860
U N I T 1 0
Nervous System
Merkel cells) also signal postural information. Signals
from these receptors travel through the discrimina-
tive pathway and are processed in the thalamus before
reaching the cerebral cortex. Lesions affecting the pos-
terior column of the spinal cord impair position sense.
The vestibular system (see Chapter 38) also plays an
essential role in position sense.
Pain Sensation
Pain is an unpleasant sensory and emotional sensation
associated with actual and potential tissue damage.
1–4
Unlike other somatic modalities, pain has an urgent and
primitive quality, a quality responsible for the psycho-
logical, social, cultural, and cognitive aspects of the pain
experience. Despite its unpleasantness, pain can serve
a useful purpose in that it warns of impending tissue
injury, motivating the person to seek relief. For example,
an inflamed appendix could progress in severity, rup-
ture, and even cause death were it not for the warning
afforded by pain.
Pain is a common symptom that varies widely in
intensity and spares no age group. It can be equally
devastating for infants and children, young and middle-
aged adults, and the elderly. Both acute pain and chronic
pain can be major health problems. It is the most com-
mon reason for visits to health care facilities. Acute pain
often results from injury, surgery, or invasive medical
procedures. It also can be a presenting symptom of
infections, such as otitis media. Chronic pain can be
symptomatic of a wide range of health problems includ-
ing arthritis, back injury, and cancer.
PainTheories
Many theories, including the specific and pattern theo-
ries, have been offered to explain the physiologic basis
for the pain experience. The
specificity theory
regards
pain as a separate sensory modality evoked by the activ-
ity of specific receptors that transmit information by
special nerve endings to pain centers or regions in the
forebrain where pain is experienced.
5
Pattern theory
proposes that pain receptors share endings or pathways
with other sensory modalities, but that different patterns
of activity (i.e., spatial or temporal) of the same neurons
can be used to signal painful and nonpainful stimuli.
5
For example, light touch applied to the skin would pro-
duce the sensation of touch through low-frequency fir-
ing of the receptor; intense pressure would produce pain
through high-frequency firing of the same receptor. Both
of these theories focus on the neurophysiologic basis of
pain, and aspects of both probably apply. Specific noci-
ceptive afferents have been identified; however, almost
all afferent stimuli, if driven at a very high frequency,
can be experienced as painful.
Gate control theory, a modification of specificity the-
ory proposed by Melzack and Wall in 1965, postulated
that the presence of a neural gating mechanism at the
segmental spinal cord level could block projection of
pain information to the brain.
6
According to this theory,
internuncial neurons involved in the gating mechanism
are activated by large-diameter, faster-propagating fibers
that carry tactile information capable of blocking the
transmission of impulses from small-diameter myelinated
and unmyelinated pain fibers. Pain therapists have long
known that pain intensity can be temporarily modified
by the stimulation of other sensory fibers. For example,
repeated sweeping of a soft-bristled brush on the skin
(i.e., brushing) over or near a painful area may result in
pain reduction for several minutes to several hours.
Pain modulation is now known to be a much more
complex phenomenon than that proposed by these the-
ories. Tactile information is transmitted by small- and
large-diameter fibers. Major interactions between sensory
modalities, including the so-called gating phenomenon,
SUMMARY CONCEPTS
■■
The somatosensory component of the nervous
system relays information about four major
sensory modalities of touch, temperature, pain,
and body position.
■■
Somatosensory information is sequentially
transmitted over three types of neurons: first-
order neurons, which transmit information
from receptors in the sensory units of the
system to dorsal horn neurons; second-order
association neurons in the spinal cord, which
communicate with various reflex circuits and
transmit information to the thalamus, where it
is roughly localized and perceived as a crude
sense; and third-order neurons, which forward
the information from the thalamus to the
somatosensory cortex, where full localization,
intensity discrimination, and interpretation occurs.
■■
The somatosensory system is organized
segmentally into dermatomes, with each segment
supplied by a single dorsal root ganglion that
contains the neuronal cell bodies for the sensory
units of the segment.
■■
There are two ascending pathways for transmission
of somatosensory information: the discriminative
and anteriolateral pathways.The discriminative
pathway, which crosses at the medulla, uses only
three neurons to rapidly transmit information
such as position sense and discriminative touch
from sensory receptors to somatosensory cortex.
The anterolateral pathway, which crosses within
the first few segments of entering the spinal cord,
consists of bilateral, multisynaptic, slow-conducting
tracts that transmit information such pain, thermal
sensation, crude touch, and pressure.
