THERMAL SENSATIONS 



435 



upon stimulation of the nerve ending may be more 

 closely correlated with the most usual mode of stimu- 

 lation — which, for the superficially situated 'cold' 

 endings, is cooling of the skin. The discharges from 

 these endings are then transmitted to specific cells of 

 the cortex the activity of which will be labelled cold. 

 So far it is easy to follow the idea of Lele et al. (65). 

 But when these authors maintain that the thermal 

 nerve endings respond even to mechanical and noxi- 

 ous stimuli they diverge from the experimental evi- 

 dence, in that the activity of single temperature fibers 

 of the cat and the dog cannot be influenced by me- 

 chanical stimulation of their receptive field at least 

 within reasonable limits of stimulus strength. The 

 nonspecific response of certain mechanoceptive fibers 

 to cooling, as demonstrated by Hensel & Zotterman 

 (53), requires a sudden temperature rise of more than 

 8°C. Since the cold fibers from the facial region of the 

 cat which have been more closely studied (46) do not 

 behave differently from those of the tongue, there can 

 be hardly any doubt that these fibers possess endings 

 which are specifically stimulated when the layer of 

 the skin in which thev are situated is cooled. 



Afferent Nerve Paths 



Judging from the relative spike height Zotterman 

 (96) suggested that the cold fibers of the tongue of the 

 cat were fairly thin myelinated fibers belonging to the 

 6-group of the class A fibers according to Gasser & 

 Erlanger's nomenclature (fig. 10). Direct measure- 

 ments of isolated single cold fibers from the saphenous 

 nerve of the cat by Maruhashi et al. (68) gave diam- 

 eters of 1.5 to 3 ti. These fibers showed a punctiform 

 receptive field and were the smallest of the myelinated 

 fibers. They were sensitive neither to light touch nor 

 to pinprick. As the warm fibers give rise to spikes of 

 somewhat higher amplitudes, they are considered to 

 be of slightly greater diameters (54, 96). 



The central course of the temperature fibers in 

 man is only roughly known. After entering the spinal 

 cord via the dorsal roots the thermal fibers form a 

 lateral division which enters the dorsolateral fasciculus 

 or the tract of Lissauer. The fibers ascend only one to 

 three segments before terminating in the substantia 

 gelatinosa Rolandi, a cell column capping the poste- 

 rior horn with a .seemingly uniform texture containing 

 small cell bodies only and with no large myelinated 

 fibers traversing it. The axons of its small cells cross 

 the cord in the anterior gray commissure and ascend 

 in the lateral spinothalamic tract (76). In syringo- 

 myelia the fibers crossing in the narrow space of the 



anterior gray commissure are often destroyed which 

 leads to a well-known clinical syndrome characterized 

 by loss of pain, warmth and cold on both sides of the 

 body at the level of the segments involved while the 

 sense of touch and pressure is preserved. In some 

 patients there may be a dissociation between the 

 degree of impairment of heat and cold sensation. 



According to Haggqvist (36) as well as to Bailey & 

 Glees (3) the majority of the fibers in the spinothalamic 

 tract are 2 to 4 /x in diameter, 35 per cent are 4 to 6 /i 

 and only a few fibers run up to to fi in diameter. Thus 

 the dimensions of thermal and nociceptive peripheral 

 fibers seem to be preserved in the second order of 

 neurons. The spinothalamic tract is .so organized that 

 fibers ascending from the caudal region are pushed 

 outwards by the accretion of crossing fibers at each 

 successive segment (93). Fibers from the cervical 

 part are thus situated most anterior and medially. 

 This arrangement seems to maintain the topographi- 

 cal organization of the fibers into the cortical projec- 

 tion. 



The small-sized temperature fibers of the trigeminal 

 nerve follow the course of the pain fibers after entering 

 the brain stem into the elongated spinal nucleus which 

 extends through the medulla to meet the substantia 

 gelatinosa Rolandi (28). Division of this tractus 

 spinalis of the trigeminal nerve, the trigeminal tractot- 

 omy of Sjoqvist, in the medulla leads to an analgesia 

 and also to a fairly complete thermal anesthesia in 

 the opposite half of the face as well as to failure of 

 tickling sensations (79, 97). The exact localization of 

 the third thermoceptive neurons in the thalamus is 

 not known. The spinothalamic tract fiiaers from dififer- 

 ent levels of the spinal cord terminate in the postero- 

 ventral nucleus of the thalamus but in doing so they 

 interdigitate so much that the original peripheral 

 topography of fibers mediating different modalities 

 seems to be regained. In the ventral nuclei of the 

 thalamus the finer topographical organization has 

 been worked out by studying degeneration of the 

 fibers in the medial lemniscus and the spinothalamic 

 tract (13) but a still more detailed map was obtained 

 by Mountcastle & Henneman (69) by studying the 

 electric response appearing in the thalamus on stimu- 

 lation of points on the body surface. The body surface 

 is projected onto the thalamus, specifically onto the 

 posteroventral nucleus which is the only part in which 

 stimulation of the skin evoked any electric response, in 

 such a way that the head is represented posteromedi- 

 ally, the tail anterolaterally, the back superiorly and 

 the feet inferiorly. According to Ruch (76) this 

 topography manifested in the thalamic terminations 



