THERMORECEPTOR PHYSIOLOGY 
On cooling, there is a transient inhibition of the steady discharge, 
and on rewarming the discharge comes again. 
As far as we know, all specific thermoreceptors in the cat's 
external skin are connected with C fibers. This is quite surprising 
because, according to the classic concept, the C fibers were sup- 
posed to be connected with pain. Later it was found that they were 
sensitive to mechanical stimulation. Now we know that the C fibers 
respond also to cooling and warming. 
At constant temperatures the non- myelinated cold and warm 
fibers of the skin show a steady discharge, the frequency of which 
is dependent on theabsolutetemperatureof the skin. Figure 9 shows 
an example for three different thermosensitive C fibers. The max- 
imum of the warm fiber is above 40 C, whereas the cold fibers 
have maxima at lower temperatures. It is difficult to define from 
the steady discharge alone whether a receptor is a cold or a warm 
because the frequency curve has a positive and a negative tempera- 
ture coefficient. But if we considerthe discharge during temperature 
changes, a cold receptor will react in the whole temperature range, 
with an increase in frequency during cooling and a decrease during 
warming. The opposite is true for a warm receptor. 
I think it is quite important to get some information not only 
about the impulse pattern in thecat, inthe dog, or in other animals, 
but also in human subjects, because it is very difficult to draw any 
conclusion from animal experiments as to the behavior of human 
cutaneous receptors. First, the whole pattern of receptors might be 
quite different in human skin as compared with the cat's skin. The 
cat has a fur coat and probably a different distribution of receptors. 
Second, the cat would hardly tell you about its sensations. This 
problem becomes especially difficult in the case of non-specific 
receptors respondingto cooling as well as to mechanical stimulation. 
Therefore, we have tried recently to record afferent impulses from 
human cutaneous nerve fibers. This work has been done in coordin- 
ation with Dr. Boman during the last summer in our laboratory. 
Figure 10 shows the first record from a single specific cold 
fiber in the human hand. The impulses were recorded from fibers 
running in the superficial branch of the radial nerve. The general 
behavior of this receptor is very similar to that of the cold receptors 
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