Page 327 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Sensory Functions of the Skin AP frequency is proportional to the vibration
frequency (! B3 ).
Somatovisceral sensibility is the collective Resolution. RA and SA1 sensors are densely
term for all sensory input from receptors or distributed in the mouth, lips and fingertips,
sensors of the body (as opposed to the sensory especially in the index and middle finger
organs of the head). It includes the areas of (about 100/cm ). They can distinguish closely
2
proprioception (! p. 316), nociception adjacent stimuli as separate, i.e., each afferent
axon has a narrow receptive field. Since the sig-
(! p. 318), and skin or surface sensitivity.
Central Nervous System and Senses objects (stereognosis). Tactile sensors are lo- and fingertips to distinguish between two
The sense of touch (taction) is essential for
nals do not converge as they travel to the CNS,
perception of form, shape, and spatial nature of
the ability of these sensors in the mouth, lips
closely adjacent tactile stimuli, i.e. their resolu-
cated predominantly in the palm, especially in
the fingertips, and in the tongue and oral cav-
tion, is very high.
ity. Stereognostic perception of an object re-
The spatial threshold for two-point discrimina-
quires that the CNS integrate signals from ad-
tion, i.e., the distance at which two simultaneous
jacent receptors into a spatial pattern and
stimuli can be perceived as separate, is used as a
coordinate them with tactile motor function.
measure of tactile resolution. The spatial thresholds
Mechanosensors. Hairless areas of the skin
are roughly 1 mm on the fingers, lips and tip of the
the arm, and over 60 mm on the back.
which are afferently innervated by myelinated
SA2 receptors and pacinian corpuscles have a
nerve fibers of class II/A! (! p. 49 C):
broad receptive field (the exact function of SA2 re-
corpuscle
! The
Ruffini’s
spindle-shaped
12 contain the following mechanosensors (! A), tongue, 4 mm on the palm of the hand, 15 mm on
ceptors is not known). Pacinian corpuscles are there-
(! A3) partly encapsulates the afferent axon
branches. This unit is a slowly adapting (SA) fore well adapted to detect vibrations, e.g., earth
tremors.
pressosensor of the SA2 type. They are P sen-
sors (! p. 312). Thus, the greater the pressure Two types of thermosensors are located in the
on the skin (depth of indentation or weight of skin: cold sensors for temperatures ! 36 "C and
an object), the higher the AP frequency (! B1). warm sensors for those # 36 "C. The lower the
! Merkel’s cells (! A2) are in synaptic contact temperature (in the 20–36 "C range), the
to meniscus-shaped axon terminals. These higher the AP frequency of the cold receptors.
complexes are pressure-sensitive SA1 sensors. The reverse applies to warm receptors in the
They are PD sensors (combination of B1 and 36–43 "C range (! C). Temperatures ranging
B2) since their AP frequency is not only de- from 20" to 40 "C are subject to rapid adapta-
pendent on the pressure intensity but also on tion of thermosensation (PD characteristics).
the rate of its change (dp/dt; ! p. 312). Water warmed, for example, to 25 "C initially
! Meissner’s corpuscles (! A1) are composed feels cold. More extreme temperatures, on the
of lamellar cell layers between which club- other hand, are persistently perceived as cold
shaped axons terminate. This unit represents a or hot (this helps to maintain a constant core
rapidly adapting pressure sensor (RA sensor) temperature and prevent skin damage). The
that responds only to pressure changes, dp/dt density of these cold and warm sensors in
(pure D sensor or velocity sensor). The RA sen- most skin areas is low as compared to the
sors are specific for touch (skin indentation of much higher densities in the mouth and lips.
10–100µm) and low-frequency vibration (10– (That is why the lips or cheeks are used for
100 Hz). Hair follicle receptors (! A5), which temperature testing.)
respond to bending of the hairs, assume these Different sensors are responsible for thermoception
functions in hairy areas of the skin. at temperatures exceeding 45 "C. These heat sen-
! Pacinian corpuscles (! A4) are innervated sors are also used for the perception of pungent sub-
by a centrally situated axon. They adapt very stances such as capsaicin, the active constituent of
rapidly and therefore respond to changes in hot chili peppers. Stimulation of VR1 receptors
pressure change velocity, i.e. to acceleration (vanilloid receptor type 1) for capsaicin mediates the
314 (d p/dt ), and sense high-frequency vibration opening of cation channels in nociceptive nerve
2
2
(100–400 Hz; indentation depths ! 3µm). The endings, which leads to their depolarization.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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