858
U N I T 1 0
Nervous System
Central Processing of Somatosensory
Information
Perception, or the final processing of somatosensory
information, involves awareness of the stimuli, local-
ization and discrimination of their characteristics, and
interpretation of their meaning.
1,2
As sensory informa-
tion reaches the thalamus, it begins to enter the level of
consciousness, is roughly localized, and is perceived as
a crude sense. The full localization, discrimination of
the intensity, and interpretation of the meaning of the
stimuli require processing by the somatosensory cortex.
The somatosensory cortex is located in the parietal
lobe, which lies behind the central sulcus and above the
lateral sulcus (Fig. 35-5). The strip of parietal cortex that
borders the central sulcus is called the
primary somato-
sensory cortex
because it receives primary sensory infor-
mation by direct projections from the thalamus. Parallel
to and just behind the primary somatosensory cortex
(i.e., toward the occipital cortex) lies the somatosensory
association area, which is required to transform the raw
sensory information into a meaningful learned percep-
tion. Most of the perceptive aspects of body sensation,
or somesthesia, require the function of this association
area. The perceptive aspect, or meaningfulness, of a
stimulus pattern—such as the perception of sitting on a
soft chair rather than on a hard bicycle seat—involves
the integration of present sensation with past learning.
Somatosensory Modalities
Earlier, we noted that somatosensory experience can be
divided into modalities, qualitative distinctions between
the sensations of touch, temperature, position, and
pain.
1,2
Somatosensory experience also involves quanti-
tative discrimination; that is, the ability to distinguish
between different intensities of sensory stimulation.
Although sensory receptors can respond to many forms
of sensory information at high levels, they are highly
sensitive to low levels of a particular type of sensation.
For example, a receptor may be particularly sensitive
to a small increase in local skin temperature, yet stimu-
lation with strong pressure also can result in receptor
stimulation. Cool versus warm, sharp versus dull pain,
and delicate touch versus deep pressure are all based on
different populations of afferent neurons or on central
integration of simultaneous input from several differ-
ently tuned afferents.
Tactile Sensation
The tactile system, which relays sensory information
regarding touch, pressure, and vibration, is considered
the basic somatosensory system. Loss of temperature
or pain sensitivity leaves the person with no awareness
of deficiency. If the tactile system is lost, however, total
anesthesia (i.e., numbness) of the involved body part
results.
Touch sensation results from stimulation of tactile
receptors in the skin and in tissues immediately beneath
the skin, pressure from deformation of deeper tissues, and
vibration from rapidly repetitive sensory signals. There
are at least six types of specialized tactile receptors in the
skin and deeper structures: free nerve endings, Meissner
corpuscles, Merkel disks, pacinian corpuscles, hair fol-
licle end-organs, and Ruffini end-organs
1,2
(Fig. 35-6).
Free nerve endings
are found in skin and many other
tissues, including the cornea. They detect touch and
pressure.
Meissner corpuscles
are elongated, encapsu-
lated nerve endings present in nonhairy parts of the skin.
They are particularly abundant in the fingertips, lips,
and other areas where the sense of touch is highly devel-
oped.
Merkel disks
are dome-shaped receptors found
in nonhairy and hairy parts of the skin. In contrast to
Meissner corpuscles, which adapt within a fraction of
a second, Merkel disks transmit an initial strong signal
that diminishes in strength but is slow in adapting. They
are responsible for giving steady-state signals that allow
for continuous sense of touch against the skin.
Pacinian corpuscles
are located immediately beneath
the skin and deep in the fascial tissues. They are stimu-
lated by rapidmovements of the tissues and are important
in detecting tissue vibration. The
hair follicle end-organs
consist of afferent unmyelinated fibers entwined around
most of the length of the hair follicle. These receptors,
which are rapidly adapting, detect movement on the
surface of the body.
Ruffini end-organs
are found in the
skin and deeper structures, including the joint capsules.
These receptors, which have multibranched encapsu-
lated endings, have very little adaptive capacity and are
important for signaling continuous states of deforma-
tion, such as heavy and continuous touch and pressure.
Almost all the specialized touch receptors, with the
exception of free nerve endings, transmit their signals
through large myelinated nerve fibers (i.e., types A
α
,
A
β
) that have transmission velocities ranging from 25 to
70 m/second. Most free nerve endings transmit signals
Primary sensory cortex
Central sulcus
Somatosensory
association area
Lateral sulcus
FIGURE 35-5.
Primary somatosensory cortex and
somatosensory association area.
