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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



sector of the cord contralateral to the nerves; c) it 

 was increased in amplitude by such painful maneu- 

 vers as spinal root manipulation; and a') it was 

 promptly depressed by a deepening of anesthesia 

 toward a surgical level. 



By his special technique of stimulation Delgado 

 (62) has also obtained in monkeys evidence that the 

 inferomedial part of the mesencephalic central gray 

 matter is concerned with pain. The lateral part of 

 the mesencephalic tegmentum in the region occupied 

 by the spinothalamic tract and trigeminal lemniscus 

 has yielded similar responses. Delgado has im- 

 planted tiny multilead electrodes and then stimu- 

 lated via external leads from these after the wound 

 is closed and the animal is relatively free to move 

 about. At the aforementioned points stimulation 

 evokes the same sort of complex response that the 

 normal monkey makes to a peripheral annoyance 

 such as pinching his tail. Moreover in monkeys in 

 which such responses had been elicited electrically 

 the animal developed "conditioned anxiety,' i.e. as 

 soon as placed on the stimulation stage he screeched, 

 bit and tried to escape. This did not happen in mon- 

 keys in which purely somatic motor or autonomic 

 effects had been elicited. Hence Delgado assumed 

 that the sensations evoked from the mesencephalic 

 zones were painful and were remembered. 



The possible significance of these pathways may 

 be considered in relation to curious sensory changes 

 which rarely appear following thoracic or cervical 

 cordotomy, perhaps more often after bulbar spino- 

 thalamic tractotomy for pain and in many patients 

 after mesencephalic tractotomy. Dogliotti (67), the 

 first surgeon to divide pain pathways in the mid- 

 brain, reported that his three surviving patients had 

 "diffused disagreeable sensations" in the half of the 

 body contralateral to the incision. As described by 

 Drake & McKenzie (69), in all six of their patients 

 after the operation in the midbrain there was anal- 

 gesia and thermanesthesia throughout the opposite 

 side of the body and head for 3 to 15 days after 

 operation. Then pinprick, deep pressure or thermal 

 stimuli in all six patients and even light touch in 

 one of them caused deep diffuse poorly-localized 

 agonizing pain with strong withdrawal and grimac- 

 ing. In three of the patients there was spontaneous 

 burning pain in some part of the formerly analgesic 

 area. Drake suggests that the impulses causing these 

 pains traverse a secondary route via relay in the 

 reticular formation which is not cut by the incision 

 in the midbrain. Walker (279) had already noted 

 "the diffuse, disagreeable sensation which may be 



elicited by cold, extreme heat or pinprick, especially 

 by repeated stimulation" in some of his patients 

 after mesencephalic tractotomy. He suggested that 

 spinotectal tracts may be carrying such painful 

 impulses to higher centers. Bowsher (31) also pointed 

 out that bulbothalamic and tegmentothalamic tracts 

 running in the reticular formation are separate from 

 direct spinothalamic fibers between the level of the 

 inferior olive and the thalamus. He suggests that the 

 direct spinothalamic system transmits impulses for 

 pain which is felt at once, is sharply localized and 

 does not outlast the stimulus. He attributes to the 

 medially placed .spinoreticulothalamic system the 

 diffuse poorly-localized pain with an appreciably 

 slower conduction time which does outlast the stimu- 

 lus. Since mesencephalic incisions in man have 

 missed these fibers, this explanation would account 

 for the type of persistent pain shown by such patients. 

 One further observation of Drake & McKenzie 

 also fits in with this concept. One of their patients 

 preoperatively had had severe pain in the face. 

 Mesencephalic tractotomy replaced the original 

 pain by a diffuse facial burning sensation. Division 

 of all of the primary pain pathways from the face 

 by bulbar trigeminal tractotomy then gave complete 

 relief — perhaps because reticulothalamic pathways 

 could no longer be activated. 



TH.'^L.AMUS 



The fibers for touch and proprioception in the 

 medial lemnisci mix with those for pain and tem- 

 perature as they all terminate at the thalamic level. 

 In man when vascular lesions destroy the nucleus 

 ventralis posterolateralis severe sensory loss is found 

 in the contralateral limbs and trunk; the facial 

 fibers terminate in the nucleus ventralis postero- 

 medialis (200, 229}. These inferences from human 

 material have been confirmed and extended by the 

 more critical studies on Marchi material in lower 

 primates carried out by Clark (43) and Walker 

 (275, pp. 63 to 93). Moreover Walker's (275, p. 

 1 72) observations using the same technique have 

 revealed that these same thalamic nuclei project in 

 corresponding fashion to the postcentral gyrus of 

 the same cerebral hemisphere. The nucleus ventralis 

 posteromedialis sends fibers to the lowest or facial 

 sector of the postcentral gyrus, and the most lateral 

 parts of the nucleus ventralis posterior project to the 

 superior part of the gyrus. 



Foerster & Gagel (80), Rasmussen & Peyton 



