S om atic se n satio n s are se n se d by m any ty p e s o f re c e p to rs in th e skin
• Figure 8.2
Epidermis
Dermis
Subcutaneous layer
Nociceptors are free nerve endings that
sense pain rapidly. Other free nerve endings
sense tickle, temperature, itch, and some
touch sensations.
Meissner corpuscle senses
touch near surface.
Merkel (tactile) disc senses
touch near surface.
Ruffini corpuscle senses
stretching in deep layers of skin.
Hair
root plexus senses
movement of the hair.
Pacinian corpuscle
senses pressure rapidly.
c.
Special senses
Smell
Hearing and balance
Taste
Sight
changes in blood pressure and chemical levels (Fig-
ure 8.1b). Like somatic senses, visceral senses have
receptors that are spread throughout the organs of
the body. In contrast, the special senses, such as smell,
sight, hearing, balance, and taste, have receptors that
are localized in the nose, eyes, ears, and mouth (Fig-
ure 8.1 c). Let’s look first at the somatic senses.
Receptors Help Monitor the
Internal and External Environments
The
dermis
of
the
skin
contains various tactile re-
ceptors, which include
en-
capsulated
nerve
endings
and free nerve endings (bare
dendrites) located at various
levels
(Figure
8.2).
With
prolonged stimulation,
adap-
tation
occurs, causing a loss
of sensation. Some receptors
can adapt rapidly (desensitize quickly) and some
adapt more slowly (take longer to adjust). Either type
of adaptation results in desensitization to a stimu-
lus. Four types of receptors sense touch:
Meissner
corpuscles
,
hair root plexuses, Merkel (tactile)
discs
, and
Ruffini corpuscles
. Pressure is sensed
by rapidly adapting
Pacinian corpuscles
(
p a -S IN -
e-an
), which are widely distributed throughout the
body. Pacinian corpuscles also sense high-frequency
encapsulated
nerve ending
A receptor enclosed
in a connective tissue
capsule.
adaptation
A de-
crease in the strength
of a sensation during
a prolonged stimulus.
Somatic Senses Are Related to Detection of Pressure, Chemicals, Proprioception, and Touch 229
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