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Chapter 6:
Somatosensory System
Copyright © 2013 by Saunders, an imprint of Elsevier Inc.
Introduction
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Somatosensation is sensory information from
the skin and musculoskeletal systems.
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Information from the skin is superficial (e.g.,
touch, pain, temperature) or cutaneous.
Information from the musculoskeletal system
includes proprioception and pain.
Information in the somatosensory system
proceeds from the receptor through a series
of neurons to the brain.
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Introduction—cont’d
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Speed of information processing is
determined by the following:
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Diameter of the axons
Degree of axonal myelination
Number of synapses in the pathway
Distinction is between sensory information
(nerve impulses generated from the original
stimuli) and sensation (awareness of stimuli
from the senses).
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Sensory Receptors

Are specialized and respond only to a
specific type of stimulus, adequate stimulus,
and under normal conditions.
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Mechanoreceptors are the mechanical
deformation of the receptor by touch, pressure,
stretch, or vibration.
Chemoreceptors are substances released by
cells, including damaged cells after injury or
infection.
Thermoreceptors transmit information regarding
heat or cold.
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Sensory Receptors: Nociceptors

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Are a subset of somatosensory receptors.
Stimulation results in the sensation of pain.

Example: Pressure mechanoreceptors are
stimulated by stubbing a toe; the sensation
experienced is pain rather than pressure.
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Somatosensory Peripheral Neurons
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Peripheral sensory neurons have two axons.
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Distal: Conduct messages from the receptor to the
cell body.
Proximal: Project from the cell body into the spinal
cord or brainstem.
Afferents are classified according to axon
diameter (Ia, Ib, II or A-β, A-δ, C).
Larger-diameter axons transmit information
faster than smaller-diameter axons.
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Cutaneous Innervation
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Cutaneous Innervation—cont’d
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Receptive fields tend to be smaller distally
and larger proximally.
Distal regions of the body have a greater
density of receptors than proximal areas.

Example: The combination of smaller receptive
fields and greater density of receptors distally
enables us to distinguish between two closely
applied stimuli on a fingertip; however, the same
stimuli cannot be distinguished on the trunk.
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Cutaneous Innervation—cont’d
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Touch is categorized as fine or coarse.
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Fine touch includes a variety of receptors and
subsensations.
Coarse touch is mediated by free endings
throughout the skin.
Cutaneous receptors respond to touch,
pressure, vibration, stretch, noxious stimuli,
and temperature.
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Cutaneous Innervation—cont’d
Note differences between nerve root
and peripheral nerve
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Muscle Spindles

Are the sensory organs in muscle.
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Intrafusal and Extrafusal Fibers
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Intrafusal fibers are contractile only at the ends;
the central region cannot contract.
Two types of intrafusal fibers include:
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Nuclear bag fibers, which have clumps of nuclei.
Nuclear chain fibers, which have nuclei arranged in
single file.
Two different sensory endings include:
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Primary endings of type Ia neurons wrap around the
central region of each intrafusal fiber.
Secondary endings of type II afferents end mainly on
nuclear chain fibers adjacent to the primary endings.
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Golgi Tendon Organs
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Joint Receptors
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Joint Receptors—cont’d
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Muscle spindles respond to quick and
prolonged stretches of the muscle.
Tendon organs signal the force generated by
the muscle contraction or by a passive
stretch of the tendon.
Joint receptors respond to mechanical
deformation of joint capsules and ligaments.
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Summary: Function
of Different-Diameter Axons
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Pathways to the Brain
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Are important distinctions among the types of
pathways; fidelity of information is conveyed.
Have high-fidelity transmission that provides
accurate details regarding the location of the
stimulation.
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Pathways to the Brain—cont’d
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When describing pathways in the nervous
system, only the neurons with long axons
that connect distant regions of the nervous
system (projection neurons) are counted.
A tract is the bundle of axons with the same
origin and a common termination.
Somatosensory pathways are often named
for the origin and termination of the tract that
contains the second neuron in the series.
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Pathways to the Brain—cont’d
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Three types of pathways bring sensory
information to the brain:
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Conscious relay
Divergent (diffuse systems)
Unconscious relay
Table 6-2
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Conscious Relay Pathways
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Transmit information to many locations in the
brainstem and cerebrum and use pathways
with varying numbers of neurons (though
usually three)
Information in conscious relay pathways is
transmitted with high fidelity.
Information in these pathways allows
individuals to make fine distinctions about
stimuli.
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Divergent Pathways
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Information is transmitted to many locations
in the brainstem and cerebrum and use
pathways with varying numbers of neurons.
Sensory information is used at both the
conscious and unconscious levels.
Aching pain is a form of sensation that is
transmitted via divergent pathways in the
central nervous system.
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Unconscious Relay Pathways
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Unconscious proprioceptive and other
movement-related information is carried to
the cerebellum.
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Does not reach consciousness
Information plays an essential role in
automatic adjustments of our movements
and posture.
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Conscious Relay Pathways
to the Cerebral Cortex
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All four types of somatosensation reach
conscious awareness.
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Touch
Proprioception
Temperature
Pain
Pathways to consciousness travel upward in
the spinal cord via two routes:
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Dorsal columns
Anterolateral tracts
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Rules for Conscious Relay
Pathways to the Cerebral Cortex
1.
There are 3 neurons in the pathway
1.
2.
Only count neurons with long axons
The first neuron is in the peripheral nervous
system
1.
2.
DRG
Trigeminal ganglia
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3. The second neuron is in the segmental
nervous system
1. spinal cord
2. Brainstem
4. The axon of the second neuron crosses the
midline
1. DC-ML: Caudal medulla
2. ALS: Spinal cord
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5. The third neuron is located in the thalamus
1. VPL for the body
2. VPM for the head
6. The third neuron terminates in a specific
region of somatosensory cortex (post-central
gyrus)
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Discriminative Touch and Conscious
Proprioception
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Discriminative touch is the localization of
touch and vibration and the ability to
discriminate between two closely spaced
points touching the skin.
Conscious proprioception is the awareness
of the movements and relative position of
body parts.
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Discriminative Touch and
Conscious Proprioception
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Stereognosis: the ability to use touch and
proprioceptive information to identify an
object; for example, a key in the hand can be
identified without vision.
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Discriminative Touch and
Conscious Proprioception
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Pathways for discriminative touch and
conscious proprioception use a three-neuron
relay.
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Primary conveys information from the receptors to
the medulla.
Secondary conveys information from the medulla
to the thalamus.
Tertiary conveys information from the thalamus to
the cerebral cortex.
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Discriminative Touch and
Conscious Proprioception
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Dorsal Column/Medial
Lemniscus System
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Primary neurons include:
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Many collateral branches entering the gray matter
of the spinal cord
• Some collaterals that contribute to motor control, some
that influence activity in neurons in other sensory
systems, and others that influence autonomic regulation
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Axons in gracile fascicle (<T6) or cuneate fascicle
(>T6)
Secondary neurons include:
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Cell bodies located in the nucleus gracilis or
cuneatus
Axons that cross the midline as the internal
arcuate fibers,
then ascend to the thalamus
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Dorsal Column/
Medial Lemniscus System
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Third-order neurons include:
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Those that connect the thalamus to the sensory
cortex
Those that have axons forming part of the
thalamocortical radiations, which are fibers
connecting the thalamus to the cerebral cortex
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Discriminative Touch Information
from the Face
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Somatotopic Arrangement
of Information
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Somatosensory Areas
of the Cerebral Cortex
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Primary sensory cortex discriminates among
the size, texture, or shape of objects.
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What is the object like?
Somatosensory association areas analyze
the information from the primary sensory
area and the thalamus and provide
stereognosis and memory of the tactile and
spatial environment.
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What is the object?
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Temperature Sensation
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Heat and cold are detected by specialized
free nerve endings of small myelinated and
unmyelinated neurons.
A-delta fibers carry impulses produced by
cooling.
C fibers carry information regarding heat.
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Pain
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Pain is composed of both sensation and the
emotional response to the sensation.
Nociceptive: receptors or neurons that
receive or transmit information about stimuli
that damage or threaten to damage tissue.
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Fast pain (spinothalamic pain) is the initial and
immediate sharp sensation that indicates the
location of the injury.
Slow pain (spinolimbic pain) is the dull, throbbing
ache following fast pain that is not well localized.
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Fast, Localized
Nociception:
Lateral System
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Fast pain uses a three-neuron system:
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Primary neurons bring information into the dorsal
horn of the spinal cord.
 Axons of secondary neurons cross the midline
and project from the spinal cord to the thalamus.
 Tertiary neurons project from the thalamus to the
cerebral cortex.
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Fast, Localized System:
Lateral System—cont’d
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Dorsal Column/Medial Lemniscus
and Spinothalamic Systems
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Both systems consist of three-neuron relay
pathways.
In contrast to the discriminative touch and
conscious proprioceptive information,
anterolateral white matter contains axons
transmitting information about pain,
temperature, and coarse touch.
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Dorsal Column/Medial Lemniscus
and Spinothalamic Systems—cont’d
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Functions of the dorsal and anterolateral
columns are not rigidly segregated;
information about nondiscriminative (coarse)
touch travels in the anterolateral system, and
some pain and temperature information
ascends in the dorsal columns.
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Fast Pain Information
From the Face
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Travels in the trigeminal nerve.
Lesions that interrupt the pathways
conveying nociceptive information produce
analgesia
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Analgesia is the absence of pain in response to
stimuli that would normally be painful.
 Crossed analgesia: a single lesion can cause pain
sensation to be lost on the side of the face
ipsilateral to the lesion and the contralateral side
of the body
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Fast Versus Slow Pain
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When fast pain information reaches the
somatosensory cortex, the individual is
consciously aware of sharp pain in a specific
location.
If tissue damage has occurred, fast pain is
followed by a slow, aching pain.
Onset of slow pain is later than fast pain
because the impulses travel on smaller,
unmyelinated axons.
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Medial Pain System
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Medial pain system: divergent ascending
network of neurons
Uses several pathways with variable
numbers of projection neurons, not a threeneuron pathway like fast pain.
Activity of the medial pain system elicits
affective, motivational, withdrawal, arousal,
and autonomic responses.
Information is not somatotopically organized,
so slow pain cannot be precisely localized.
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First Neuron
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First neuron: a small, unmyelinated C fiber
Receptors are free nerve endings, sensitive
to noxious heat, chemical, or mechanical
stimulation
Has high-threshold C fiber endings that
become sensitized with repeated stimulation.
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Ascending Projection Neurons
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Axons of ascending projection neurons reach
the midbrain, reticular formation, and limbic
areas via three tracts in the anterolateral
spinal cord.
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Spinomesencephalic
Spinoreticular
Spinolimbic
Tracts are parallel ascending tracts.
Only information in the spinolimbic tract is
perceived as pain.
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Spinomesencephalic Tract
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Carries nociceptive information to the
superior colliculus and periaqueductal gray.
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Both are in the midbrain
• Superior colliculus – tectum (visual map of world)
• Periaqueductal gray – around cerebral aqueduct
Periaqueductal gray is part of the descending pain
control system.
Is involved in turning the eyes and head
toward the source of noxious input and in
activating descending tracts that control pain.
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Spinoreticular Tract
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These ascending neurons synapse in the
reticular formation.
Reticular formation: A neural network in the
brainstem that includes the reticular nuclei
and their connections
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Modulates arousal, attention, and sleep-waking
cycles.
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Spinolimbic Tract
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Axons of the spinolimbic tract transmit slow
pain information to the medial and
intralaminar nuclei in the thalamus.
Slow pain pathways provide information that
produces automatic movements and
autonomic and emotional responses to
noxious stimuli.
Activity in the spinoreticular and spinolimbic
tracts result in arousal, withdrawal,
autonomic, and affective responses to pain.
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Trigeminoreticulolimbic Pathway
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Slow pain information is transmitted via this
pathway from the face.
Although intact sensory and parietal cortex is
required for the localization of pain, crude
awareness of slow pain can be achieved in
many cortical areas and possibly in the
thalamus and basal ganglia.
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Temperature Information

Information is transmitted in phylogenetically
older pathways to the following:
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Reticular formation
Nonspecific nuclei of the thalamus
Subcortical nuclei
Hypothalamus
Information that does not reach conscious
awareness contributes to arousal, provides
gross localization, and contributes to
autonomic regulation.
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Unconscious Relay Tracts
to the Cerebellum
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Transmit information from proprioceptors and
information about activity in spinal
interneurons.
Relay information critical for adjusting
movements.
Inadequate proprioceptive input can cause
ataxia because the loss of sensory feedback
disrupts movement control.
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High-Fidelity Pathways

Two pathways relay high-fidelity,
somatotopically arranged information to the
cerebellar cortex.
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Posterior spinocerebellar pathway
• Transmits information from the legs and the lower half of
the body.

Cuneocerebellar pathway
• Begins with primary afferents from the arm and upper
half of the body; the central axons travel via the posterior
columns to the lower medulla.
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Internal Feedback Tracts

Tracts monitor the activity of spinal
interneurons and of descending motor
signals from the cerebral cortex and
brainstem:
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Anterior spinocerebellar tract
• Transmits information from the thoracolumbar spinal
cord.

Rostrospinocerebellar tract
• Transmits information from the cervical spinal cord to the
ipsilateral cerebellum.
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Function of Spinocerebellar Tracts
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Information in the spinocerebellar tracts
comes from the proprioceptors, spinal
interneurons, and descending motor
pathways.
Information, which does not reach conscious
awareness, contributes to automatic
movements and postural adjustments.
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