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Heat curve
Cold curve
Temperature (C)
Figure 3.20 Response of thermoreceptors
varies with the temperature. Thermal receptors are formed by free nerve endings.
The arguments provided to separate these two sensitivities are (Ottensmeyer &
Salisbury, 1997):
For example, the nipples have more cold detecting points than the fingertips. The
forehead, cornea and conjunctiva have only the cold detecting points. The uvula
(the small piece of soft tissue that can be seen dangling down from the soft palate)
has only the heat detecting points;
Cold detection is quicker than heat detection, probably because the cold receptors
are in the upper layers compared to the heat receptors (cold receptors are found
at 0.15mm under the skin, whereas the heat receptors are at about 0.3mm).
Figure 3.20 shows the response of thermoreceptors (heat and cold) to a temper-
ature stimulus. The interesting point which we can observe on the left curve (for
cold receptor) is, since the response is maximum at 20 C, the neighbouring tem-
peratures (symmetric with respect to the maximum) and each temperature other
than 20 C provoke the same response and thus the same sensation for two differ-
ent temperatures. Another interesting observation is that thermoreceptors detecting
cold respond to stimuli that are higher than 45 C and provoke a cold sensation (called
paradoxical cold).
A trained observer can detect very small changes in temperature θ provided that
they are quick (for example, on 1 cm 2 of the palm, the observer can differentiate a drop
in temperature of
0.05 C/sec). By increasing area S of simulation, the differential
sensitivity will be more given the higher degree of spatial summation. The relation
obtained is θ
K log S (where K is a constant in C/m 2 ). On the other hand, a slow
variation (6 C-7 C in 20 to 30 minutes) will not be perceived by the observer. The
heat receptors will be activated only between 32 C and 55 C. A burning sensation
is observed beyond these values and is relayed by the nociceptors. Finally, there are
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