SC Proprioception/Tactile Pathways

Question 1

2 general types of somatic sensation

Answer 1

Discriminative tough, flutter-vibration, proprioception

Crude touch, thermal sensation, nociception

Question 2

What brain area is composed of postcentral gyrus and posterior paracentral gyrus?

Answer 2

Primary somatosensory cortex

[bordered anteriorly by central sulcus and posteriorly by postcentral sulcus]

Question 3

2 aa. supplying the primary somatosensory cortex

Answer 3

Anterior cerebral a.

Middle cerebral a.

Question 4

What effects would lesions to the anterior or middle cerebral aa. have on the primary somatosensory cortex?

Answer 4

Lesions to anterior cerebral a. —> tactile loss over contralateral lower extremity

Lesions to middle cerebral a. —> tactile loss over contralateral upper extremity and face

Question 5

Axons from ______-order thalamic neurons terminate in primary somatosensory cortex

Answer 5

Third

Question 6

Subdivisions of primary somatosensory cortex from anterior to posterior

Answer 6

Brodmann area 3a
Brodmann area 3b
Brodmann area 1
Brodmann area 2

Question 7

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

Answer 7

3a; primary motor

3b; 1

2; somatosensory association

Question 8

Effects of lesions in primary somatosensory cortex — specifically areas 3b, 1, or 2

Answer 8

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)

Question 9

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

Answer 9

Contralateral

Question 10

What part of the brain lies deep in the inner face of the upper bank of the lateral sulcus?

Answer 10

Secondary somatosensory cortex

Question 11

What are the 2 primary inputs to secondary somatosensory cortex?

Answer 11

Ipsilateral primary somatosensory cortex

Ventral posterior inferior nucleus of thalamus

Question 12

What general regions of the brain are located posterior to area 2, and include area 5 and 7b?

Answer 12

Parietal cortical regions

Question 13

Parietal cortical regions receive some tactile inputs from ______ _____ input and _______ ______ cortex

Answer 13

Medial-lemniscal; primary somatosensory

Question 14

Effects of lesions in parietal association area

Answer 14

Agnosia (contralateral body parts lost from personal body map)

Question 15

What are the 4 main pathways associated with the primary somatosensory cortex?

Answer 15

Posterior column-medial lemniscal pathway

Trigeminothalamic pathways

Spinocerebellar pathways

Anterolateral system

Question 16

What is the primary pathway for discriminative touch, flutter-vibration, and proprioception?

Answer 16

Posterior-column medial lemniscal pathway

[trigeminothalamic pathways and spinocerebellar pathways also participate, just aren’t the primary paths for this info]

Question 17

Characteristic features of posterior column-medial lemniscal pathway

Answer 17

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

Question 18

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?

Answer 18

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

Question 19

Order of events in activation of PCML pathway

Answer 19

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

Question 20

3 components of primary afferent fibers in PCML

Answer 20

Peripheral process = extends from DRG (mechanoreceptor)

Central process = extends from DRG into CNS

Pseudounipolar cell body = in DRG

Question 21

2 components of posterior columns

Answer 21

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

Question 22

What is the difference between core clusters of fibers vs. outer shell fibers in the fasciculus gracilis or fasciculus cuneatus?

Answer 22

Core clusters receive input from rapidly and slowly adapting afferents

Outer shells receive input from muscle spindles, joints, and pacinian corpuscles

Question 23

Effects of lesions in either fasciculus gracilis or cuneatus

Answer 23

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

Question 24

Where do second-order neurons of posterior column nuclei (in the medulla) send axons?

Answer 24

Contralateral thalamus

Question 25

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 ________

Answer 25

Internal arcuate; medial lemniscus

Medially; laterally

Question 26

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?

Answer 26

Ventral posteromedial nucleus (VPM) — receives head proprioceptive info

Question 27

Both the VPL and VPM are supplied by thalamogeniculate branches of what a?

Answer 27

Posterior cerebral a.

Question 28

Compromise of posterior cerebral a. have what effect on the ventral posterior nuclei?

Answer 28

Loss of all tactile sense over contralateral body and head

Question 29

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 ______

Answer 29

Core; shell

Question 30

2 populations of neurons associated with ventral posterolateral nucleus

Answer 30

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

Question 31

Lesions of the posterior column nuclei at the midbrain level have what effect?

Answer 31

Deficits in discriminitve touch, vibratory, and proprioception over contralateral side of body

Question 32

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?

Answer 32

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]

Question 33

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?

Answer 33

Proprioceptive deficits on the right, but anesthesia on the left (or vice versa)