402 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



demonstrated anatomically. The termination of 

 gracilis neurons in the lateral and of those of the 

 cuneate in the medial parts of the external component 

 of the ventrobasal complex, and of the trigeminal 

 tracts in its arcuate component, has been established 

 by degeneration experiments (47, 67, 203, 253, 260). 

 The same pattern is shown by the locations of retro- 

 grade degenerations produced by lesions of the face, 

 arm or leg areas of the postcentral gyrus (49, 50, 143, 



249. 253)- 



An important confirmation of the location of the 



somatic relay nucleus of the thalamus and the pattern 

 in it has come from the study of patients in whom the 

 ventral thalamic nuclei were stimulated by means of 

 stereotactically placed electrodes in the course of 

 thalamotomy for intractable pain (174, 175)- Stimu- 

 lation of the ventrobasal complex produced somatic 

 sensations referred to bodily parts in topographic 

 patterns similar to those in the monkey. The sensa- 

 tions produced by thalamic stimulation were referred 

 onl\' to the contralateral side of the bodv. 



Patterns in Postcentral Humologue oj Cerebral Cortex 



Since sensory somatic cortical projection patterns 

 are described by Terzuolo & Adey, Chapter XXXIII 

 of this work, it may he read for a surves' of this in- 

 formation and the description of relations between the 

 sensory somatic fields and the motor areas. 



Here we wish to stress a few generalizations impor- 

 tant for our considerations. The first essential point 

 which emerges from the extensive mapping studies 

 made mainly by Woolsey and his collaborators is that 

 the cortical pattern in every mammal studied is a 

 representation of the body form itself, with distor- 

 tions which are almost certainly due to differences in 

 the peripheral innervation density. These in turn ap- 

 pear clearly related to the development of one or an- 

 other part of the body of a given mainmal as a tactile 

 organ. Hence, the share of a body part in cortical 

 representation apparently reflects the relative value 

 of this part for tactile discriminatory acuity. 



The second point to be made is that the thalamic 

 pattern is projected in toto upon the cortical receiving 

 area with only such further distortions as could be 

 expected by the transfer of a three-dimensional pat- 

 tern upon essentially a two-dimensional surface. 

 While this statement is made on the basis of studies 

 done only in rabbit, cat and monkey, there is no rea- 

 son to doubt that it is true for other mammals as well. 

 There appears to be no part of the thalamic relay 



nucleus which is functionally independent of the 

 cortex, a finding which is concordant with the obser- 

 vation that an adequate cortical removal results in 

 virtually complete retrograde degeneration of the 

 ventrobasal complex. There is, therefore, no reason to 

 assume that this complex is a terminal station for any 

 sensory somatic processes in any mammal, even 

 though a contrary thought in this respect was fre- 

 quently entertained in the past, and is implicit in the 

 concept of 'thalamic sensations'. For any given mam- 

 mal, the pattern of cortical representation is probably 

 essentially similar not only to the respective repre- 

 sentation in the ventrobasal complex but to that in 

 the dorsal column nuclei as well. The a\ailai3le data 

 in this regard are very limited. It is clear enough, 

 however, that this holds true for the cat and the same 

 can be deduced for the macaque. Moreover, the long, 

 sentient and prehensile tail of the spider monkey 

 Ateles is known to have a large representation in the 

 dorsal column nuclei ('43) and this rather unusual dis- 

 tortion of the pattern is fully reflected in the cortical 

 representation (45). It appears then that all relay 

 nuclei of the system participate fully in elaboration 

 of the projectional pattern, as must be the ca.se if the 

 organization of this projection is correlated with the 

 peripheral innervation density. While this conclusion 

 appears to be almost self-evident, it may be useful to 

 stress it since even modern neurological thinking is 

 often unduly dominated by the concept of different 

 fimctional levels. This tends to neglect the viewing of 

 the synaptic regions of a system as integral parts of 

 the whole, if such regions happen to lie at different 

 topographical levels. 



An important question as to the functional mean- 

 ing of a morphological pattern is posed by the c\to- 

 architectonic differentiation of the postcentral hom- 

 ologue. While the number of fields distinguished here 

 may vary according to different criteria of various 

 workers, there is hardly any doubt that this region 

 possesses a definite gradient of morphological change. 

 In primates, areas 3, i, and 2 are classically distin- 

 guished in an orocaudal sequence and all these fields 

 together form the substrate for the representation pat- 

 tern of the body as determined by the evoked poten- 

 tial technique. It is possible that differences in or- 

 ganization of thalamocortical projections underlie 

 the cytoarchitectural differentiation of these fields. 

 In this relation it was reported recently (50) that at 

 least areas 3 and 2 differ substantially in this regard 

 from each other. Area 3 has been shown to receive 

 exclusive projections from the ventrobasal complex. 



