CHAPTER XII 



The evoked potentials 



HSIANG-TUNG CHANGE 



Rockefeller Institute, New York City, and the Institute of Neurophysiology, 

 University 0/ Copenhagen, Denmark 



CHAPTER CONTENTS 



Introduction 



Definition 



Limitations of Evoked Potentials as Tools for Anatomical 

 Study- 

 Components of Evoked Potentials and Their Identification 



The Latent Period 



Effect of Repetitive Stimulation 



Effect of Changes in Internal and External Milieu 



Anatomical Considerations 

 Neural Mechanisms for the Elaboration of Evoked Cortical 

 Potentials 



Types of Neural Elements Involved and Their Mode of 

 Action 



The Initiation of Postsynaptic Impulses in Pyramidal 

 Neurons 



Apical Dendrites and Electrical Signs of the Evoked Poten- 

 tial 



Microelectrode Findings Concerning the Mechanism of Im- 

 pulse Initiation in Single Neurons 

 After-Discharges 



Repetitive Firing of Individual Neurons 



Local After-Discharges Involving Intrinsic Neuronal Circuits 



Rhythmic After-Discharges Involving Long Neuronal 

 Circuits: Corticothalamic Reverberatory Activity 

 Excitability Changes Accompanying and Following the Evoked 

 Potential 



Refractory Periods 



Postexcitatory Depression 



Periodic Variation in Cortical Excitability 



Interaction of Afferent Impulses in the Cerebral Cortex 



Modification of Cortical Excitability by Constant Inflow of 

 Afferent Impulses 

 Summary 



INTRODUCTION 



Definition 



BY AN EVOKED POTENTIAL IS meant the detectable 

 electrical change of any part of the brain in response to 



' Present address: Academia Sinica, Shanghai, China. 



deliberate stimulation of a peripheral sense organ, a 

 sensory nerve, a point on the sensory pathway or any 

 related structure of the sensory system. Although 

 observations of evoked potentials have most fre- 

 quently been made in the sensory system, potentials 

 produced by other means such as by direct electrical 

 stimulation or antidromic stimulation of the neuron 

 fall into the same category. 



In physiology the term 'potential' is often very 

 loosely used as signifying merely the electrical change. 

 Strictly speaking, the potential at a point of the tissue 

 implies the same meaning as in physics and thus 

 denotes the work necessary to bring a unit charge from 

 infinity up to the point in question which is located in 

 an electrostatic field. Its absolute value can be 

 measured only with one electrode placed on the active 

 tissue and the other grounded. It would be necessary 

 to record the current rather than the potential if the 

 two recording electrodes were placed on the same 

 active tissue. However, it is possible to obtain a close 

 approximation of the potential value by means of 

 Laplacian placement of electrodes in which one active 

 electrode is surrounded by a number of .similar 

 electrodes combined together as a single pole. This 

 method of recording seems to have certain advantages, 

 especially when precise localization and determination 

 of the distribution of the action potential are desired 

 (58). The evoked potential diflfers from the so-called 

 spontaneous electrical changes in iTiany respects, 

 notably the following, a) It bears definite temporal 

 relationship to the onset of the stimulus. In other 

 words, it has a definite latent period determined by 

 the conduction velocity of the nerve impulses, the 

 conduction distance between the point of stimulation 

 and the point of recording, the synaptic delay and the 

 number of synapses involved. In a given system the 

 latency is generally fixed and consistent under 

 similar experimental conditions. 6) It has a definite 

 pattern of response characteristic of a specific system 



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