somatic sensory system
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
PowerPoint Presentation - Somatic Sensory System
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 differentroutes 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. slowLocation: skin, muscle, tendon, hairRate 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
neurons 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:
Meissners corpusclePacinian corpuscle Merkels disk Ruffinis
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.Doesnt 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 pathwayPain and temperature use
spinothalamic (anterolateral pathway).
The Main Mechanosensory Pathways
cross in the medulla
upper and lower bodyuse slightly different pathways.
Medial lemniscus
Trigeminal pathway
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info from head and face
Somatic sensory cortex
Located in parietal lobe-post central gyrusDivided into regions,
Broadmanns areas 3b and 1: cutaneous3a: proprioception2: tactile
and proprioceptionEach area contains a somatotopic map of the
bodyhomunculous
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 medialLower body lateral
Human Primary Somatoensory Cortex
somatotopy
areas of high receptor densityget more cortical space
The Humunculus reflects sensory receptor density
receptive fields of somatosensory cortex
Area 3b- simple, responses generally from stimulations of a single
finger1 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
gangliaSecond order: brainstem nucleiThird order: thalamus .project
to SIFourth order; SI projects to SIITopographical 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 extrastimulation of
touch receptors.
scheme for transcutaneousnerve recording.
Nociceptor doesntfire until pain is felt.Other thermorecptorsfire
at all temps and atabout 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 viadorsal 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
morphine-induced 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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