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HANDBOOK OF PH'iSIOLOOV 



NEUROPHYSIOLOGY I 



ber of reasons actually not realized, and that tactile 

 and kinesthetic stimuli activate usually only a limited 

 number of synaptic regions, even though this number 

 varies considerably under difTerent experimental con- 

 ditions. 



In the sections which follow we shall concern our- 

 selves almost exclusively with pathways and synaptic 

 regions \vhich are demonstrably significant for tactile 

 and kinesthetic sensations. We shall not consider the 

 problem of how stimulation of tactile and kinesthetic 

 receptors may affect other activity in the central 

 nervous system. 



Classification oj Central Tactile and fiiiieslhetic Systems 



It is well known that tactile or kinesthetic dis- 

 charges or both are conducted centripetally in the 

 posterior columns of the spinal cord, in its antero- 

 lateral columns and in the trigeminal pathways. It 

 also seems evident that such impulses can enter the 

 central nervous system through the roots of the ninth 

 and tenth cranial nerves, and there is good evidence 

 (197) that some chorda tympani fibers may be acti- 

 vated by mechanoreceptors. 



It is both convenient and almost certainly correct 

 to consider together the systems arising in the posterior 

 column nuclei and the one arising in the maiia sensory 

 trigeminal nucleus. We shall refer to them as com- 

 ponents of the inedial lemniscal system. Likewise, we 

 shall consider the spinothalamic system as consisting 

 of two components. The first is the spinothalamic 

 tract arising in the posterior horns of the spinal cord 

 and the second is the spinothalamic tract arising in 

 the spinal nucleus of the fifth nerve. W'e shall refer to 

 the latter, following White & .Sweet (273), as the 

 bulbothalamic tract. 



MEDI.'^L LEMNISCAL SYSTEM 



Anatamical Definition 



This system is the better known of the two, both 

 anatomically and functionally. Anatomically we shall 

 mention here only the centripetal terminations of the 

 successive axons in the system. Some collateral con- 

 nections relevant for our considerations will be dis- 

 cussed later. The spinal component of the system is 

 formed by axons emanating from the cells of the 

 spinal ganglia and ascending on the homolateral side 

 of the cord in the posterior column and synapsing on 

 cells in GoU and Burdach's nuclei; by axons originat- 



ing from the cells of these nuclei, crossing (as far as is 

 known, entirely) to the opposite side and ascending in 

 the medial lemniscus to end upon the cells of the 

 external component of the thalamic ventrobasal 

 complex; and by axons originating in the cells of the 

 latter element and projecting upon the postcentral 

 cortex or its homologue.' 



The trigeminal component of the lemniscal system 

 arises in the main sensory nucleus of the fifth nerve. 

 There is complete agreement among most observers 

 that the main outflow of this nucleus consists of axons 

 crossing to the opposite side. The pathway adjoins 

 mediodorsally the medial lemniscus, forms an integral 

 part of it and terminates in the arcuate component of 

 the ventrobasal complex. The cells of the latter 

 project, as do the cells of the external element, upon 

 the postcentral cortex. 



In addition to the crossed \entral pathways men- 

 tioned aijo\e, a dorsal pathway originating in the 

 main sensory nucleus and reaching the thalamus via 

 a tegmental route is frequently described. Consider- 

 able uncertainty prevails, however, about the origin 

 of this tract, its components and its terminations in 

 the thalamus. Wallenberg (261) described an un- 

 crossed and a crossed component and believed that 



' It is customary to consider n. vcntialis posteromedialis and 

 n. ventralis posterolateralis as the two tactile thalamic nuclei. 

 We refer, however, to the principal tactile thalamic region as 

 the ventrobasal complex and distinguish within this complex the 

 arcuate or medial component, which receives the trigeminal 

 projection, and the external or lateral component, which 

 recei\es projections from the rest of the body (207). The reasons 

 for this nomenclature are as follows. First, the two components 

 of the ventrobasal complex are almost identical in their archi- 

 tecture and for that reason should not be divided into two 

 separate nuclei. Actually in the rabbit such a separation is very 

 difficult while in the cat and monkey it is best done on the basis 

 of a dividing fibrous lamella. Second, most workers include 

 into their n. ventralis posteromedialis a ventromedial element 

 which is not activated by tactile stimuli and which displays 

 structural characteristics of its own, which are different from 

 those of the arcuate portion of the ventrobasal complex. Only 

 Jimenez-Castellanos (136) and Jasper & Ajmone-Marsan (135) 

 do not include this element in the n. ventralis posteromedialis. 

 The latter workers, however, consider as a part of this nucleus a 

 portion of the posterior thalamic group. Likewise, n. ventralis 

 posterolateralis is with many workers not coextensive with the 

 lateral component of the \'entrobasal complex. Thus Olszewski 

 (i8g), for example, distinguishes within his n. ventralis posterior 

 lateralis an oral part which is not activated by tactile stimuli 

 and a caudal part which corresponds probably exactly to the 

 lateral component of the ventrobasal complex. 



In respect to the sensory somatic cortical field we shall use 

 interchangeably the following terms: first somatic field, post- 

 central cortex (areas i to 3 in primates), postcentral homologue 

 and primary receiving area. 



