SC Proprioception/Tactile Pathways Flashcards
2 general types of somatic sensation
Discriminative tough, flutter-vibration, proprioception
Crude touch, thermal sensation, nociception
What brain area is composed of postcentral gyrus and posterior paracentral gyrus?
Primary somatosensory cortex
[bordered anteriorly by central sulcus and posteriorly by postcentral sulcus]
2 aa. supplying the primary somatosensory cortex
Anterior cerebral a.
Middle cerebral a.
What effects would lesions to the anterior or middle cerebral aa. have on the primary somatosensory cortex?
Lesions to anterior cerebral a. —> tactile loss over contralateral lower extremity
Lesions to middle cerebral a. —> tactile loss over contralateral upper extremity and face
Axons from ______-order thalamic neurons terminate in primary somatosensory cortex
Third
Subdivisions of primary somatosensory cortex from anterior to posterior
Brodmann area 3a
Brodmann area 3b
Brodmann area 1
Brodmann area 2
Brodmann area ____ is located in depths of central sulcus and abuts area 4 which is the ________ cortex
Brodmann areas ____ and ____ extend up the bank of the central sulcus onto the shoulder of the postcentral gyrus
Brodmann area _____lies on the postcentral gyral surface and abuts area 5 which is the _______ cortex
3a; primary motor
3b; 1
2; somatosensory association
Effects of lesions in primary somatosensory cortex — specifically areas 3b, 1, or 2
Area 3b — profound deficits in both texture AND size/shape discrimination
Area 1 — deficit in texture discrimination
Area 2 — loss of size/shape discrimination (astereognosis)
Typically, primary somatosensory lesions include larger areas and frequently result in more global deficits like loss of proprioception, vibratory sense, pain, and thermal sensations on the _____ side of the body
Contralateral
What part of the brain lies deep in the inner face of the upper bank of the lateral sulcus?
Secondary somatosensory cortex
What are the 2 primary inputs to secondary somatosensory cortex?
Ipsilateral primary somatosensory cortex
Ventral posterior inferior nucleus of thalamus
What general regions of the brain are located posterior to area 2, and include area 5 and 7b?
Parietal cortical regions
Parietal cortical regions receive some tactile inputs from ______ _____ input and _______ ______ cortex
Medial-lemniscal; primary somatosensory
Effects of lesions in parietal association area
Agnosia (contralateral body parts lost from personal body map)
What are the 4 main pathways associated with the primary somatosensory cortex?
Posterior column-medial lemniscal pathway
Trigeminothalamic pathways
Spinocerebellar pathways
Anterolateral system
What is the primary pathway for discriminative touch, flutter-vibration, and proprioception?
Posterior-column medial lemniscal pathway
[trigeminothalamic pathways and spinocerebellar pathways also participate, just aren’t the primary paths for this info]
Characteristic features of posterior column-medial lemniscal pathway
Afferent fibers with fast conduction velocity
Limited number of synaptic relays
Precise somatotopic organization
Signals by use of frequency and population codes
2-point discrimination
Accuracy of 2-point discrimination via PCML depends on receptor density and receptive field size. Which areas have high vs. low density and large vs. small receptive fields?
High density areas/small receptive fields = digits and perioral region (lips); take up large portion of somatosensory cortex
Low density areas/large receptive fields = back; take up small region of somatosensory cortex
Order of events in activation of PCML pathway
Activation of peripheral mechanoreceptors —> transduced to electric signal by primary afferent neuron
Primary afferent fibers enter SC via medial division of posterior root then branch
One set of branches terminates on second-order neurons in spinal cord gray matter at, above, or below level of entry (most ascend cranially to posterior columns = fasciculus gracilis + fasciculus cuneatus)
Second-order neurons of posterior column nuclei send axons to contralateral thalamus. Internal arcuate fibers loop anteromedially in medulla — cross midline as sensory decussation and ascend as medial lemniscus on opposite side
Medial lemniscus terminates in ventral posterolateral nucleus (VPL) of the thalamus, third-order neurons go to SI
3 components of primary afferent fibers in PCML
Peripheral process = extends from DRG (mechanoreceptor)
Central process = extends from DRG into CNS
Pseudounipolar cell body = in DRG
2 components of posterior columns
Fasciculus gracilis (includes sacral level fibers positioned medially and more superior fibers up to T6 added laterally) — transmits to nucleus gracilis in posterior medulla
Fasciculus cuneatus (includes levels T6 and above, added laterally to fasciculus gracilis) — transmits to nucleus cuneatus in posterior medulla
What is the difference between core clusters of fibers vs. outer shell fibers in the fasciculus gracilis or fasciculus cuneatus?
Core clusters receive input from rapidly and slowly adapting afferents
Outer shells receive input from muscle spindles, joints, and pacinian corpuscles
Effects of lesions in either fasciculus gracilis or cuneatus
Ipsilateral reduction/loss of discriminative, positional, and vibratory tactile sensations at and below the segmental level of injury
Sensory ataxia = loss of DTRs and proprioceptive losses from extremities
Where do second-order neurons of posterior column nuclei (in the medulla) send axons?
Contralateral thalamus
Second-order neurons of posterior column nuclei (in medulla) send axons to contralateral thalamus.
This is done via _____ ______ fibers which loop anteromedially in the medulla, crossing midline as sensory decussation and ascending as the _____ _____ on the opposite side.
As the medial lemniscus extends rostrally, it rotates laterally in the pons. Upper extremity fibers lie ________, lower extremity fibers lie ________
Internal arcuate; medial lemniscus
Medially; laterally
The medial lemniscus terminates in ventral posterolateral nucleus (VPL) of the thalamus in the caudal thalamus. What is the other component of the ventral posterior nuclei and what is its function?
Ventral posteromedial nucleus (VPM) — receives head proprioceptive info
Both the VPL and VPM are supplied by thalamogeniculate branches of what a?
Posterior cerebral a.
Compromise of posterior cerebral a. have what effect on the ventral posterior nuclei?
Loss of all tactile sense over contralateral body and head
Within the ventral posterolateral nucleus, fibers from the contralateral nucleus cuneatus terminate medial to those of the nucleus gracilis. Rapidly and slowly adapting inputs target the VPL ______; pacinian and joint/muscle inputs target VPL ______
Core; shell
2 populations of neurons associated with ventral posterolateral nucleus
Third-order neurons = large-diameter axons that traverse posterior limb of the internal capsule and terminate in the primary somatosensory and secondary somatosensory cortices
Local circuit interneurons = inhibitory; receive excitatory corticothalamic inputs and influence the firing rates of third-order neurons
Lesions of the posterior column nuclei at the midbrain level have what effect?
Deficits in discriminitve touch, vibratory, and proprioception over contralateral side of body
Brainstem or SC lesions that result in deficits that differ between each half of the body — dependent upon where fibers cross
What does this mean for brainstem lesions?
Brainstem lesions can result in sensory deficits of trunk/extremities contralateral to the lesion, but sensory deficits of face/CN ipsilateral to the lesion
[i.e., the right face and the left arm/leg lack proprioceptive information]
Brainstem or SC lesions that result in deficits that differ between each half of the body — dependent upon where fibers cross
What does this mean for spinal cord lesions?
Proprioceptive deficits on the right, but anesthesia on the left (or vice versa)
