9. Somatosensory: Proprioception/Tactile Flashcards
what is the role of the somatosensory system?
transmit and analyze touch/tactile info from interna/external locations
what are the pathways of the somatosensory system
- posterior column - medial lemniscal pathway
- trigeminothalamic pathway
- spinocerebellar pathway
- anterolateral system
what type of stimuli does the PCMLS percieve
mechanical stimuli
discriminative touch, vibration, proprioception on BODY
what can the PCMLS discriminate between
size, shape & texture
recognize 3D shapes
conscious awareness of body position & limb movement in space
what is two point discrimination
ability to discriminate btn 2-stimuli
- varies over diff areas of the body
- related to density of peripheral N endings
what are the receptor density gradients in the the body
high density of tactile receptors: digits and oral
low density: other regions like back
how is receptive field relate to receptor density
- small receptive field -> high receptor density (small spaces on skin –> need more neurons to cover all these spaces)
- large field -> low density (largers spaces -> less neurons needed to cover whole body region)
receptive field = skin innervated by somatic afferent fibers
what are the properties of primary afferent fibers
- sensory axons w/ cell bodeis in DRG –> enter sp.cord and terminate in grey matter
- distributed in periphery to form dermatomes (which are associated w fibers/pathways that relay pain/temp info)
what do the large diameter-primary afferent fibers from PCML do
relay discriminative touch, flutter/vibration & proprioception
enter sp.cord thru medial division of post root –> branch
—> stay there
or ascend (majority)
or descend
where in the spinal cord can a fiber synapse with 2nd order neurons
at
above
below levels of entry
most of the primary afferent fibers of PCML will enter the spinal cord and do what ….
ascend cranially & form posterior column
Post column =
- fasciculus gracilis (sacral - T6) (LE)
- fasciculus cuneatus (T6-superior) (UE)
how are fibers organized in the post column
organized topographically
- sacral level fibers - positioned medially
- fibers from rostral levels added laterally
what occurs when there is a lesion in the post. column (sp. cord)
ipsilat. reduction/loss of:
discriminative, positional & vibratory/tactile sensation
at or below segments of injury
what would happen if you had a tumor in the medial posterior column of the sp. cord?
first have LE extremity loss (fasciculus gracile)
then as tumor grows & hits fasciculus cuneatus (lateral part) –> get UE loss
what is sensory ataxia
neurosyphilis
loss of M stretch reflex
& loss of proprioception from extremites
-may have wide stance, place feet on floor with force
what contain the 2nd order neurons of the PCMLS
gracile nucleus (sacral -T6)
cuneate nucleus (T6 and above)
get info from primary afferent from ipsilat DRG
where do 2nd order neurons in the PCML send info ?
- cross at sensory ducussation = internal arcuate fibers
- & ascend as medial lemniscus to the thalamus *contralateral*
what is the path of medial lemniscus fibers
terminate in ventral posterolateral nucleus (VPL) of thalamus
medially = UE fibers
laterally = LE fibers
what are the ventral posterior nuclei & what is its blood supply
area in caudal thalamas
= ventral posterolateral (VPL) & ventral posteromedial (VPM)
- VPL - maintain somatotopic arrangement used in PCML path!
- BS = thalamogeniculate branches of PCA
what occurs when thre is a lesion of the thalamogeniculate branches of PCA
loss of all tactile sensation over contralat body/head (depending on if VPL or VPM)
where do 3rd order neurons start and end
start: VPL or VPM
thru post limb of internal capsule
send S1 or S2
what is S1 & what is it made of
primary somatosensory cortex
-postcentral gyrus & post. paracentral gyrus
BS = ACA & MCA
what are brodmann’s areas
subdivisions of S1
= Brodmann areas 3a, 3b, 1 & 2
= specific dedicated fxns for higher order processing
how would lesions of ACA & MCA affect the primary motor cortex
ACA: contralateral lower limb
MCA: tactile loss - contralat upper body/face
(contralat bc crossed medulla - any lesion at or above medulla in this path = contralat damage)
