491 



(220} as well as Gardner & Cuneo (87) have been 

 able to follow only a few degenerating spinothalamic 

 fibers beyond the midbrain and into the thalamic 

 nucleus ventralis posterolateralis after thoracic 

 cordotomy in man. By the same Marchi method 

 Glees (98) insists that the degeneration after such an 

 operation is not in the posteroventral portion of the 

 lateral nucleus but dorsal to it; he sees the terminat- 

 ing fibers lying close to the nucleus lateralis posterior. 



Using stains for a.xonal degeneration Mehler (187) 

 has found that true spinothalamic fibers to the nu- 

 cleus ventralis posterolateralis constitute 30 per cent 

 of the ascending afferent fibers in the chimpanzee. 

 Moreover he also saw terminations in these thalamic 

 nuclei : parafascicularis, paracentralis and the small- 

 celled component of the nucleus centralis lateralis. 

 Bowsher (31) studying four patients following 

 thoracic, cervical or bulbar spinothalamic tractotomy 

 by either the a.xonal degeneration stain of Glees or 

 that of Nauta found 'a large amount of degenera- 

 tion' in the nucleus ventralis posterolateralis on the 

 side of the surgical lesion as well as a little in the 

 same nucleus on the other side. The fibers reach the 

 contralateral thalamus by way of the dorsal part of 

 the posterior commissure. Moreover Bowsher found 

 terminations in the relatively large nucleus centrum 

 medianum of man not hitherto described even in 

 lower primates. He also saw a few degenerating fibers 

 in the rostral part of the thalamic reticular nucleus. 



Although there are numerous careful studies of 

 thalamic action potentials evoked by electrical 

 stimulation of afferent nerves these have been cor- 

 related mainly with touch and have dealt largely 

 with the potentials conducted by the fastest fibers 

 presumably related to touch or proprioception. 

 However Gaze & Gordon (93, 94) recorded simul- 

 taneously the electrical activity of single neural 

 units in the thalamus and the compoimd action po- 

 tential from the saphenous nerve after stimulation 

 of this uncut nerve in the cat and monkey. Having 

 found an active thalamic unit, the investigators 

 then sought to determine the form of cutaneous 

 stimulus which would cause it to fire. The units 

 responding to electrical stimulus of alpha, beta or 

 gamma saphenous nerve fibers usually responded 

 also to light touch. Only a few of them required 

 strong mechanical stimuli. These comprised 80 per 

 cent of the total of 63 thalamic units found, whereas 

 only I 7 per cent responded to the stronger electrical 

 stimulus required to activate saphenous delta fibers. 

 Six-sevenths of these required stronger stimuli like 

 squeezing, pinching, tapping or pricking to activate 



them. Three units were found which responded to 

 stimulation of saphenous 'C fibers, but the cutane- 

 ous stimulus which would fire them was not identi- 

 fied. The mean latencies — knee to thalamus — were 

 23, 47 and 630 msec., respectively, for the three 

 groups. Some representation ipsilateral as well as 

 that contralateral to the stimulated nerve was found 

 in the monkey thalamus. There was no anatomical 

 segregation among the different types of unit de- 

 scribed and there was likewise a huge overlap be- 

 tween regions for face, forelimb and hindlimb both 

 in cat and monkey, responses from the face even 

 being obtained in the leg area. Upon subtraction of 

 the peripheral conduction time from the total la- 

 tency one obtained a central conduction time in- 

 cluding synaptic dela\' averaging 15 m per sec. for 

 the alpha, beta, gamma group, 7.9 m per sec. for the 

 delta groups and 0.66 m per sec. for C fibers. 



Dclgado's (62) monkeys with chronic implanted 

 electrodes also exhibited behavior suggesting pain 

 when the thalamic nucleus ventralis posterior was 

 stimulated. However, electrical stimulation within 

 the thalamus of conscious man as reported by 

 Talairach et al. (260) and by Hecaen et al. (124) did 

 not cause actual pain, although the centrum me- 

 dianum, the nucleus ventralis posteromedialis and the 

 nucleus medialis dorsalis were the presumed sites of 

 stimulation in five of their patients. Nevertheless the 

 making of electrical lesions mainly in the region of 

 the centrum medianum caused hypalgesia to anal- 

 gesia over varying extents of the contralateral half 

 of head, limbs or torso along with reduction or 

 elimination of the clinical complaint of contralateral 

 pain. In agreement with the observations that the 

 medial lemniscus terminates close to this area there 

 were in these patients also varying degrees of contra- 

 lateral loss of touch, position and vibratory sen.se 

 and stereognosis. 



Following thrombosis of the thalamogeniculate 

 artery in man there is typically an extensive de- 

 struction of the posterior part of the lateral nuclear 

 mass of the thalamus which contains the nuclei 

 receiving fibers from the general afferent systems and 

 projects to the postcentral gyrus of the cerebral 

 cortex. This lesion produces among other signs a 

 transitory complete contralateral hemianalgesia as 

 part of the classical thalamic syndrome of Dejerine 

 & Roussy (61). This sign soon gives way to painful 

 sensations upon noxious stimulation; later these 

 occur upon milder stimulation such as touch, vibra- 

 tion, pressure or sound; and finally there may ap- 

 pear a state of spontaneous, constant or paroxysmal 



