300 



HANDBOOK OF PHYSIOLOGY' 



NEUROPHYSIOLOGY I 



which is more or less predictable and reproducible 

 under similar conditions, c) It usually appears in a 

 circumscribed area of the central nervous system 

 where the active tissue is located. 



Identification of the evoked potential requires 

 knowledge of anatomical connections between the 

 site of stimulation and the point of recording. As 

 distinct from spontaneous electrical activity, po- 

 tentials evoked by deliberate stimulation of peripheral 

 sensory nerves are sharply localized in the central 

 nervous system. The procedure of evoked potential 

 registration thus becomes a useful tool for the in- 

 vestigation of sensory pathways. It has been particu- 

 larly fruitful when applied to the study of cortical 

 representation of the auditory, visual and various 

 cutaneous sense organs. 



l.iimlations oj Evoked Potentials as Tools Jor 

 Anatomical Study 



It is not intended at present to discuss in detail 

 sensory localization demonstrated by the evoked 

 potential technique. This is discussed in the chapters of 

 this handbook dealing with the various sensory 

 mechanisms. However, in order to caution against the 

 misuse of the technique it may be pertinent to men- 

 tion briefly here some fundamental aspects of the 

 evoked potential, especially those which limit its 

 usefulness in anatomicophysiological study. First, the 

 method is valuable in determining the area of sensory 

 projection on the cerebral cortex only when the 

 observation is made under such conditions that the 

 cerebral cortex is not in an e.xalted state of excitation, 

 if the true sensory projection area is to be determined. 

 It is an obvious fact that in so complex an organiza- 

 tion as the cerebral cortex each neuron is potentially 

 related to any other neuron through a vast nuinber 

 of chains of synaptic connections. When a given neu- 

 ron is activated by an afferent impulse, almost any 

 other neuron may become excited unless some re- 

 strictive influence is exerted to curb the spread of the 

 evoked potential to remote regions where ito afferent 

 fibers terminate directly. Second, the appearance of 

 an electrical change in a given area of the cerebral 

 cortex does not necessarily indicate the presence of 

 neuronal activity underlying that area. As pointed 

 out long ago by Helmholtz, measurement of the 

 external field of electrical current on the surface of a 

 living tissue is not adequate to ascertain the location of 

 the internal electromotive force. The electrical 

 changes detected from the surface of the brain may 

 be derived from a purelv phssical process such as the 



potential field created by the pa.ssage of electrical 

 current along a nerve bathed in a conducting medium. 

 According to the \oiume conductor principle (47, 49), 

 when a synchronous volley of impulses passes along a 

 nerve embedded in a \olume of conducting medium, 

 there will appear in the medium a travelling electric 

 field around the ner\e. .Such a field is caused by the 

 flow of electric current from the inactive region to the 

 depolarized region of the ner\e. The region occupied 

 by the nerve impulse serves as a fictitious sink of 

 current flow^ and the regions lying ahead and behind 

 the impulse as fictitious sources. Thus, the sign of the 

 action current recorded from the active elements in 

 the brain is negati\c-positi\e diphasic at the point 

 where the nerve impulse is initiated, positive-negative 

 diphasic at the point where the conducting path ends 

 and positive-negative-positive triphasic at the middle 

 of the conducting path. At points away from the 

 acti\e element the sign of the detectable action po- 

 tential will depend on the position of the electrode 

 relative to the direction and the pattern of the iso- 

 potential lines of the traveling electric field. 



COMPONENTS OF EVOKED POTENTI.^LS 

 AND THEIR IDENTIFICATION 



The action potential in the brain evoked either by 

 electrical stimulation of the ascending pathways or by 

 adequate stimulation of the sense organs consists of 

 two components, the presynaptic and the post- 

 synaptic. The former indicates the arrival of impulses 

 passing along the axon and their terminals, and the 

 latter the acti\ities of the cell body and dendrites. 

 For the purpose of illustration, the evoked potential 

 of the sensory cortex may be taken as an example. 

 Following the arrival of a volley of afferent impulses 

 from the thalamus, the projection area of the cerebral 

 cortex gives rise to a surface-positive primary response 

 followed sometimes by a series of rhythmic after- 

 discharges. The primary response is made up of the 

 presynaptic potential produced by the activity of 

 thalamocortical fibers and the postsynaptic potential 

 produced by the discharge of intracortical neurons. 



The incoming impulses from the thalamus are often 

 blended with, and obscured by, the powerful post- 

 synaptic discharge of the cortical neurons. This is 

 especially true when the afiferent impulses are ini- 

 tiated by stimulation of the peripheral sense organs 

 or of the pathway far away from the cerebral cortex. 

 In that event, the afferent impulses usually arrive at 

 the cortex asynchronously clue to temporal dispersion. 



