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39
CHAPTER
Pain
Headache
Migraine Headache
Migraine Headache in Children
Cluster Headache
Tension-type Headache
Temporomandibular Joint Pain
Pain in Children and Older Adults
Pain in Children
Pain in Older Adults
Organization and Control of Somatosensory
Function
Sensory Systems
The Sensory Unit
Dermatomal Pattern of Dorsal Root Innervation
Spinal Circuitry and Ascending Neural Pathways
Central Processing of Somatosensory Information
Sensory Modalities
Tactile Sensation
Thermal Sensation
Position Sensation
Pain
Pain Theories
Pain Mechanisms and Pathways
Pain Receptors and Mediators
Spinal Cord Circuitry and Pathways
Brain Centers and Pain Perception
Central Pathways for Pain Modulation
Pain Threshold and Tolerance
Types of Pain
Cutaneous and Deep Somatic Pain
Visceral Pain
Referred Pain
Acute and Chronic Pain
Pain Management
Assessment
Treatment
Alterations in Pain Sensitivity and Special Types
of Pain
Alterations in Pain Sensitivity
Special Types of Pain
Neuropathic Pain
Neuralgia
Complex Regional Pain Syndrome
Phantom Limb Pain
S
ensory mechanisms provide individuals with a continu-
ous stream of information about their bodies, the outside
world, and the interactions between the two. The somato-
sensory component of the nervous system provides an aware-
ness of body sensations such as touch, temperature, limb posi-
tion, and pain.
ORGANIZATION AND CONTROL OF
SOMATOSENSORY FUNCTION
The somatosensory system is designed to provide the central
nervous system (CNS) with information about the body.
Sensory neurons can be divided into three types that vary in
distribution and the type of sensation detected: general so-
matic, special somatic, and general visceral afferent neurons
(see Chapter 36).
General somatic afferent neurons
have branches
with widespread distribution throughout the body and with
many distinct types of receptors that result in sensations such
as pain, touch, and temperature.
Special somatic afferent neu-
rons
have receptors located primarily in muscles, tendons, and
joints. These receptors sense position and movement of the
body.
General visceral afferent neurons
have receptors on various
visceral structures and sense fullness and discomfort.
Sensory Systems
Sensory systems can be conceptualized as a serial succession of
neurons consisting of first-order, second-order, and third-order
neurons. The
first-order neurons
contain the sensory receptors
and transmit sensory information from the periphery to the
725
726
Unit Ten: Alterations in the Nervous System
CNS. The
second-order neurons
communicate with various reflex
networks and sensory pathways in the spinal cord and contain
the ascending pathways that travel to the thalamus.
Third-order
neurons
relay information from the thalamus to the cerebral
cortex (Fig. 39-1). Many interneurons process and modify the
sensory information at the level of the second- and third-order
neurons, and many more participate before coordinated and
appropriate learned-movement responses occur.
KEY CONCEPTS
THE SOMATOSENSORY SYSTEM
The somatosensory system relays information to the
CNS about four major body sensations: touch, tem-
perature, pain, and body position. Stimulation of re-
ceptors on regions of the body wall is required to
initiate the sensory response.
■
The Sensory Unit
The somatosensory experience arises from information pro-
vided by a variety of receptors distributed throughout the body.
There are four major modalities of sensory experience: (1) dis-
criminative touch, which is required to identify the size and
shape of objects and their movement across the skin; (2) tem-
perature sensation; (3) sense of movement of the limbs and
joints of the body; and (4) nociception or pain sense.
Each of the somatosensory modalities is mediated by a dis-
tinct system of receptors and pathways to the brain. However,
all somatosensory information from the limbs and trunk shares
a common class of sensory neurons called
dorsal root ganglion
neurons
. Somatosensory information from the face and cranial
structures is transmitted by the trigeminal sensory neurons,
which function in the same manner as the dorsal root ganglion
neurons. The cell body of the dorsal root ganglion neuron, its
receptor (which innervates a small area of periphery), and its
central axon (which projects to the CNS) form a
sensory unit
.
Individual dorsal root ganglion neurons respond selectively to
specific types of stimuli because of their specialized peripheral
terminals, or receptors.
The system is organized into dermatomes, with each
segment supplied by a single dorsal root ganglion
that sequentially relays the sensory information to
the spinal cord, the thalamus, and the sensory
cortex.
■
Two pathways carry sensory information through
the CNS. The discriminative pathway crosses in the
medulla and relays touch and body position. The
anterolateral pathway crosses in the spinal cord
and relays temperature and pain sensation from
the opposite side of the body.
■
innervated strips occur in a regular sequence moving upward
from the second coccygeal segment through the cervical seg-
ments, reflecting the basic segmental organization of the body
and the nervous system (Fig. 39-2). The cranial nerves that in-
nervate the head send their axons to equivalent nuclei in the
brain stem. Neighboring dermatomes overlap one another suf-
ficiently so that a loss of one dorsal root or root ganglion re-
sults in reduced but not total loss of sensory innervation of a
dermatome (Fig. 39-3). Dermatome maps are helpful in inter-
preting the level and extent of sensory deficits that are the re-
sult of segmental nerve and spinal cord damage.
Dermatomal Pattern of Dorsal Root Innervation
The somatosensory innervation of the body, including the
head, retains a basic segmental organizational pattern that
was established during embryonic development. The region
of the body wall that is supplied by a single pair of dorsal root
ganglia is called a
dermatome
. These dorsal root ganglion-
Spinal Circuitry and Ascending Neural Pathways
On entry into the spinal cord, the central axons of the somato-
sensory neurons branch extensively and project to nuclei in
the spinal gray matter. Some branches become involved in
local spinal cord reflexes and directly initiate motor reflexes
(
e.g.
, flexor-withdrawal reflex). Two parallel pathways, the
rapid conducting
discriminative pathway
and the slower con-
ducting
anterolateral pathway,
transmit information from the
spinal cord to the thalamic level of sensation, each taking a
different route through the CNS.
Somatosensory
cortex
Third-
order
Thalamus
The Discriminative Pathway.
The discriminative pathway,
which crosses at the base of the medulla, is used for the rapid
transmission of sensory information such as discriminative
touch (Fig. 39-4). It contains branches of primary afferent
axons that travel up the ipsilateral (
i.e.
, same side) dorsal col-
umns of the spinal cord white matter and synapse with highly
evolved somatosensory input association neurons in the me-
dulla. The discriminative pathway uses only three neurons to
transmit information from a sensory receptor to the somato-
sensory strip of parietal cerebral cortex of the opposite side of
the brain: (1) the primary dorsal root ganglion neuron, which
projects its central axon to the dorsal column nuclei; (2) the
dorsal column neuron, which sends its axon through a rapid
Second-
order
Receptor
First-
order
■
FIGURE 39-1
■
Arrangement of first-order, second-order, and
third-order neurons of the somatosensory system.
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Chapter 39: Pain
C2
C3
C2
C3
T2
C4
C4
T2
C4
C4
T
T4
T
T6
T7
T8
T9
T10
T11
T12
L1
L2
L3
C5
C
5
C
5
C
5
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
T2
T2
T2
T2
C
6
T1
T
1
T1
C
6
T
1
C
6
C
S5
C
6
C
8
C
7
7
S3
C
8
8
S
3
7
Coccyx
C
8
C
7
S4
S4
L2
L2
S2
L3
L3
S2
L3
L
5
L4
L4
L
4
L
4
L
5
L
5
S1
S1
■
FIGURE 39-2
■
Cutaneous distribution of spinal
nerves (dermatomes). (Barr, M. [1993].
The human
nervous system.
New York: Harper & Row)
L5
S1
S1
conducting tract, called the
medial lemniscus
, that crosses at the
base of the medulla and travels to the thalamus on the oppo-
site side of the brain, where basic sensation begins; and (3) the
thalamic neuron, which projects its axons through the somato-
sensory radiation to the primary sensory cortex. The medial
lemniscus is joined by fibers from the sensory nucleus of the
trigeminal nerve (cranial nerve V) that supplies the face. Sen-
sory information arriving at the sensory cortex by this route can
be discretely localized and discriminated in terms of intensity.
One of the distinct features of the discriminative pathway is
that it relays precise information regarding spatial orientation.
This is the only pathway taken by the sensations of muscle and
joint movement, vibration, and delicate discriminative touch,
as is required to differentiate correctly the location of touch on
the skin at two neighboring points (
i.e.
, two-point discrimina-
tion). One of the important functions of the discriminative
pathway is to integrate the input from multiple receptors. The
sense of shape and size of an object in the absence of visual-
ization, called
stereognosis
, is based on precise afferent infor-
mation from muscle, tendon, and joint receptors. For example,
a screwdriver is perceived as being different from a knife in
terms of its texture (tactile sensibility) and shape based on the
relative position of the fingers as they move over the object.
This complex interpretive perception requires that the discrim-
inative system must be functioning optimally and that higher-
order parietal association cortex processing and prior learning
must have occurred. If the discriminative somatosensory path-
way is functional but the parietal association cortex has be-
come discretely damaged, the person can correctly describe the
object but does not recognize that it is a screwdriver. This deficit
is called
astereognosis
.
Central
processes
Dorsal
root ganglia
Peripheral
processes
The Anterolateral Pathway.
The anterolateral pathways (ante-
rior and lateral spinothalamic pathways), which crosses within
the first few segments of entering the spinal cord, consists of bi-
lateral multisynaptic slow-conducting tracts (Fig. 39-5). These
pathways provide for transmission of sensory information such
as pain, thermal sensations, crude touch, and pressure that
does not require discrete localization of signal source or fine
discrimination of intensity. The fibers of the anterolateral path-
way originate in the dorsal horns at the level of the segmental
nerve, where the dorsal root neurons enter the spinal cord. They
cross in the anterior commissure of the cord, within a few seg-
ments of origin, to the opposite anterolateral pathway, where
they ascend upward toward the brain. The spinothalamic tract
■
FIGURE 39-3
■
The dermatomes formed by the peripheral
processes of adjacent spinal nerves overlap on the body surface.
The central processes of these fibers also overlap in their spinal
distribution.
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Unit Ten: Alterations in the Nervous System
Primary
somatosensory
cortex
Thalamus
Somatosensory
and other areas
of cerebral cortex
Medulla
Reticular formation
Paleospinothalamatic tract
Dorsal root
ganglion
Neospinothalamatic tract
Dorsal
columns
Dorsal horn
Spinal cord
■
FIGURE 39-5
■
Neospinothalamic and paleospinothalamic sub-
divisions of the anterolateral sensory pathway. The neospinothal-
amic tract runs to the thalamic nuclei and has fibers that project
to the somatosensory cortex. The paleospinothalamic tract sends
collaterals to the reticular formation and other structures, from
which further fibers project to the thalamus. These fibers influence
the hypothalamus and the limbic system as well as the cerebral
cortex.
Spinal cord
■
FIGURE 39-4
■
Discriminative pathway. This pathway is an as-
cending system for rapid transmission of sensations that relate
joint movement (kinesthesis), body position (proprioception), vi-
bration, and delicate touch. Primary afferents travel up the dorsal
columns of the spinal cord white matter and synapse with so-
matosensory input association neurons in the medulla. Secondary
neurons project through the brain stem to the thalamus and
synapse with tertiary neurons, which relay the information to the
primary somatosensory cortex on the opposite side of the brain.
systems. This circuitry provides touch with its affective or
emotional aspects.
fibers synapse with several nuclei in the thalamus, but en route
they give off numerous branches that travel to the reticular
activating system of the brain stem. These projections provide
the basis for increased wakefulness or awareness after strong
somatosensory stimulation and for the generalized startle re-
action that occurs with sudden and intense stimuli. They also
stimulate autonomic nervous system responses, such as an in-
crease in blood pressure and heart rate, dilation of the pupils,
and the pale, moist skin that results from constriction of the
cutaneous blood vessels and activation of the sweat glands.
There are two subdivisions in the anterolateral pathway:
the outer
neospinothalamic tract
and the inner
paleospinothala-
mic tract
(Fig. 39-5). The neospinothalamic tract, which carries
bright pain, consists of a sequence of at least three neurons
with long axons. It provides for relatively rapid transmission
of sensory information to the thalamus. The paleospino-
thalamic tract, which is phylogenetically older than the neo-
spinothalamic system, consists of bilateral, multisynaptic slow-
conducting tracts that transmit sensory signals that do not re-
quire discrete localization of signal source or discrimination
of fine gradations in intensity. This slower-conducting path-
way also projects into the intralaminar nuclei of the thala-
mus, which have close connections with the limbic cortical
Central Processing of Somatosensory Information
Perception, or the final processing of somatosensory informa-
tion, involves awareness of the stimuli, localization and dis-
crimination of their characteristics, and interpretation of their
meaning. As sensory information reaches the thalamus, it be-
gins to enter the level of consciousness. In the thalamus, the
sensory information is roughly localized and perceived as a
crude sense. The full localization, discrimination of the inten-
sity, 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 sul-
cus (Fig. 39-6). The strip of parietal cortex that borders the cen-
tral sulcus is called the
primary somatosensory cortex
because it re-
ceives primary sensory information by way of direct projections
from the thalamus. A distorted map of the body and head sur-
face, called the
sensory homunculus
, reflects the density of corti-
cal neurons devoted to sensory input from afferents in corre-
sponding peripheral areas. As depicted in Figure 39-7, most of
the cortical surface is devoted to areas of the body such as the
thumb, forefinger, lips, and tongue, where fine touch and pres-
sure discrimination are essential for normal function.
729
Chapter 39: Pain
Sensory Modalities
Somatosensory experience can be divided into
modalities
, a term
used for qualitative, subjective distinctions between sensations
such as touch, heat, and pain. Such experiences require the
function of sensory receptors and forebrain structures in the
thalamus and cerebral cortex. Sensory experience also involves
quantitative sensory discrimination or the ability to distinguish
between different levels of sensory stimulation.
The receptive endings of different afferent neurons are par-
ticularly sensitive to specific forms of physical and chemical
energy. For instance, a receptive ending may be particularly
sensitive to a small increase in local skin temperature. Other af-
ferent sensory terminals are most sensitive to slight indenta-
tions of the skin, and their signals are subjectively interpreted
as touch. Cool versus warm, sharp versus dull pain, and deli-
cate touch versus deep pressure are all based on different pop-
ulations of afferent neurons or on central integration of si-
multaneous input from several differently tuned afferents. For
example, the sensation of itch results from a combination of
high activity in pain- and touch-sensitive afferents, and the sen-
sation of tickle requires a gently moving tactile stimulus over
cool skin.
When information from different primary afferents reaches
the forebrain, where subjective experience occurs, the qualita-
tive differences between warmth and touch are called
sensory
modalities
. Although the receptor-detected information is re-
layed to the thalamus and cortex over separate pathways, the
experience of a modality, such as cold versus warm, is uniquely
subjective.
Central sulcus
Primary sensory cortex
Somatosensory
association area
Lateral sulcus
■
FIGURE 39-6
■
Primary somatosensory and association somato-
sensory cortex.
The somatosensory association cortex, which lies parallel to
and just behind the primary somatosensory cortex, is required
to transform the raw material of sensation into a meaningful
experience. It is here that the stimulus pattern from the present
sensory experience is integrated with past learning. For in-
stance, a person’s past learning plus present tactile sensation
provide the perception of sitting on a soft chair, rather than on
a hard bicycle seat.
Tactile Sensation
The tactile system, which relays sensory information regarding
touch, pressure, and vibration, is considered the basic somato-
sensory system. Loss of temperature or pain sensitivity leaves
the person with no awareness of deficiency. However, if the tac-
tile system is lost, total anesthesia (
i.e.
, numbness) of the in-
volved body part results.
Touch sensation results from stimulation of tactile receptors
in the skin and in tissues immediately beneath the skin, pres-
sure 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’s corpuscles, Merkel’s disks, pa-
cinian corpuscles, hair follicle end-organs, and Ruffini’s end-
organs
1,2
(Fig. 39-8).
Free nerve endings
are found in skin and many other tissues,
including the cornea. They detect touch and pressure.
Meissner’s
corpuscles
are present in nonhairy parts of the skin. They are par-
ticularly abundant in the fingertips, lips, and other areas where
the sense of touch is highly developed.
Meissner’s corpuscles
are
particularly sensitive to movement of very light objects over
the surface of the skin and to low frequency vibration.
Merkel’s
disks
are found in nonhairy areas and in hairy parts of the skin.
They are responsible for giving steady-state signals that allow
for continuous determination of touch against the skin.
The
pacinian corpuscle
is located immediately beneath the
skin and deep in the fascial tissues of the body and is impor-
tant in detecting tissue vibration. The
hair follicle end-organs
de-
tect movement on the surface of the body.
Ruffini’s end-organs
are found in the skin and deeper structures, including the joint
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