722 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



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FIG. 7. Diagrammatic reconstruction of typical records, led 

 from surface to white matter, by addition of what are inferred 

 to be their chief potential components, a. Diphasic response 

 assignable to conduction along apical dendrites when these only 

 are activated below the surface of the cortex; negative phase 

 is early negative component, b. Summation of surface-positive 

 potentials inferred to be caused by activity of successively 

 activated basal dendrites at successively higher levels of the 

 cortex, f. .Spike sequence detectable in responses to near- 

 threshold stimulation of the optic nerve, later responses of 

 which are usually obscured following stronger stimuli, d. Slow, 

 presumably diphasic process arising from deeper layer of cor- 

 tex, the second phase of which appears as a late negativity in 

 many respon.ses. /. From addition of all four of above com- 

 ponents, a reconstruction of a record of response to optic nerve 

 stimulation which shows considerable early negativity. 2. 

 Similar reconstruction of response to direct stimulation fairly 

 deep in the cortex, but above layers V and VI, eliminating 

 component d\ negative phase of diphasic component a is 

 exaggerated, as appears to be the case in records from direction 

 stimulation, and the first spikes are telescoped. 3. Reconstruc- 

 tion as in ;, but for record showing minimal or no early nega- 

 tivity, from which therefore component a is eliminated. [From 

 Bishop & Clare (22).] 



to the way the apical dendrites behave (see fig;. 7). 

 When these structures are directly activated from the 

 region of the neuron's cell bodies, the dendrites 

 always conduct toward the surface of the cortex. 

 When indirectly activated, by way of the radiation, 

 this conduction often does not materialize, even fol- 

 lowing maximal optic ner\e stimulation. It generally 

 fails in connection with very weak stimulation. They 

 deduced that in normal (nonsynchronou.s) activation 

 conduction does not occur from cell ijody to apical 

 dendrite. On the other hand, supposedly when a 

 sufiicient number of dendrites are activated, some 

 kind of mutual facilitation provides for antidromic 

 conduction. 



These authors believe that when strychnine or any 

 other convulsive drug is applied, the spike manifesta- 

 tion is primarily the indication of a conducted re- 

 sponse in apical dendrites. The essence of the convul- 

 sive state lies both in the heightened irritability of the 

 dendrites and in the mutual facilitation and activa- 

 tion to the point of a massive and well synchronized 

 discharge. 



The cortical response, recorded from leads from 

 brain surface to white matter, is a composite 

 (summation) of many primary sources of potential. 

 The response to a single brief stimulus to the optic 

 nerve producing a volley of impulses may produce a 

 record that is an inadequate representation of cortical 

 function in normal behaxior. The brief stimulus pro- 

 duces a degree of synchronization that, in itself, is an 

 artificial distribution of impulses from the start. This 

 sort of volley could be considered more appropriate 

 for producing con\ulsions than for the usual response 

 (23). One of the justifications for this, however, aside 

 from procedural necessity, is that the normal ob.ser\er 

 can make a visual discrimination from such stimuli 

 which are shorter than might ordinarily be thought 

 efTective. Actually, for certain comparisons between 

 perceptions very short pulses of photic stimulation are 

 found usable and analytically helpful. Of course, 

 none of such stimuli is actually as brief as the electric 

 shocks used. 



While the investigations of Bishop & Clare cited in 

 this chapter indicate a general propagation of mass 

 impulse from cell to cell upward from the neighbor- 

 hood of the terminals of afferent fibers and apparently 

 downward also to lower cortical layers, the norma! 

 stratification may be less sharp than has been de- 

 scribed. Apparently, as more and more minute 

 regions are explored with closely located microelec- 

 trodes, an increasing heterogeneitv in the directions 



