Somatosensory System

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Somatosensory System
dorsal root and trigeminal ganglia receive all
information from the peripheral nerves with 2
types of cells- large and small form essentially
T-junctions-- branches to periphery and CNS
without soma being involved in
signaling different conduction velocities and
spiking patterns in different types of
ganglion neurons whose axons are mixed in the
peripheral nerves
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Somatosensory System
4 types of mechanosensory neurons ennervate the
skin Pacinian corpuscles detects low frequency
mechanical stimuli and sustained pressure--
onion skin-like structure can react to high
frequency (200 Hz) as well spikes related to
the intensity of the pressure
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Somatosensory System
Meissner's Corpuscles have two axons with broad
flattened disks each neuron ennervates from
20-50 corpuscles with their broad receptive
fields and shallow location, they detect small
stretching within the skin, ie. slippage of
something in the hand
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Somatosensory System
Merkel's disk axon terminal wrapped by an
epithelial cell collection of Merkel cells
results in a touch dome role of the epithelial
Merkel cell is unclear used to determine
texture, form, or curvature
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Somatosensory System
hair follicle afferents ennervate hair follicles
of the skin most sensitive of the
mechanoreceptors-- 10x more than Merkel's
disks receptive field size varies for all
according to body location- smaller on
fingertips vs wrists, for example the more
dense the receptive field, the finer the size
discrimination
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Somatosensory System
muscle and tendon receptors are used to determine
body position muscle receptors are parallel and
measure stretch in the muscle fiber tendon
(golgi) receptors also measure stretch stretch
measured opposite from muscle stretch kinesthesia
sense of body position combines stretch,
touch, and various other sensory systems can
be mimicked by vibrating the tendon-- seems as
if movement is taking place even though it
isn't
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Somatosensory System
nociceptor any neuron which senses something
noxious includes heat, pain, sharp punctures,
swollen ankle, etc often called free nerve
endings lack the capsule cells of other sensors
response is determined solely by the type of
receptor on the cells first pain rapidly
carried, closely localized second pain extended
pain, not localized pain to change
behavior CGRP and substance P are neuropeptides
essential for pain receptors
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Somatosensory System
axon reflex swelling and sensitivity around a
damaged region induced by CGRP and substance
P causes release of histamine from mast
cells prostaglandins (lipids) and bradykinin
(peptide) cause intense pain prostaglandins are
formed from arachadonic acid in membranes
reacting with cyclooxygenase-2 (COX2) pain
relievers aspirin, ibuprofin, NSAIDS
(nonsteroidal antiinflamatories) all target
COX2
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Somatosensory System
nociception mediated by transient receptor
potential (TRP) channels TRP-V in particular
(ie. vanilloids such as in chilli peppers) TRP-V
also opens in response to high temperature ie.
'hot' foods
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CNS Pathways of Somatosensation
separate pathways are activated by
mechano/proprioception and thermo/nociception
pain/heat is detected in spinal cord
interneurons mechano/proprioception make synapse
onto cells in the medulla both project to
different regions of the thalamus, then to
cortical regions in general thalamus acts as
a relay to cortex distinct pathways are kept
separate
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Mechanosensation Pathway
each dorsal root ganglia collects signals from a
band of cells in the body called a dermomere
(ie. set up as the rhombomeres during
development) large diameter, heavily myelinated
axons enter cord and turn anteriorly- form 2
fasciculi in dorsal columns depending whether
above/below T7 more anterior fibers are layered
to the lateral sides in the column layering
maintains somatotopic map to medulla axons enter
dorsal column nuclei according to type one-one
connections are maintained inhibitory
interneurons prevent a response from all but
the strongest driving neuron
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Mechanosensation Pathway
lemniscal properties features of thalamic
processing static lemniscal properties place
specificity, modality specficity thalamic
'rods' narrow rostral-caudal expansion of
axonal projection no detectable use of
interneurons to increase contrast
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Nociception Pathway
nociceptive axons have small myelin coatings and
travel as an outer edge group of axons
referred to as Lissauer's tract (go up and down)
synapse on dorsal horn neurons over several
segments secondary neurons project anteriorly to
the thalamus as first (discriminatory) pain
and second (agonizing) pain some first pain
projection neurons also receive signals aiding
in localization of the pain
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Nociception Pathway
acute pain neurons from the dorsal horn project
to the thalamus in separate cell types than
mechanosensory neurons-- still segregated exact
projection regions of several pain types still
not defined well but targets to project to SI
of the cerebral cortex MRI show 4 areas of pain
in brain SI, SII with discriminative pain
(first) rostral anterior cingulate rostral
insula are emotional or anticipatory
blue- SI red- SII
placebos work on emotional pain
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Nociception Pathway
periaquaductal gray (PAG) is a brain region
receiving innervation from raphe magnus and
locus coeruleus drives inhibitory connections
to axon terminals of both first and second
pain neurons, suppressing pain ie. part of shock
or adaptation also releases met-enkephalin
pentapeptide which is an opiate
activator referred pain is where pain in one
location seems to be in another caused by the
overlap of initial connections over several
segments phantom limb pain comes from
adjacent body parts expanding their areas into
silenced SI regions caused by synaptic plasticity
in cortex as well as spinal cord,thalamus and
medulla
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Trigeminal System
sensory processes in the face (mechano, noci, and
temperature) are ennervated from the
trigeminal ganglia which forms trigeminal
nerve principal sensory nucleus of the pons
works like the dorsal column cells on the same
side of the face project from the pons to the
VPM of the thalamus near hand targets sensory
modalities are still separated in the
trigeminal ganglia and pons although there is
some mixing (ie. tooth pulp mechanosensitive/pain)
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Cortical Representation of Touch
unlike the rest of the somatosensory system, the
cortex has strong lateral inhibition to
sharpen boundaries of touch receptive fields
have 3 characteristics 1. central region of
rapid excitation 2. rapidly occurring lateral
inhibition 3. delayed inhibition overlapping
excitation tied to spatial details of touch--
emergent properties based on cortical
connections and not seen earlier in the system
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Cortical Representation of Touch
in SI, spatial information is independent of
velocity temporal inhibition, based on the
slower response, can be overcome by velocity--
we feel bumps when moving our fingers over a
surface that might not be detectable just
touching it 70 cortical responses also become
direction sensitive- velocity vectors only
shows up in later stages of cortical
processing area 3 of SI is an early stage in
processing- other areas have distinct defects
when damaged while area 3 is more general SI
areas 3 and 1 play a role in determining
vibration frequency- if the cells can't fire
that fast, they can't make the discrimination
artificial stimulation using that frequency
encodes the answer
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Cortical Representation of Touch
SII also has 4 functionally distinct areas in
macaques (and humans?) SII integrates various
inputs for representing sizes and shapes of
objects also includes texture
discrimination SII activates in response to more
complex features of a stimulus very late
processing stage in somatosensory processing SII
activity is modulated by attention- concentration
improves tactile discrimination and increases
the response rate to 90 of neurons the more
fine the discrimination task, the more attention
affects it and the more synchronous firing
happens SII appears to be a major area of brain
function improved by attention