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HANDBOOK OF PHYSIOLOCV 



NEUROPHVSIOLOOV I 



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FIG. 2. Cortical cell responding specifically to cooling of the 

 tong\ie (cat). A, water of 1 1 °C and B, water of 37 °C were 

 applied to the tongue (signal on lower beam). Time, 50 cps. 

 C and D show the same cell responding to electrical stimulation 

 of the tip of the tongue. .S', stimulus artifact. Note that the 

 first spikes do not appear until the falling phase of the primary 

 cortical response. Time, 5 msec. Negativity upwards in all 

 records. [From Landgrcn, S-, personal communication.] 



of sensory systems is preserved in the thalamocortical 

 projections. The medially situated arcuate nucleus 

 receiving impulses from the face projects near the 

 Sylvian fissure. The lateral part of the posterolateral 

 nucleus, receiving impulses from the leg, projects 

 near the mid-line. The projection from the arm is 

 intermediate in both thalamus and cortex. As far as 

 anatomical studies have revealed, the sensory body 

 surface is projected upon the postcentral gyrus with 

 spatial relations preserved but in an opposite direction 

 compared with that in the thalamus. Lacking any 

 direct evidence of the localization of the third thermal 

 neuron in the thalamus we have to suppose that the 

 original peripheral topographical organization of the 

 thermal units is regained at this level. 



Electrical stimulation of the somesthetic areas of 

 the cortex made on conscious patients (16, 71) gives 

 rise to localized sensations. The most usual responses 

 are numbness, tingling and a feeling of movement 

 and only more rarely warmth and cold are experi- 

 enced. When recording from single cortical cells in 

 the cat by means of fine microelectrodes, Cohen et al. 

 (15) have found cells in the tongue sensory area which 

 respond specifically to cooling of the tip of the tongue 

 but not to mechanical or taste stimuli. Further investi- 

 gations by Landgren (63) show that in response to 

 cooling of the tongue cortical cold cells produce a 

 discharge, the latency, frequency and duration of 

 which is dependent upon the strength of the thermal 

 stimulus. The shortest recorded latency of the specific 



cortical cold cells to an electric .shock to the tongue 

 was 0.015 sec. compared to 0.005 ^ec. of a cortical 

 touch cell within the same area. The shortest latency 

 recorded to cooling of the tongue was about 0.02 sec. 

 (fig. 2). The receptive fields of the specific cold cells 

 were limited to the tip or the lateral edge of the 

 tongue. Besides these specific cells other cells were 

 found which responded to mechanical as well as to 

 thermal stimuli, occasionally also to taste stimuli. 

 These nonspecific cortical cells showed much longer 

 latencies (0.08 to 0.30 sec.) which suggests that they 

 cannot be primary. So far only one cortical cell 

 responding specifically to warming the tongue was 

 found. It thus looks as though the thermosensitive 

 imits are represented in the cortex topographically in 

 much the same way as on the surface of the body. The 

 fact that we have found in the somesthetic cortical 

 areas cells which respond specifically to cold or to 

 warmth does not exclude the possibility that there are 

 peripheral afferent neurons which respond to thermal 

 as well as to mechanical and noxious stimuli. Such 

 neurons can, however, scarcely contribute to the 

 specific thermal discrimination. For that purpose we 

 have to reckon with the activity of specific peripheral 

 neurons finally activating specific cortical neurons. 

 All previous speculation of a possible frequency code 

 is not only incompatible with Johannes Miiller's 

 law of specific sensory energies as currently conceived 

 but also with recent electrophysiological investiga- 

 tions of the impulse traffic in sensory nerve fibers. 

 Although many have looked for facts indicating some 

 kind of frequency code there is to date very little 

 evidence that frequency modulation in the sensory 

 nerve can influence anything but the intensity of the 

 cortical events underlying the sensation. This opinion 

 will not be changed if nerve fibers are found with 

 endings which are not strictK functionally specific. 

 Some of the unmvelinated afferent cutaneous fibers 

 are most probably acti\ated by strong abnormal 

 stimulation as well as by inechanical stimulation. 

 The interference of these fibers with the activity 

 of more strictly specific fibers inay very well under- 

 lie such cutaneous sensations as hot and tickling 

 which possess something more than one sensory 

 quality. No nerve endings are aijsolutely specific as 

 they are all excitable by electrical stimulation or by 

 strong mechanical or chemical stimulation. Thus 

 when we speak of specific nerve endings from a func- 

 tional point of view, we refer only to such sensory 

 end organs as are specific within reasonable limits. 



