HENSEL, H. 
Now, the question remained open whether specific cold recep- 
tors might be connected with non-myelinated C fibers; about 80 per 
cent of the afferent fibers in the cutaneous nerves are C fibers. 
Recently, we had the opportunity of recording afferent impulses 
from single C fibers in the external skin of the cat. This work has 
been done together with Dr. Iggo, Edinburgh, and Dr. Witt in our 
laboratory, and we found that many cold receptors as well as warm 
receptors are connected with non-myelinated fibers. 
Figure 3 illustrates the method. The saphenous nerve was dis- 
sected into very thin filaments in order to record impulses from 
single non-myelinated C fibers. The C fiber was identified by 
measuring its conduction velocity. This was accomplished by apply- 
ing electrical stimuli to the nerve dist ally from the recording point. 
The impulses were recorded by means of two oscillographs; one 
oscillograph was used for normal recording of the spike, whereas 
the sweep of the second oscillograph was triggered by the electric 
stimulus. The conduction velocity could be found by measuring the 
distance between the stimulating and the recording point and by 
measuring the time between stimulus and the onset of the C fiber 
impulse at the recording electrode (Fig. 4). By means of this method, 
the conduction velocity of the non-myelinated temperature fibers of 
the skin turned out to be in a range of 0.5 to 1.5 m/sec. 
Figure 5 shows the discharge of a single non-myelinated cold 
fiber when cooling the skin. After the onset of the discharge, a 
partial adaptation can be seen. Even cooling by several tenths of a 
degree centigrade is sufficient to cause a marked increase in the 
impulse frequency. The C fiber also shows a steacfy discharge at 
constant skin temperatures. The short inhibition of the discharge 
before the onset of cooling is due to a slight increase in temperature. 
Plotting the discharge frequency of a single C fiber against time, 
we get curves as shown in Figure 6. Sudden cooling causes a phasic 
increase in frequency. On rewarming, there is a transient inhibition 
and then the discharge reappears and finally reaches the initial level. 
Figure 7 shows the discharge frequency of a specific non- 
myelinated warm fiber. The plots of the impulse frequency against 
time of this warm fiber show a phasic increase in frequency during 
warming (Fig. 8). Even warming by several tenths of a degree 
centigrade is sufficient to cause a marked increase in frequency. 
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