312 



HANDBOOK OF PHVSIOLOGV -^ NEUROPHYSIOLOGY I 



Such potentials differ from the spontaneous electrical 

 activity in that they have a definite temporal relation- 

 ship to the onset of the stimulus, a constant pattern of 

 response and a focus of maximal response in the brain. 

 The technique of evoked potential registration has 

 been widely used as a tool for anatomical studies of 

 the central nervous system. However, the conclusions 

 drawn from the results of such studies are justified 

 only when the limitations of this technique are duly 

 considered. 



The primary response of the evoked cortical poten- 

 tial consists of a presynaptic component produced by 

 the impulses from the afferent fibers and a post- 

 synaptic component produced by the discharge of 

 intracortical neurons. These two components can be 

 differentiated from each other by various experimental 

 procedures, such as by study of a) latency, A) the 

 effect of repetitive stimulation, <) the relative tolerance 

 to changes in internal and external milieu and above 

 all d) the anatomical considerations. 



The neural mechanism for elaboration of the 

 evoked cortical potential is formulated on the basis of 

 the histological organization of the cerebral cortex 

 and the general principles of neurophysiology. In a 

 proposed scheme it is suggested that the afferent im- 

 pulses from the thalamocortical fibers first excite the 

 Golgi type II cells in the fourth cortical stratum which 

 in turn transmit the postsynaptic impulses to star 

 pyramids and the star cells in the same layer, then to 

 small and medium pyramidal neurons in the supra- 

 granular layers and finally to the large pyramidal 

 neurons in the deep layers. The surface-positive deflec- 

 tion of the evoked potential is attributed to the 

 synchronized propagation of impulses along the 

 apical dendrites from the cell body of pyramidal 

 neurons inward to the cortical surface. The depolar- 

 ization process of the cortical pyramidal neurons is 

 believed to start always at the cell body due to the 

 effective excitatory action of the pericorpu.scular 

 synapses. The subliminal excitation of the paraden- 

 dritic synapses has the effect onl\ of modifying the 

 excitability state of the neur(jn. This hypothesis is 



supported by the results of microelectrode findings of 

 single neurons. 



After-discharges can be classified into three kinds: 

 /) the self-sustained repetitive firing of single elements, 

 2) persistent local after-discharges involving the 

 activity of closely situated intrinsic neurons and 3) 

 the periodic after-discharges involving the activity 

 of reverberating circuits interconnecting distant 

 structures. Of these three, the most frequently ob- 

 served in the central nervous system is the local after- 

 discharge, maintained by a mechanism of self-re- 

 excitation through collaterals and numerous closed 

 neuronal circuits within the cortex. The activities of 

 the long reverberating circuit are not to be confused 

 with other kinds of periodic waves which may happen 

 to have similar frequency and similar wave form. 



The ijrain undergoes a cycle of excitability changes 

 accompanying and following the evoked potential. 

 The true refractory period which lasts for less than a 

 millisecond is thought to be related to the repolariza- 

 tion process of the neuron. The postexcitatory de- 

 pression which may last for as long as 100 msec, is 

 proi:)ably a functional manifestation of the hyper- 

 polarization process. Because of the supersession of 

 the rcfraciorv phase by the process of postexcitatory 

 depression, the value of the true refractory period of 

 the neuronal aggregate cannot be accurately deter- 

 mined. In addition to the regular cxcitabilit\- c\cle 

 there is a periodic variation of excitability accom- 

 panying the reverberating activity of the sensory 

 cortex. 



Although the ev'oked potentials in different systems 

 are independent processes, they do show interaction 

 proi^ably due to the overlapping of their fiber distribu- 

 tion or the con\crgence of the afferent impulses on the 

 common neurons, or through the integration in a 

 general activating system such as the reticular forma- 

 tion. Such interaction of afferent impulses in the 

 cerebral cortex makes it possible for the constant 

 afferent inflow in any particular sensory system to 

 modif\ the lc\ei of cortical excitability as a whole. 



REFERENCE. S 



1. Amassian, V. E. .-1. Rei. Nerv. & Ment. Dis., Proc. 30: 371, 



■952- 



2. Amassian, V. E. and J. L. Devito. Fer/. Proc. 13:3, 1954- 



3. Amassian, V. E. and R. V. Devito. J. .Neurophysiol . 17; 575, 



1954- 



4. Barron, D. H. and B. H. C. Matthews. J. Phy.uul. 92: 



276, 1938. 



5. Bartlev, ,S. H. and G. H. Bishop. Am. J. Plmio!. 103 159, 



'933- 



6. Beecher, H. K., F. K. McDonougii and .\. Forbes. J. 



Neurophysiol. i: 324, 1938. 



7. Bernhard, C. G. J. .Neurophysiol. 7: 397, 1944. 



8. Bishop, P. O. and J. G. McLeod. J. .Neurophysiol. 17: 387, 

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