42-2 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



that the fibers of the medial lemniscus emit any sub- 

 stantial number of collaterals. Ramon y Cajal (201) 

 was emphatic in this respect and believed that only a 

 few collaterals were given off to the region of the red 

 nucleus and a somewhat larger number to the pretec- 

 tal region. He seems to have been hesitant in respect 

 to the medial lemniscal contribution to the mamillary 

 peduncle. Other workers occasionally described some 

 collaterals to other regions as well but there seems to 

 be no convincing evidence that the medial lemniscus 

 gives off any substantial number of collaterals below 

 the midbrain. Matzke (170) recently stressed that the 

 medial lemniscus does not decrease perceptibly in 

 size from its origin to its termination. 



The situation is different for the fibers arising in the 

 posterior horns and ascending in the anterolateral 

 column. We have considered thus far exclusively one 

 component of the group, the spinothalamic tract, 

 since it reaches farthest orally. However, it is well 

 known that there are other fibers accompanying the 

 spinothalamic tract which terminate below the thal- 

 amus and it is likely that at least some of them conduct 

 impulses evoked by cutaneous stimuli. The fact that 

 after anterolateral cordotomy only a small fraction of 

 the degenerating fibers reach the thalamus led, as 

 was discussed earlier, to doubts as to the existence of 

 the spinothalamic tract in some mammals. It appears 

 then that collaterals from the spinothalamic tract or 

 other anterolateral column fibers terminating at 

 lower levels could provide obvious pathways for 

 relaying sensory-somatic activity to the brain-stem 

 structures without any strain on the known anatomical 

 facts concerning the medial lemniscus. 



Second, it has been shown that the brainstem 

 potentials evoked by stimulation of the sciatic nerve 

 occur bilaterally (230). This observation is quite in 

 harmony with the presumed existence of the ipsi- and 

 contralateral tracts in the anterolateral columns. On 

 the other hand, the occurrence of such ipsilateral po- 

 tentials would be rather puzzling if they were medi- 

 ated by the lemniscal system since, despite some 

 protestations to the contrary, there is, we believe, no 

 evidence for existence of an ipsilateral lemniscus aris- 

 ing in the posterior column nuclei. 



Third, it has been shown that the potentials evoked 

 in the ascending reticular system by stimulation of the 

 sciatic nerve possess substantially longer latencies than 

 do responses recorded at virtually any level of the 

 lemniscal system (73). It is not at all clear why this 

 should be so if a collateral inflow for the ascending 

 activating system were indeed available almost at all 

 levels from the medial lemniscus. If, on the other 



hand, these potentials were evoked by mediation of 

 predominantly small fibers of the anterolateral col- 

 umns their long latencies could be readily understood 

 even if the number of intercalated synaptic regions 

 was quite small. That some potentials in the brain 

 stem are, as could be expected, relayed through the 

 anterolateral column has been shown recently by 

 Collins & O'Leary (54). These workers studied a 

 small region in the midbrain which was activated 

 when smaller fibers (gamma and delta groups) of the 

 radial or sciatic nerves were excited. They could show 

 that the midbrain potentials survived (in contrast to 

 the potential evoked in the ventrobasal complex) a 

 destruction of the homolateral posterior column 

 (homolateral to the stimulated nerve) but were 

 abolished (again in contrast to the potentials evoked 

 in the ventrobasal complex) when the contralateral 

 anterolateral column was destroyed. It may be em- 

 phasized that the midbrain potentials displayed much 

 longer latencies than did the potentials evoked in the 

 medial lemniscal system, and in contrast to the latter 

 were sensitive to anesthetic agents. 



In summary, it appears that the sources of the 

 sensory somatic inflow which activate the various 

 brain stem structures are not yet established une- 

 quivocally. The evidence at hand seems to imply that 

 the potentials recorded by the California workers in 

 the reticular activating system relay mainly or solely 

 through the anterolateral columns and the tracts aris- 

 ing from them rather than through the posterior 

 column and the medial lemniscal system. However, 

 these are indirect conclusions and it would \x desir- 

 al^le to test them experimentally. 



Cortical Fields Olliir Than the Primary Receiving Area 

 Which Are Activated by Tactile Stimuli 



We ha\e thus far proceeded on the assumption 

 that, among all the discharges in the central nervous 

 system which are provoked by cutaneous and deep 

 stimuli, only tho.se which occur in the medial lem- 

 niscal or spinothalamic systems are relevant for the 

 arousal of tactile and kinesthetic sensations. Clinical 

 evidence suggests that this is likely to be so for all the 

 synaptic regions below the cortical le\el. For the cor- 

 tex itself the situation is less clear mainly because one 

 is uncertain as to the extent of the cortical fields which 

 are directly activated by the subcortical components 

 of the classical systems. There is, of course, no doubt 

 that the postcentral region in primates or its homo- 

 logue in other mammals (which can be defined as the 

 projection field of the ventrobasal complex of the 



