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Adaptation
Slow
static responses
Fast
dynamic responses
Variable
Small, clear boundaries
Ridges and curvatures
Friction and variation
of forces
(43%)
Meissner
(25%)
Merkel
FA I
SA I
Large, unclear boundaries
Uniform
Lateral shearing and
forces
Mechanical transitions
and vibrations
(13%)
Pacini's
Golgi-Mazzoni
(19%)
Ruffini
FA II
SA II
Figure 3.22 Classification of mechanoreceptors on the basis of their time response
The mechanical variations are transferred into nerve impulses by the mechanorecep-
tors located on the peripheral layers of the skin. The exteroceptive nerve endings
are essentially located in the two main layers of the skin - the epidermis and the
papillary dermis. Four types of exteroceptive mechanoreceptors have been identified
(Johanson&Vallbo, 1979; Johnson&Phillips, 1981; Johansson et al., 1982; Johnson,
1983).
The first two are the Meissner endings located in our hand's skin between the
papillary ridges and the dermis and the Merkel endings located at the end of these
ridges. These two types are characterised by a large spatial resolution. The remaining
two types are Pacini's corpuscles and Ruffini endings. They are found deeper in the
skin (i.e. below the dermis) with larger reception fields.
We can create two major categories of human mechanoreceptors on the basis of
the nature of their response to a deformation experienced by the skin with respect
to the time factor (time function). The first category, slow adaptation units, includes
44% of the cases. These units respond with a steady release all through the stimulus.
The remaining 56% are the units having a faster adaptation; they respond with a
gust of impulses only when the state of the stimulus changes and not on its stages. In
each of these categories, we can further differentiate between two different types of
mechanoreceptors on the basis of the properties of their receptive fields. The following
cases can be observed (Johansson & Vallbo, 1983):
The Fast-Adapting Type I units (FA-I) and the Slow-Adapting Type I units (SA-I):
These two types have small and well-defined receptive fields (Meissner and Merkel
endings);
The Fast-Adapting Type II units (FA-II) and the Slow-Adapting Type II units
(SA-II): Unlike type I, type II are units having larger receptive fields with unclear
boundaries (Pacini's corpuscles and Golgi and Ruffini endings).
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