THE EVOKED POTENTIALS 



309 



between single neurons and neuronal aggregates is not 

 entirely understood. Perhaps in the case of neuronal 

 aggregates the processes of postexcitatory depression 

 have superseded the phase of true refractoriness and 

 therefore make it appear that the refractory period is 

 prolonged. The true refractory period is believed to 

 be the same as the period during which the repolariza- 

 tion process of the membrane potential of the active 

 neurons is taking place. Its value is determined by 

 the rate of the repolarization process. 



Poitexcitatury Di'pressiun 



The phase of postexcitatory depression may be 

 defined as the period immediately following the initial 

 recovery from refractoriness during which the nervous 

 tissue undergoes various degrees of lowered excitabil- 

 ity. The basic pattern of the event exists in the ex- 

 citability cycle of any kind of nervous tissue including 

 the cerebral cortex, the spinal cord, peripheral 

 nerves and sympathetic ganglia. 



The degree and duration of the postexcitatory de- 

 pression vary more or less proportionately with the 

 stimulus strength and the number of neurons pre- 

 viously activated by the conditioning stimulus. There 

 may be no obvious secondary depression following the 

 refractory period if the previous response is weak. 

 After an intense response of the cortex the postexcita- 

 tory depression may last for several hundred milli- 

 seconds. Extremely severe depression lasting for a 

 second or more has been observed following the 

 respon.se of the cortex treated with strychnine. This is 

 true for cortical responses to topical stimulation as 

 well as for those to afferent impulses (15). By u.sing 

 paired electric shocks applied to the optic nerve, both 

 Marshall (53) and Clare & Bishop (26) demonstrated 

 the existence, in the visual cortex and in the lateral 

 geniculate body, of a typical excital:)ility cycle with the 

 supernormal phase followed by a long period of post- 

 excitatory depression. When the retina is continuously 

 illuminated, not only the onset but also the cessation 

 of the photic stimulus produce the phenomenon of 

 postexcitatory depression of the cerebral cortex. The 

 visual cortex undergoes a period of severe depression 

 immediately after the recovery of the cortex from the 

 refractoriness caused by retina excitation. In extreme 

 cases this period of temporary depression may last for 

 as long as several seconds. A similar phenomenon is 

 present in the auditory system. Rosenblith el al. (61) 

 observed that the neural respon.se to a click as re- 

 corded from the round window and from the auditory 

 cortex of the cat were depressed within the first 40 sec. 



after sudden exposure to continuous tones. The post- 

 excitatory inactivation of the cortex discus.sed here 

 may constitute a physiological basis for the temporary 

 blindness and deafness following a sudden exposure to 

 strong light and loud sound. The diininished excitabil- 

 ity of neurons during the period of postexcitatory 

 depression is l)elieved to be associated with the mem- 

 brane potential changes of the neuron following the 

 discharge. From recent microelectrode studies of the 

 electrical properties of single neurons it has been 

 observed that the repolarization of the membrane 

 potential may develop into a phase of hyperpolariza- 

 tion which reaches a maximum at 5 to 10 msec, and 

 may last for as long as 100 msec. During the period of 

 hyperpolarization the action potential of the neuron 

 is inhibited (27, 28). The time courses of the repolari- 

 zation and the hyperpolarization processes bear a 

 close relationship to the refractoriness and the post- 

 excitatory depression of the neuron. ' 



Pinodic \ nnatuin in Cortical Excitability 



Unique to the sensory area, the e.xcitability state of 

 the cortex does not always return to the normal level 

 following the completion of a usual excitability cycle 

 but undergoes a further cyclic waxing and waning 

 with regular intervals. The periodic excitability 

 change of the visual cortex was described bv Bishop in 

 '933 Co)- However, he believed it was an indication of 

 the excitability change of the optic pathways rather 

 than of the cortical neurons them.selves. The periodic 

 \ ariation in excitability of the auditory cortex beyond 

 the unresponsive period caused by a sound stimulus 

 was obser\-ed by Jarcho (38). He noticed the periodic 

 depression of the cortex at a frequency coincidental 

 with the repetitive corticothalamic after-discharges. 

 No increased excitability was seen at any time, how- 

 ever. Jarcho's finding was soon confirmed with the 

 further disclosure that in company with the rising and 

 falling of the corticothalamic reverberating waves, 

 there are concomitant increase and decrease of cortical 

 excitability (14, 15). The temporal relation between 

 the repetitive waves and the excitability change of the 

 cerebral cortex resulting from the corticothalamic 

 reverberating activity was found to be such that the 

 cortical excitability is increased during the developing 

 phase of the re\-erberating wave and decreased during 

 the returning phase of the reverberating wave. An 

 alignment of the excitability curve and the contour of 

 the reverberating waves on the same time scale show 

 that the maximum of the increased excitability is 

 reached in the middle of the developing phase, that 



