like peripheral nerves or other excitable tissues the 

 aggregate of neurons in the central nerxous system, 

 after being activated either by direct or synaptic 

 stimulation, undergoes a cycle of excitability change 

 consisting of a refractory phase and a recovery phase. 

 After recovery, it may go into another period of 

 secondary depression during which the neurons fail to 

 respond or respond with less vigor. Uniquely in the 

 sensory cortex, a periodic variation in excitability may 

 deselop as a result of the corticothalamic re\erberat- 

 insf activity. 



RefraclDiy Periods 



The alisolute refractory period following the re- 

 sponse of the somesthetic corte.x to an aflferent stimulus 

 was about 8 msec, and the total recovery time was i 7 

 msec, as determined in monkeys under ether anes- 

 thesia. Barbiturates have the effect of lengthening the 

 recovery time. For example., under pentobarbital 

 anesthesia the absolute and the relative refractory 

 periods were found to be of the order of 25 to 50 msec, 

 and 87 to 144 msec, respectively (52, 55). Forbes & 

 Morison (33) found that the amplitude of the primary 

 response of the somesthetic cortex was reduced to 50 to 

 70 per cent of its initial value when the sciatic nerve 

 of a cat was stimulated at a frequency of 5 to 7 per 

 sec, implying that the relative refractory period was 

 much longer than the value obtained by Marshall 

 et al. (55). Forbes et al. (34) later reported that stimula- 

 tion of the sciatic nerve at a frequency of 60 per sec. 



produced no detectable cortical response after the re- 

 sponse to the first stimulus of the series. They also 

 reported the decrease in size of the primary response to 

 repetitive stimuli at a frequency of 5.5 per sec. and the 

 phenomenon of alternative response to repetitive 

 stimuli delivered at the rate of 14 per sec. 



The values of the absolute and relative refractory 

 periods in the \isual cortex of man and animals as 

 determined by Gastaut et al. (37) were 20 and 40 

 msec, respectiveh'. According to Tunturi (67), the 

 absolute refractory period in the auditory cortex to a 

 click lasts 20 to 100 msec, and the duration of the 

 relative refractory period is 100 to 250 msec. A glance 

 at the figures obtained by various investigators makes 

 one immediately realize the impossibility of finding 

 standard values for these events since the experimental 

 conditions which determine the results are extremely 

 variable. Among the more important factors affecting 

 the excitability of the brain are the anesthetics used in 

 the experiment (40), the depth of anesthesia during 

 which the observations are made (29, 33. 62), the 

 arterial pressure (6), the moisture (54), the tempera- 

 ture, etc. The level of tonic reticular activity is also a 

 factor of major importance (40). 



The effect of barbiturates on the excitability of the 

 nervous system is particularly interesting. It has been 

 suggested that barbiturates act selecti\ely on inter- 

 nuncial neurons in the corte.x rather than on the 

 afTercnt pathway. The suggestion remains to be 

 reconciled with the fact that under barbiturate 

 anesthesia the primary response of the exoked poten- 

 tial is little affected as compared with the marked 

 suppression of the spontaneous cortical waves. As is 

 known, the primary response of the ex-oked cortical 

 potential consists largely of the actixity of cortical 

 internuncial ncin-ons. 



The total period of refractoriness of the auditory 

 cortex following a direct electric shock was about 44 

 msec. The absolute refractory period was estimated as 

 about 7 or 8 msec. This value as compared with that 

 of the peripheral nerve or with that of the individual 

 neurons in the central nervous system is indeed very 

 large. The absolute refractory period of a nerve fiber 

 is known to occupy about the same time as the rising 

 phase of the action potential, xvhich usuallx' does not 

 exceed i msec. It has been frequently ob.serxed that 

 the minimal interval between successix-e spikes in a 

 train of unit discharges may be as brief as i m.sec. or 

 less, implying that the refractory period of single 

 neurons following a discharge is substantially shorter 

 than that of the potential recorded from the ag- 

 gregates of neurons. The reason for this difference 



