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Chapter 12: Somatosensory (Four qualities of a stimulus (location:…
Chapter 12: Somatosensory
mechanoreceptors: sensitive to binding, stretching, pressure, vibration
free nerve endings
specialized nerve endings
Merkel's disk
small, slow
Meissner's corpuscle
small, rapid
pacinian corpuscle
large, rapid
capsule makes corpuscle sensitive to VIBRATING, HIGH FREQUENCY stimuli
intact corpuscle --> large receptor potential at stimulus on and stimulus off
without corpuscle --> slow adaptation
Ruffini ending
large, slow
Four qualities of a stimulus
modality: touch
location: receptive field - awareness of spatial aspects depends on spatial distribution of activated receptors
receptive field of mechanoreceptor: certain area of skin where it can transduce pressure or vibration
surround inhibition of RF ENHANCES SPATIAL RESOLUTION
two-point discrimination: ability to distinguish 2 points as distinct
two-point discrimination is best for body regions with the highest innervation density and amount of cortical area devoted to them (fingers and lips)
intensity: firing rate
duration: time course of response
Mechanosensitive ion channels are sensitive to:
stretch of lipid bilayer
force applied to extracellular proteins linked to channel
force applied to intracellular proteins linked to channel
Primary afferent axons enter through DORSAL roots
from skin (areas 3b and 1)
AS
pain, temperature
C
temperature, pain, itch
AB
touch vibration, myelinated
from muscle spindles and Golgi tendon organs (areas 3a and 2)
I
II
Segmental organization of spinal cord and sensory innervation of skin are related
dermatome: area of skin and tissues innervated by a single DORSAL ROOT
Pathways for touch and vibration
Dorsal column-medial lemniscal pathway
touch, vibration, conscious proprioception information ascends through
dorsal columns (spinal cord)
dorsal column nuclei (medulla)
medial lemniscus (brainstem) and then decussates
VPL (thalamus)
S1
Trigeminal Touch Pathway
somatic sensation of the face
cranial nerve V, from face to Principal sensory nucleus of V
VPM
S1
Pain and temperature: activate transient receptor potential ion channels (TRP)
Nociceptors
mediate transduction of pain
activated by stimuli that could cause nerve damage (scalding temperature, strong mechanical)
free nerve endings
Thermal receptors
mediate transduction of cold, cool, warm, hot
free nerve endings
Pathways for pain and temperature
Spinothalamic tract (for pain and temperature)
AS and C afferents synapse in the substantia gelatinosa of dorsal horn
decussate and travel up the cord in the spinothalamic tract to the VPL
S1
Trigeminal Pain pathway: pain and temperature of the FACE
Referred pain: crosstalk between visceral afferent and cutaneous nociceptors
when VISCERAL nociceptor activation is perceived as a CUTANEOUS sensation
Disorders of Posterior Parietal Cortex: analysis of "where" things are from visual inputs
agnosia: inability to recognize objects
astereognosia: loss of stereognosis (ability to perceive form of an object by using touching)
neglect syndrome: part of body or world is ignored
functional columns for slowly vs. rapidly adapting neurons