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various conditions of the skin's thermal stimulation, but we can define the following
two main conditions:
Exposure to a radiant source: In these conditions, the skin temperature increases
gradually in 5 to 10 seconds, more or less linearly, depending on time (ramp
function);
A direct contact suddenly made with a cold or hot surface: In this case, the skin
temperature changes rapidly.
3.3.2.3 Mechanoreceptors
The mechanoreceptors play an important role in tactile perception. These receptors
are the ones that are most frequently called into action when a contact is established
between the skin and an object (or material) from the outside world. These neurons
are located in the entire body and can be free nerve endings or can also be closed
in the form of small bulbar, axial or cylindrical corpuscles. The distribution of these
mechanoreceptors and the combination of their efferent signals in response to the
stimuli involve a specialisation of the sensory structure.
Density of distribution
The skin has a very rich afferent innervation; its density varies significantly from one
place to other. For example, the tips and the flesh of our fingers have a large num-
ber of nerves (about 2500 receptors/cm 2 ) 4 . The surface of our hand that is used to
grip is estimated to have about 17000 mechanoreceptors. The distance between them
varies between 0.7mm on the fingers and 2mm on the palm. These different densi-
ties of receptors (depending on the skin area) lead to different sensitivity thresholds
(Moss-Salentijn, 1992). The skin's sensitivity threshold to a mechanical stimulation
corresponds to about 6
m deep static push on the skin and varies tremendously with
the location of the stimulus. The just-noticeable difference (or differential threshold)
in pressure also varies significantly from one area to other. The lowest differential
fraction, also known as the Weber ratio 5 , is about 5% for the skin's pressure sensitiv-
ity. The spatial discrimination threshold measured using compass dividers (Johnson &
Phillips, 1981) also varies with the location of the stimulus (Figure 3.21). The lowest
thresholds are located at the tip of the tongue and fingers; the back has the highest
spatial discrimination threshold.
µ
Classification of mechanoreceptors
Mechanoreceptors in the human body are characterised by their functional aspect, their
layout (distribution, location, etc.) or even by the nature of their temporal responses. It
is thus necessary to introduce different mechanoreceptors considering all these aspects.
We have classified the mechanoreceptors on the basis of three notions - space, time and
4 Including all types of receptors: thermal, mechanical, etc.
5 Corresponds to the fraction closest to the value of stimulus perceived as being different with
respect to the nominal value of stimulus.
 
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