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Somatic Sensory System
Light touch, vibration, pressure, position of
limbs (sense of self), pain, temperature.
• Leads to the ability to identify shapes and
textures of objects.
• Monitors internal and external forces acting
on the body.
• Detects potentially harmful circumstances.
Overview
• Receptor neurons located in skin or joints receive
stimuli.
• Information is carried to brain via the spinal cord>
brainstem> thalamus> postcentral gyrus of the
parietal lobe> other cortical areas.
• Projections are topographic with respect to body
and the amount of cortical space allocated to
various body parts is proportional to the
density of sensory receptors in that area.
Somatic Sensory System
Touch and Pain have different
routes to the brain.
General Organization of the
Somatic Sensory System
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Different receptors are classified by
different properties
• Function- Pain, temperature, touch, muscle length.
• Morphology- free nerve endings or encapsulated.
Nociceptors and thermoceptors have free nerve
endings, most others encapsulated.
• Conduction velocities , fast vs. slow
• Location: skin, muscle, tendon, hair
• Rate of adaptation: slow vs fast
General Properties
• stimuli applied to skin change the nerve
endings, produce graded receptor potentials
that trigger action potentials.
• quality of stimulus (what it represents and
where it is) is determined by the neuron’s
targets in the brain.
• quantity or strength of stimulus signalled by
rate of action potentials.
Slowly Adapting and Rapidly Adapting Mechanoreceptors
Respond Differently to Stimulation
Low threshold (or high sensitivity)
mechanoreceptors
Provide information about touch, pressure, vibration,
and skin tension.
Four major types of encapsulated mechanoreceptors:
• Meissner’s corpuscle
• Pacinian corpuscle
• Merkel’s disk
• Ruffini’s corpuscle.
Called low-threshold because even weak stimulation
causes action potentials. Innervated by large
myelinated axons.
Receptive fields
• Receptive field is the region of the skin
within which a tactile (touch) stimulus
evokes a sensory response in the cell or its
axon.
– Receptive fields are small in the finger tip (1-2
mm), but larger (5-10 mm) in the palms.
The Skin Mechanoreceptors
Differences in mechanosensory
discrimination across the body surface
• Accuracy of of touch varies over the body.
• Tested with two-point discrimination.
• Fingers can distinguish things 2mm apart, forearms 40 mm
apart.
• Mechanoreceptors are more numerous in finger tips and
have smaller receptive fields.
• Doesn’t explain everything. Two-point thresholds vary
with practice (can be learned), and depend on the stimulus.
• Phantom limbs- central processing can occur in the
absence of peripheral receptors.
Sensitivity of Tactile Discrimination Varies
with Location on the Body Surface
The route of sensory information
• Mechanoreceptors and propioception receptors use
the Dorsal-column-medial lemniscus pathway
• Pain and temperature use spinothalamic
(anterolateral pathway).
The Main Mechanosensory Pathways
Medial lemniscus
cross in the medulla
upper and lower body
use slightly different
pathways.
Trigeminal pathway
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info from head and face
Somatic sensory cortex
• Located in parietal lobe-post central gyrus
• Divided into regions, Broadmann’s areas
– 3b and 1: cutaneous
– 3a: proprioception
– 2: tactile and proprioception
• Each area contains a somatotopic “map” of
the body
– homunculous
Schematic Representation of the
Main Mechanosensory Pathways
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Somatotopic Order in Human
Primary Somatosensory Cortex
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The Somatic Sensory Portions of
the Thalamus and Cortical
Targets
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Upper body medial
Lower body lateral
Human Primary Somatoensory Cortex
areas of high receptor density
get more cortical space
somatotopy
The Humunculus reflects sensory receptor density
receptive fields of somatosensory cortex
• Area 3b- simple, responses generally from
stimulations of a single finger
• 1 and 2- respond to stimulation of multiple
fingers. 1- often a particular direction is
important, 2- particular shapes.
• SI is organized in columns, by receptive field and
modality. Stick an electrode vertically, all neurons
share same region of body and slowly adapting vs
fast adapting mechanosensory neurons
Higher order processing
• SI sends out projections to other areas of
cortex.
• SII, adjacent to SI. Receives info from SI
and sends it to amygdala and hippocampus.
Plays roles in fear conditioning and tactile
learning and memory.
Summary - organization
– First order neurons: cell bodies in dorsal root
and cranial nerve ganglia
– Second order: brainstem nuclei
– Third order: thalamus ….project to SI
– Fourth order; SI projects to SII
• Topographical arrangement throughout
Pain
• Submodality of the sense of touch, warns of
injury and things that should be avoided.
• More subjective that the other senses. The
same stimulus can produce different
responses in different individuals, or in the
same individual in different circumstances.
Pain involves specialized neurons not just extra
stimulation of touch receptors.
scheme for transcutaneous
nerve recording.
Nociceptor doesn’t
fire until pain is felt.
Other thermorecptors
fire at all temps and at
about the same frequency.
Pain threshold is about 45
degrees C
Detecting pain
• Ion channel receptors open in response to
heat as well as capsaicin, called TRP
(transient receptor potential) channels.
• When open allow in Ca++ and Na+ that
generate action potentials.
Heat gated ion channels
• Capsaicin receptors are
nonselective cation
channels opened by heat,
low pH, and capsaicin (the
hot in hot peppers).
• Mice without VR1
(vanilloid receptor – 1 or
TRPV1) have impaired
sensitivity to pain. Can
drink capsaicin as if it were
water.
Nociceptors
• Activation of VR1
channels by pure
capsaicin and extracts
of various peppers.
Nature 1997 Oct 23;389(6653):816-24
Two types of pain: Two types of axons
• first pain (sharp), Ad fibers
• second pain (dull, longer lasting) C-fibers
Referred pain
Few if any dorsal horn neurons
are specialized solely for the
transmission of visceral pain.
It is conveyed relayed to brain via
dorsal horn neurons that also get
inputs from skin.
Therefore a person may feel pain at
a site far from its source.
Descending Systems Modulate Transmission
of Ascending Pain Signals
• Descending pathways
from cortex and
hypothalamus…
Descending Systems Modulate Transmission
of Ascending Pain Signals
• Through
periaqueductal gray
rostral medulla reduces
spinothalamic tract
activation.
opioids
• Opioid receptors (metabotropic) in descending
pain pathway.
• Ligands- enkephalins, endorphins, and dynorphin.
Found in all descending pain areas.
• Opiate antagonist naloxone blocks morphineinduced analgesia.
• Opioids decrease the chance that a nociceptor will
fire, cause inhibition.
Endogenous opioid peptides hyperpolarize cells to
decrease action potential firing
Placebo Effect
• Sugar pills can reduce perception of pain.
• The effect can be blocked by naloxone, a
competitive antagonist of opioid receptors.
• The placebo effect is based on a biochemical
change in the brain.
Endogenous Opioids
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