4i6 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



spinothalamic system is quite inadequate since many 

 basic questions concerning it are still not solved. As 

 we define the system it consists of the spinothalamic 

 tract arising in the posterior horns and of the bulbo- 

 thalamic tract originating in the spinal nucleus of the 

 fifth nerve. ^ The system is known to transmit impulses 

 provoked by painful and thermal stimuli but there is 

 adequate evidence as well that some tactile impulses 

 are also relayed through it. 



Location of Tactile Fibers m Sjntuithalamic System 



It is customary to distinguish within the spinotha- 

 lamic system of the spinal cord (though not in the 

 bulbothalamic tract) a ventral and a lateral spino- 

 thalamic pathw-ay. The first is assumed to conduct 

 tactile impulses, the second is known to be important 

 for arousal of painful and thermal .sensations. The 

 ventral spinothalamic tract is usually believed to lie 

 in the medial aspect of the anterolateral column. 

 Clinical experience in man indicates that some fibers 

 in this column must be concerned with touch since 

 tactile anesthesia results only if in addition to a de- 

 struction of a posterior column on one side a contra- 

 lateral injury is present .somewhere in the region of the 

 anterolateral column. On the other hand, only 

 partial impairment in tactile sensation occurs when 

 either of these columns is .selectively injured. In fact, 

 it has been frequently believed in the past that a 

 destruction of the anterolateral column alone does 

 not lead to any deficits in tactile sensations. Foerster 

 & Gagel (71), Foerster (70) and KroU (141), however, 

 using finer testing techniques were able to determine 

 some such deficits after anterolateral cordotomies. 



- Similar to the uncertainty which prevails in respect to the 

 secondary trigeminal pathways arising in the main sensory 

 nucleus of the fifth nerve (sec p. 396), a considerable confusion 

 and controversy exists in regard to the central course of the 

 bulbothalamic tract. Wallenberg (259) who described this 

 pathway maintained that it ascends in the dorsolateral portion 

 of the reticular substance of the brain stem tegmentum. He 

 believed that it terminates in the region of the centrum medi- 

 anum and in the arcuate component of the ventrobasal com- 

 plex. Biirgi's observations (36) imply that some of these fibers 

 may end also in the n. lateralis posterior. Although Wallen- 

 berg's findings as to the course of this tract in the brain stem 

 have been repeatedly confirmed (93, 241), many observers 

 conclude that the bulbottialamic tract crosses to join the medial 

 lemniscus and separates from it again at the level of the mid- 

 brain to join the spinothalamic tract. We believe that Wallen- 

 berg's original description is likely to be correct and that con- 

 trary results are probably due to lesions involving the posterior 

 column nuclei. For a contrary view and the review of the litera- 

 ture on this subject see Biirgi (36). 



The deficits are generally described as an increase in 

 threshold for tactile stimuli and a decrease in the 

 number of 'sensory spots' without any readily detect- 

 able iinpairment in the capacity to localize the 

 stimuli or to discriminate between them. These ob- 

 servations have been confirined at all levels of the 

 spinothalamic system by a number of subsequent 

 workers who were interested in this problem (58, 

 109-111, 256, 270, 274). A striking aspect of tactile 

 impairment is that tickle sensations disappear with 

 some lesions of the anterolateral column and that 

 with bilateral lesions severe disturbances of sensations 

 in the sexual sphere are present. 



Even though it is established that the spinothalamic 

 system must relay some tactile impulses, any exact 

 definition of the fibers concerned and therefore the 

 very existence of a separate ventral spinothalamic 

 tract as a tactile component of the spinothalamic 

 system seems to be based mainly on suppositions. 

 Foerster & Gagel (71) snd Foerster (70) concluded 

 that fibers concerned with temperature lie dorsally 

 to the fibers concerned with pain in the lateral spino- 

 thalamic tract and they assigned on a hypothetical 

 ijasis the anterior column to touch and pressure. 

 Walker (255) modified this scheme and believed the 

 fibers concerned with touch to lie in the most medial 

 aspects of the anterolateral column and, although he 

 emphasized the apparent overlap, he retained the 

 basic sequence of separate fiber systems for tempera- 

 ture, pain and touch. Many recent observers stress 

 the apparent or real overlap of fibers concerned with 

 pain and temperature [for a review of the literature 

 see White & Sweet (273)], but they are usually non- 

 committal on the problem of touch. Apparently this 

 is so because touch deficits resulting from anterior 

 cordotomy, or tractotomies performed at the level of 

 the medulla, pons or midbrain are of little or no clini- 

 cal discomfort to the patients, because they are diffi- 

 cult to detect and evaluate without special tests, and 

 because many observers were primarily interested in 

 the problem of pain. In consequence, despite the very 

 large number of operati\e procedures performed in 

 man on the spinothalamic system there is still no con- 

 clusive evidence as to whether touch deficits, such as 

 they are, result from injury of a separate sector of the 

 spinothalamic tract or whether fibers concerned with 

 touch are modality specific but are intermingled with 

 other fibers of the system. Finally, it is possible that no 

 modality specific tactile fibers exist within the system. 

 The fact that after anterior cordotomies tickle sensa- 

 tions in the analgesic areas have been reported as 

 alwavs lost (70, 71), as almost always preserved (134) 



