Sensory Physiology
203
SECTION B
Specifi c Sensory Systems
Somatic Sensation
Sensation from the skin, muscles, bones, tendons, and joints,
or
somatic sensation,
is initiated by a variety of sensory recep-
tors collectively called somatic receptors (
Figure 7–16
). Some
of these receptors respond to mechanical stimulation of the
skin, hairs, and underlying tissues, whereas others respond to
temperature or chemical changes. Activation of somatic recep-
tors gives rise to the sensations of touch, pressure, awareness
of the position of the body parts and their movement, temper-
ature, and pain. The receptors for visceral sensations, which
arise in certain organs of the thoracic and abdominal cavities,
are the same types as the receptors that give rise to somatic
sensations. Some organs, such as the liver, have no sensory
receptors at all. Each sensation is associated with a specifi c
receptor type. In other words, distinct receptors exist for heat,
cold, touch, pressure, limb position or movement, and pain.
Touch and Pressure
Stimulation of a variety of mechanoreceptors in the skin (see
Figure 7–16) leads to a wide range of touch and pressure experi-
ences—hair bending, deep pressure, vibrations, and superfi cial
touch, for example. These mechanoreceptors are highly special-
ized nerve endings encapsulated in elaborate cellular structures.
The details of the mechanoreceptors vary, but generally the
nerve endings are linked to networks of collagen fi bers within a
capsule. These networks transmit the mechanical tension in the
capsule to ion channels in the nerve endings and activate them.
The skin mechanoreceptors adapt at different rates.
About half of them adapt rapidly (i.e., they fi re only when the
stimulus is changing), and the others adapt slowly. Activation
of rapidly adapting receptors gives rise to the sensations of
touch, movement, and vibration, whereas slowly adapting
receptors give rise to the sensation of pressure.
In both categories, some receptors have small, well-
defi ned receptive fi elds and can provide precise information
about the contours of objects indenting the skin. As might be
expected, these receptors are concentrated at the fi ngertips.
In contrast, other receptors have large receptive fi elds with
obscure boundaries, sometimes covering a whole fi nger or
a large part of the palm. These receptors are not involved in
detailed spatial discrimination but signal information about
skin stretch and joint movement.
Sense of Posture and Movement
The senses of posture and movement are complex. The major
receptors responsible for these senses are the muscle-spindle
stretch receptors. These mechanoreceptors occur in skeletal
Skin
surface
Dermis
Epidermis
A. Meissner's corpuscle—rapidly adapting mechanoreceptor,
touch and pressure
B. Merkle's corpuscle—slowly adapting mechanoreceptor, touch
and pressure
C. Free nerve ending—slowly adapting, some are nociceptors,
some are thermoreceptors, and some are mechanoreceptors
D. Pacinian corpuscles—rapidly adapting mechanoreceptor,
vibration and deep pressure
E. Ruffini corpuscle—slowly adapting mechanoreceptor, skin
stretch
D
E
A
A
C
C
B
Figure 7–16
Skin receptors. Some nerve fi bers have free endings not related to any apparent receptor structure. Thicker, myelinated axons, on the other
hand, end in receptors that have a complex structure. (Not drawn to scale; for example, Pacinian corpuscles are actually four to fi ve times larger
than Meissner’s corpuscles.)
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