4i8 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



cited that the spinothalamic tract can usually be fol- 

 lowed with fair ease up to the region immediately 

 medial to the medial geniculate body. It is the area 

 between this region and the ventrobasal complex 

 itself in which so many Marchi granules disappear 

 with the result that the number of terminations in the 

 ventrobasal complex is often only scanty. The point 

 to be made is that this area, which is morphologically 

 a part of the posterior nuclear thalamic group (209), 

 is the critical region under consideration. 



In the cat; this region will remain essentially pre- 

 served after an extensive ablation of the suprasylvian 

 and lateral gyri and of the entire auditory region. It 

 will degenerate completely if in addition to this mas- 

 sive remoxal the second somatic aiVa is ablated as 

 well. Nevertheless, the removal of the second somatic 

 area alone will not cause any marked changes (209). 

 It appears then that the axons of this thalamic region 

 entertain connections (probably of collateral nature) 

 with the second somatic area. This conclusion is 

 harmonious with the findings of Knighton (138) who, 

 attempting to determine the thalamic relay nucleus 

 for the second somatic area, found that stimulation 

 of the posterior segment of what he beliexed to be the 

 n. ventralis posteromedialis activates the second 

 somatic field. From his drawings one can be fairly 

 confident that the actual locus of Knighton's critical 

 area was that segment of the posterior nuclear group 

 which is intercalated between the ventrobasal com- 

 plex and the medial geniculate. A similar interpreta- 

 tion applies, in our opinion, to the findings of Strat- 

 ford (235) who studied the corticothalamic projections 

 of the second somatic area by means of the strychnine 

 technique. 



The evidence which suggests that the second so- 

 matic area may be activated by a thalamic grouping 

 other than the classical tactile thalamic region agrees 

 also with the findings of Woolsey & Wang (278) and 

 VVoolsey (personal communication) who determined 

 that after ablation of the first somatic area in an acute 

 or chronic e.xperiment the responses in the second 

 somatic area are not detectably affected. 



It is tempting to assume that the second somatic 

 area is acti\ated solely by the spinothalamic system, 

 an assumption which, if true, could shed new light on 

 the function of this cortical region. This assumption, 

 however, implies that a destruction of both antero- 

 lateral columns should eliminate potentials evoked by 

 tactile stimuli in the second somatic cortex. While the 

 evidence in this respect is scanty, the findings at hand 

 imply that this is not the ca.se. 



Topical Orgariiz'iti'in (if Spinothalamic System 



We have already indicated that the a\ailable evi- 

 dence is inconclusive for deciding whether tactile 

 impulses are relayed within the spinothalamic system 

 in a separate spectrum of fibers or whether they are 

 transmitted partly or wholly by neurons which are 

 also utilized by discharges provoked by painful or 

 thermal stimuli. The existence of a specific tactile 

 pathway is inferred primarily from the observations 

 that thermal and painful .sensations may be affected 

 differentially by lesions of the spinal cord [for review 

 of literature see White & Sweet (273)]. The observa- 

 tions that such dissociations are not easily produced 

 even with shallow incisions into the spinothalamic 

 tract and that they are altogether rare or nonexistent 

 with extensive anterolateral cordotomies do not 

 militate, we believe, against the concept of separate 

 pain and temperature pathways as is occasionally 

 argued. 



Regardless of how the spinothalamic system may be 

 organized in respect to different modalities of sensa- 

 tions, the evidence is conclusive that it is topically 

 organized in respect to the body surface. Thus, the 

 dermatomes are described as projecting in an orderly 

 fashion upon the cells of substantia gelatinosa (237). 

 It has been deduced early (196) and since amply con- 

 firmed by virtually all who perform anterolateral 

 cordotomies that fibers concerned with the caudal 

 portions of the body lie laterally to those related to 

 more oral skin areas at any level of the spinal cord. 

 The same basic sequence prevails in the medulla (57, 

 218, 219, 271), apparently in the pons, in the mid- 

 brain (257) and in respect to the terminations in the 

 thalamus (44, 47, 263) although the details may vary 

 somewhat at different levels. 



Likewise, a topical organization of the trigeminal 

 fibers is well established. The anatomical evidence 

 indicates that the fibers of the mandibular, the maxil- 

 lary and the ophthalmic divisions of the fifth nerve 

 are arranged in a dorsoventral sequence in the spinal 

 tract [for reviews of the literature .see Astrom (19) and 

 Torvik (23B)], and this sequence has been confirmed 

 by electrophvsiological studies as well (172). In man, 

 the topical organization of this tract was inferred on 

 the basis of clinical observations (234) and these 

 deductions were proved substantially correct when 

 pain-relieving operations were introduced. Although 

 the opinions are not unanimous (64, 109, iii, 115, 

 133, 188, 202, 224), it is probable that in man, as in 

 others mammals, there is also a topical organization 



