THE EVOKED POTENTIALS 



303 



tential of surface recordings which have often been 

 designated as radiation potentials. 



The cell body discharges are usually spikes of high 

 voltage and are of quite long duration. The diflference 

 in duration between the potentials of a single axon 

 and of a single cell body has been established through 

 the microelectrode studies on spinal ganglia (65), on 

 the spinal cord (68), on the lateral geniculate body 

 and on the cerebral cortex (66). According to Wood- 

 bury & Patton (68) the duration of the spike is about 

 0.6 msec, for the axon and i msec, for the cell body- 

 Tasaki et al. (66) put the values as i msec, or less for 

 axonal response and 1.5 to 3 m.sec. for the cell body 

 respon.se. Frank & Fuortes found the respective values 

 for dorsal root fibers and cell body of the spinal 

 motoneurons to be 0.6 msec, and 1.6 msec, respec- 

 tively (35). 



NEUR.'^L MECH.^NISMS FOR THE EL.^BOR.JiTION OF 

 EVOKED CORTICAL POTENTI.ALS 



The generation of the radiation spikes of the evoked 

 potential is a relatively simple problem. It is a 

 generally accepted opinion that the initial sharp 

 spikes with short latency represent the arrival of 

 afferent impulses which are purely presynaptic in 

 nature. Sufficient evidence is available that the main 

 part of the surface positive wave of the prima r\ 

 response on which the presynaptic spikes may be 

 superimposed is made up of the discharges of the 

 cortical neurons. 



Types oj Neural Elements Im'nlvrd and 

 Their Mode of Action 



As to exactly what cortical elements are responsible 

 for the production of this potential and what is the 

 mechanism by which an afferent \olley initiates the 

 discharge of those elements, remain points to be 

 elucidated. Since there are few available data con- 

 cerning the roles played by different types of cortical 

 elements, the postulation of a mechanism for the 

 genesis of the evoked cortical potential must be made 

 on the basis of the histological organization of the 

 cerebral cortex and the estaljlished principles of 

 electrophysiology. In view of the fact that the basic 

 pattern of the cortical response to afferent impulses 

 appears remarkaijly constant throughout the sensory 

 cortex and that the sensory cortex in\'ariably receives 

 specific thalamic afferent fillers and possesses a verv 

 well developed granular layer, these latter two struc- 



tural characteristics must be taken into consideration 

 in offering any explanation of the e\oked cortical 

 potential. 



The specific afferent fibers arising from the thala- 

 mus are known to terminate mainly in the fourth 

 layer by a rich plexus of repeatedly arborized endings. 

 Afferent impulses coming along these fibers make their 

 first synaptic contact with Golgi type II cells in the 

 fourth layer. Golgi type II cells are characterized 

 by the presence of short axons terminating in profuse 

 arborizations in a localized region surrounding the 

 parent cell body. Their dendrites are rather few and 

 poorly de\eloped. By virtue of their anatomical char- 

 acteristics they are not able to transmit impulses to 

 distant regions but serve as amplifiers by which the 

 afferent impulses are reinforced. They are un- 

 doubtedly indispensable for the elaboration of the 

 evoked cortical potentials. However, since there is no 

 definite orientation of the conducting structure of 

 these cells and the electric field created by the dis- 

 charge of these cells is a closed type (48), their activitv 

 cannot be recorded as any sizable potential from the 

 cortical surface. The consequence of the discharge of 

 Golgi type II cells in la>er IV is probably to activate 

 the star p\ ramids and the star cells in the same layer 

 which in turn activate the numerous medium and 

 small p\raniids which eventually depolarize the large 

 pxramids in the fifth and sixth layers. The large 

 p\ ramids send out efferent axons to some other parts of 

 the central nervous s\stem. 



The Initiation of Postsynaptic Impulses 

 in Pyramidal Neurons 



The detectable surface-positive potential can be 

 reasonably assigned to the propagation of nerve 

 impulses along the vertically oriented apical dendrites 

 of different sized cortical pyramids. Under normal 

 conditions the depolarization process of cortical 

 pyramids resulting from a supraliminal synaptic 

 excitation is apt to start at the somatic membrane 

 around the cell body rather than at the terminal 

 portion of the dendrites. According to a recent 

 postulation (17, 23) the pericorpuscular synaptic 

 knobs constitute the inost effective apparatus for the 

 initiation of a postsynaptic discharge, whereas the 

 subliminal excitation of paradendritic synapses can 

 produce only electrotonic changes and so modifv the 

 state of excitability of the neuron. The paradendritic 

 synapses, because of their lower density of distribution 

 and their special manner of contact with the next 

 neuron, are believed to be inadequate to effect a 



