75 terms

somatosensory

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speed of info processing depends on?
-diameter of axons
-degree of myelination
-number of synapses in the pathway
proprioception
-provides info regarding stretch of muscles, tension on tendons, position of joints & deep vibrations
SS receptors
-specialized component of PNS
-located @ distal end
-function: transduction of stimulus into an electrical signal
what does being specialized mean? (SS receptors)
-only respond to a specific type of stimuli
adaptations of SS receptors
-slow adapting/tonic
-fast adapting/phasic
slow adapting/tonic
receptor continues to fire during entire presentation of the stimulus

ex: stretching during yoga
fast adapting/ phasic
receptor adapts quickly to stimuli and stops responding

ex:
receptor density
# of receptors within a given area of body surface
receptive field
area of skin innervated by single afferent neuron

-large, small
what regions have a greater density of receptors?
distal > proximal
large receptive field
borders are difficult to define; often cover a large area
small receptive field
borders are sharply defined; cover small area
mechanoreceptors
-touch
-respond to mechanical deformation of receptor

ex: hair cells in the ear --> when they bend, they respond to mechanical deformation
proprioceptors
-responds to length of muscle, tension of joints and tendons, etc
thermoreceptors
-responds to heat/cooling
chemoreceptors
-responds to substances released by cells
nocioceptors
-pain
-sensitive to stimuli that damage or can damage

**if something really hurts, will go on this pathway vs. the other pathway

-not really its own "Type;" rather it's a subtype of all of these
touch receptors
-fine touch
-coarse touch
fine touch
-Meissner's corpuscle: light touch & vibration, phasic

-Merkel's disk: pressure/tonic

-Pacinian corpuscle: vibration/touch; phasic

-Ruffini corpuscle/ending: skin stretch; tonic
coarse touch
free nerve endings; crude touch, pressure, tickle, itch
how is the SS system organized?
-3 primary components
-longitudinal and parallel systems
-3 neuron projection system

**vast majority will be sent on those pathways
3 primary components of SS pathways?
-receptors, sensory pathways, brain centers
types of sensory pathways
-conscious relay
-divergent
-unconscious
conscious relay pathway
-info about location and type of stimulation to cortex

-high fidelity: ability to get a lot of info and detail from here (accurate)
divergent pathways
transmit info to many areas of the brainstem and cerebrum
unconscious pathways
bring unconscious proprioceptive and other movement-related info to the cerebellum

-to make small automatic adjustments

-where's our body in space, etc
what kind of neurons are counted for the SS pathways?
projection
primary somatosensory cortex
(S1)
-post central gyrus

-touch: size, texture, and shape of objects

-super detailed info
somatosensory association
provides stereognosis and memory of the tactile and spatial environment
somatotopic representation of S1
lateral: face
medial: foot
stereognosis
the ability to recognize an object by touch alone
primary afferent fiber classification
-diameter (I-IV)
-conduction velocity (A-C)
first order neurons
-pseudounipolar

-doesn't receive info from other neurons, but from the environment (touch) and sends to the dorsal horn

-cell body: dorsal root ganglia
dorsal root
-afferent sensory root that enters into the SC in the dorsal horn (gray matter)
dorsal column
-white matter
-important because topographically organized
dermatomes
-area of skin innervated by axons from cell bodies in a single dorsal root

overlapping
dorsal column-medial lemniscal tract (DCML)
-function: discriminative touch, vibration, conscious proprioception

-mechano & proprioceptors

-three neuron projection system
discriminative touch
-DCML tract
-locatlization of touch and the ability to discriminate between two closely spaced points touching the skin

-highly accurate
conscious proprioception
the awareness of movements and relative position of body parts
DCML - 1st order neuron
-conveys info from receptor to medulla

-cell bodies located in the dorsal root ganglia

-ipsilateral projection into dorsal column

-synapse at dorsal column nuclei of medulla (the 2nd order neuron)
fasciculus cuneatus
-axons from upper limbs
-white matter
-more lateral on the dorsal column
fasciculus gracilis
-axons from lower
limbs

-more medial on the dorsal column
DCML - 2nd order neuron
-conveys info from medulla to thalamus

-cell bodies located in the nucelus G/C

-decussation occurs at the level of the medulla

-ascend as the medial lemniscus

-projects to VPL of the thalamus
VPL
ventral posterolateral nucleus

-where the 2nd order neuron will end
DCML - 3rd order neuron
-from VPL cell bodies in thalamus to the cortex

-projects through internal capsule to primary somatosensory cortex
spinothalamic tract
-function:
-discriminative (fast) pain
-discriminative temp
-coarse touch
receptors of the spinothalamic tract
-nocioreceptors (mechano + thermo)
STT 1st order
-conveys info from receptor to dorsal horn

-cell bodies located in DRG

-can join Tract of Lissauer (goes up/down 1-2 levels)

-synapses on the ipsilateral dorsal horn
STT 2nd order neuron
-conveys info from dorsal horn to thalamus

-cell bodies located in the ipsilateral horn

-decussates

-ascends in the anterolateral white matter tract area to the VPL of the thalamus
STT 3rd order neuron
-conveys info from VPL of thalamus to cortex

-projects through internal capsule to the primary somatosensory cortex
trigeminal thalamic tract
fast pain, discriminative touch, temp info from face
divergent anterolateral tracts
-not always somatotopically organized

**can not localize exactly where the pain is --> pain is everywhere

-spinomesencephalic
-spinoreticular
-spinolimbic
spinomesencephalic tract
involved in turning the eyes and head towards the source of the noxious input
spinoreticular tract
involved in pain stimuli commanding attention and interfering with sleep
spinolimbic tract
involved in emotions, personality, movement
spinocerebellar tracts
-imp for unconscious adjustments to movement and posture

-info from proprioceptors transmitted to the cerebellum
somatosensory testing
-if there's a sensory impairment, what type?
anesthesia
loss of feeling/awareness
hyperesthesia
exaggerated sensitivity to sensory stimuli
hypesthesia
decreased sensitivity to sensory stimuli
thermoanesthesia
inability to distinguish between hot and cold
analgesia
absence of pain in response to stimuli that would normally be painful
paresthesia
abnormal painless sensation; tingling, prickling
dyesthesia
unpleasant abnormal sensation
allodynia
pain evoked by stimulus that doesn't normally cause pain
hyperalgesia
decreased pain threshold
sensory ataxia
-incoordination that is not due to weakness

-lesion in peripheral sensory nerve, dorsal root, or column of spinal cord, or medial lemnisci

-difficulty with balance, incoordination

-impaired conscious proprioception and vibration
neuropathy sensory loss (I)
-large myelinated fibers affected first

-tingling/prickling occurs as the blood supply increases once compression is relieved
neuropathy sensory loss (III)
-lack of sensation in the dermatone

-pain
complete SCI
-loss of all sensation one or two levels below the level of the lesion

-voluntary motor control lost below level of lesion
brown-sequard syndrome
-incomplete SC lesion (hemisection) of the SC

-results in:
-interruption of pain/temp sensation contralateral to the lesion

-interruption of discriminative touch and conscious proprioception ipsilateral to the lesion

-ipsilateral paralysis
sensory deficits related to brain stem lesions
-usually mix of ipsi/contralateral signs

-only in the upper midbrain is all loss contralateral
thalamic lesions
-decreased or loss of sensation on contralateral body or face
somatosensory cortex/internal capsule lesions
-contralateral decrease or loss of discriminative sensations
tactile defensiveness
aversive response to seemingly non-noxious tactile stimuli

hypothesized to be due to lack of inhibition
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