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HANDBOOK OF PHVSIOLOGV 



NEUROPHYSIOLOGY I 



the posterior third of the complex this difference is 

 less obvious (50). It is the thalamic area as defined 

 here which receives the terminals of axons of the as- 

 cending lemniscal system (40, 47, 48, 56, 67, 1 70, 

 200, 203, 242, 252, 260), and it is this thalamic com- 

 plex alone which undergoes retrograde degeneration 

 following lesions of the cortex confined to the post- 

 central homologue (49, 50, 143, 253). The method 

 of local strychninization and observation of the in- 

 duced behavioral changes yielded results in accord 

 with these facts, though the method is too crude 

 for any detailed analysis (59, 60). Finally, electro- 

 physiological experiments are consistent with the 

 notion that the principal area of the thalamus acti- 

 vated by tactile and kinesthetic stimulation of the 

 body is coextensive with the ventrobasal complex 

 (164, 182, 184, 185, 207). Two questions in this regard 

 require further comment. 



DIRECT SPINOCORTICAL AND BULBOCORTICAL PATH- 

 WAYS. It is an old suggestion that certain ascending 

 sensory somatic fibers of spinal or dorsal column nuclei 

 origin might reach the cerebral cortex directly without 

 an intervening synaptic relay in the diencephalon. In 

 1890, Flechsig & Hosel (69) put forward this conten- 

 tion, having found some degenerations in the medial 

 lemniscus of a patient who died following a lesion be- 

 lieved to be limited to the cerebral cortex. This idea 

 was supported by Tschermak (240) but vigorously 

 opposed by other workers who failed to confirm 

 Flechsig & Hosel's observation and who concluded 

 that the fibers of the medial lemniscus all terminate 

 in the diencephalon. This latter view is widely sup- 

 ported by virtually all more extensive neuroanatom- 

 ical studies and prevails even though some dissent- 

 ing observations are occasionally described (186). 



Recently Brodal & Walberg (35) and Brodal & 

 Kaada (33) revived again the question of the exist- 

 ence of both the direct bulbocortical and the spino- 

 cortical tracts. The first is stated to arise from cells of 

 the dorsal column nuclei and to project bilaterally 

 upon the cerebral cortex by a pathway which joins 

 the pyramidal tracts of either side. The second is be- 

 lieved to derive from neurons of the spinal cord and 

 to ascend directly therefrom to the cortex in the pyram- 

 idal tracts. Both pathways are said to be activated 

 by electrical stimulation of either cutaneous or muscle 

 nerves. However, the anatomical evidence adduced 

 by Brodal & Walberg does not appear to be suffi- 

 ciently crucial to settle this old dispute, and the 

 electrophysiological observations of Brodal & Kaada 



need not imply the existence of such direct pathways 

 according to the findings of Patton & Amassian 

 (193) and of Landau (148). 



IPSILATERAL PATHWAY FROM DORSAL COLUMN NUCLEI 



TO VENTROBASAL COMPLEX. It is clear from a large 

 number of studies that, in so far as anatomical 

 methods can determine, the entire upward outflow of 

 the dorsal column nuclei ascends to the thalamus of 

 the contralateral side and terminates largely within 

 the ventrobasal thalamic coinplex. These observations 

 accord well with the results of electrophysiological 

 mapping experiments, which indicate that only the 

 contralateral body surface is projected via the lem- 

 niscal system upon the ventrobasal complex, while 

 the trigeminal component of this system does contain 

 an ipsilateral component, partially overlaid with the 

 contralateral one. This pattern of projection is fur- 

 ther confirmed by our single unit observations in the 

 thalamus (Mountcastle, V. B. & J. E. Rose, unpub- 

 lished observations). Moreover, single unit studies of 

 the postcentral homologue in cats and monkeys 

 indicate that its cells are activated only by stimulation 

 of the contralateral body surface, except for the tri- 

 geminal inflow (181; and Mountcastle, V. B. & 

 T. P. .S. Powell, manuscript in preparation). Many 

 observers do not agree with these findings, however, 

 and they report ipsilateral responses in the region of 

 the thalamus, evoked by natural stimuli or by periph- 

 eral nerve, brachial plexus or dorsal column electrical 

 stimulation (20, 51, 52, 90, 91, 116) although Berry 

 et al. (20) found that direct electrical stimulation of 

 one dorsal column evokes electrical activity only in 

 the contralateral thalamus. The latter observation is 

 of interest for it may provide a clue for the interpre- 

 tation of the divergent findings. E\idence is accumu- 

 lating (see p. 419) that in contrast to the medial 

 lemniscal system the spinothalamic system does pos- 

 sess an ipsilateral component from the body surface, 

 which may terminate partlv or wholly in the .segment 

 of the posterior thalamic group which adjoins the 

 ventrobasal coinplex posteriorly. It seems possible 

 that the workers who obtained ipsilateral responses 

 from stimulation of the body surface or nerves ob- 

 tained them actually in the region which lies pos- 

 teriorly to the ventrobasal complex. While this inter- 

 pretation would harmonize the existing discordant 

 findings, it would not immediately explain why ip- 

 silateral stimuli fail to actix'ate (at least under condi- 

 tions of moderate anesthesia) the ventrobasal complex 

 itself — as could be expected — unless one assumes that 



