Somatosensory I Flashcards
what are the two main pathways in the somatosensory system
posterior column - medial lemniscal pathway
trigeminothalamic pathway
which pathway is involved in awareness of body positions and limb movement
posterior column - medial lemniscal pathway (PMCLS)
what are the characteristic features afferent fibers of the PCMLS
fast conduction velocity,
few synaptic relays
what system allows of to localize touch with accuracy
PCMLS
what is two-point discrimination and what system is responsible for if
ability to discriminate between two stimuli simulataneously
PCMLS
what aspect of the PCMLS is two-point discrimination related to?
peripheral nerve ending density
what regions of the body have increased density of tactile receptors
hands, perioral
do small receptive fields have high or low receptor density
high receptor density
what are the three parts of primary afferent fibers
peripheral process (soma => periphery) central process (soma => CNS) pseudounipolar cell body
where is the afferent fiber cell body located
dorsal root ganglia
what information do large-diameter afferent fibers relay
discriminative touch
flutter vibration
proprioception
where do large-diameter afferent fibers enter the spinal cord
at the medial division of the posterior (AKA dorsal) root
what structure do the largest set of afferent fibers form superiorly?
posterior columns (fasciculus gracilis and cuneatus)
what two structures make up the posterior columns
fasciculus gracilis
fasciculus cuneatus
which part of the posterior column holds fibers from T6-sacrum
fasciculus gracile
which part of the posterior column holds fibers from cervical-T6
fasciculus cuneatus
how are afferent fibers within the posterior column organized?
more inferior (sacral) fibers are medial superior fibers are lateral
spinal cord lesions result in loss of tactile sensation where?
at and below segmental level of injury
what should you suspect if a patient presents with wide stance, stomping and absent DTRs
sensory ataxia, secondary to a SC lesion
where are nucleus gracile and nucleus cuneatus found
posterior medulla
where do nucleus gracile and nucleus cuneatus receive input from
primary afferents from the ipsilateral dorsal root ganglia via its corresponding fasciculus
what sensory information is received at the cores of nucleus gracilis and nucleus cuneatus
inputs from afferent fibers
what sensory information is received at the outer shells of nucleus gracilis and nucleus cuneatus
inputs from muscle spindles, joints and Pacinian corpuscles
where do second order neurons of the posterior column nuclei send their axons
through the midbrain, to the contralateral thalamus
within the medial lemniscus, how are fibers from the extremities organized
UE fibers lie medially
LE fibers lie laterally
where does the medial lemniscus terminate
ventral posterolateral nucleus
how would a patient with damage at the brainstem present
deficits in discriminative touch, sensation of position and vibration of the CONTRALATERAL side
what is the wedge-shaped group of cells in the caudal thalamus
ventral posterior nuclei
what separates the ventral posterolateral nucleus (VPL) from the ventral posteromedial nucleus (VPM)
fibers of the arcuate lamina
what is the blood supply to the ventral posterior nuclei
thalamogeniculate branches of the PCA
how would a patient present after compromise of the thalamogeniculate branches of the PCA
loss of all tactile sensation of contralateral and head
what are the inhibitory neurons that influence the firing rates of third-order neurons?
local circuit interneurons
a lesion to what structure would cause tactile loss over the contralateral upper body and face
MCA
a lesion to what structure would cause tactile loss over the contralateral LE
ACA
what are the subdivisions of the somatosensory cortex?
Brodmann areas: 3a 3b 1 2
from what structure does the secondary somatosensory cortex receive inputs
ipsilateral cortex
ventral posterior inferior nucleus of the thalamus
what are the four trigeminal nuclei
main sensory nucleus (msT)
trigeminal motor nucleus (mT)
spinal nucleus
mesencephalic nucleus (mes)
what information is carried by the main sensory nucleus
touch and pressure
what information is carried by the trigeminal motor nucleus
muscles of mastication
what information is carried by the spinal nucleus
pain and temperature
what information is carried by the mesencephalic nucleus
proprioceptive afferents from the TMJ and masticatory ms
what division of the main sensory nucleus receives afferent input from the oral cavity
dorsomedial division
what division of the main sensory nucleus receives afferent input from V1, V2 and V3
ventrolateral division
where do secondary fibers of the dorsomedial divison of the main sensory nucleus travel
ipsilaterally via the posterior trigeminothalamic tract
where do secondary fibers of the ventrolateral divison of the main sensory nucleus travel
contralaterally via the anterior trigeminothalamic tract
what is the target of the anterior and posterior trigeminothalamic tracts
the ventral posteromedial nucleus (VPM)
where do third order from the VPM project
through the posterior limb of the internal capsule to the primary somatosensory cortex
what are the sxs of a spinal cord lesion
proprioceptive defitics on one side and anestheia on the other
what sxs would be seen in a lesion to area 1 of the SI
deficit in texture discrimination
what sxs would be seen in a lesion to area 2 of the SI
loss of size and shape discrimination (astereognosis)
what sxs would be seen in a lesion to area 3b of the SI
deficits in size, shape and texture discrimination
lesion of the left posterior column at L1 produces what impairments?
loss of light touch, vibration, position sensation of the LEFT leg
lesion of the left fasciculus cuneatus at C3 produces what impairments
loss of light tough, vibration and position sensation of the left arm and trunk
afferents from what spinal levels make up the fasciculus gracilis
T6 and below
afferents from what spinal levels make up the fasciculus cuneatus
T6 and above
where are the 1st order neurons of the PCMLS
dorsal root ganglia
where and what are the 2nd order neurons of the PCMLS
medulla - nuclei gracilis and cuneatus
where and what are the 3rd order neurons of the PCMLS
thalamus, ventral posterior lateral (VPL) nucleus
where do afferents from the PCMLS go after synapsing in the thalamus
to the primary and secondary somatosensory cortices (via corona radiata)
from what vessels does the primary somatosensory cortex receive blood supply
ACA and MCA
