Module D-03

Question 1

Describe the 3 neuron chain spinal touch pathway

Answer 1

• 1st order neuron (somatosensory receptor neuron): afferent fibers in peripheral nerve (pseudounipolar with cell body in dorsal root ganglia) enter through dorsal roots
• 2nd order neuron: fibers crossing the midline(at cuneate or gracile nuclei) and terminating in the thalamus
• 3rd order neuron: fibers running in posterior limb of internal capsule and ending in S1 (primary somatosensory cortex)

Question 2

Where does the pathway for touch and vibration start ?

Answer 2

on the body surface (dermatomes)

Question 3

Where does the pathway for proprioception start?

Answer 3

on muscles

Question 4

Where are the cell bodies of the 2nd order neurons found?

Answer 4

Cuneate and Gracilis nuclei

Question 5

Which order fibers cross over in Touch pathway?

Answer 5

2nd order neurons

Question 6

Which nucleus of the thalamus do the 2nd order touch neurons synapse?

Answer 6

ventral posterior lateral (VPL) nucleus

Question 7

Where is the primary somatosensory cortex located?

Answer 7

postcentral gyrus of the parietal lobe and the part of the paracentral lobule on the medial side

Question 8

From Where does input of the trigeminal pathway for

touch, vibration and proprioception come?

Answer 8

cutaneous mechanoreceptors and muscle proprioceptors of the face

Question 9

Where are the cell bodies for the primary afferent neuron found?

Answer 9

Trigeminal ganglion

Question 10

Describe the 3 neuron pathway for the trigeminal pathway

Answer 10

1) primary afferent fibers enter the brainstem at the pons and synapse in the principal (chief) sensory nucleus of CN V in upper half of the pons
2) second order neurons fibers exit the principal nucleus of CN V and immediately cross over to the contralateral side, where they ascend in the ventral trigeminothalamic tract and synapse in the ventral posterior medial (VPM) nucleus of the thalamus.
3) Third order neurons in the VPM nucleus of the thalamus send their axons through the posterior limb of the internal capsule, close to the genu, and then through the corona radiata then synapse in the primary somatosensory cortex
(S1).

Question 11

What part of the brainstem are the fibers for somatosensory information of touch, vibration and proprioception from the face found?

Answer 11

Upper Pons and midbrain

Question 12

Name the 4 nuclei of the Trigeminal

Answer 12

1) Principal (chief) nucleus
2) Spinal nucleus of V
3) Motor nucleus of V
4) Mesencephalic nucleus of V ( in midbrain)

Question 13

Which brodmann’s areas make up the Primary somatosensory cortex

Answer 13

3,1,2

Question 14

Somatotopic map

Answer 14

the somatosensory Homonculus , shows where on the primary somatosensory cortex is which part of the body’s sensation

Question 15

Sensation of which regions of the body are found on the Lateral side of the primary somatosensory cortex?

Answer 15

from top to bottom:
Trunk
Arm
Hand
Fingers
Upper Face
Lower Face

Question 16

Sensation of which regions of the body are found on the Medial side of the primary somatosensory cortex

Answer 16

from top to bottom:
Trunk
Leg 
Foot 
Genitalia

Question 17

What are the organizing units of the sensory cortex called

Answer 17

cortical columns (slow and rapid adapting neurons processing columns)

Question 18

Which layer of the primary somatosensory cortex receives inputs from the thalamus (VPL and VPM) through thalamocortical fibers?

Answer 18

Layer 4

Question 19

Lower threshold (distance) for 2 point discrimination the ________ the spatial resolution

Answer 19

Higher

Question 20

Which are areas of highest two point discrimination thresholds (= lowest spatial resolution) ?

Answer 20

the upper arm and forearm, thigh, calf and the back of the trunk.

Question 21

Factors Contributing to High Spatial Resolution

Answer 21

• High density of cutaneous mechanoreceptors
• Small receptive fields of the sensory receptors (which is characteristic of Merkel’s disks and Meissner’s corpuscles)
• Lateral inhibition

Question 22

Which sense is often lost first in peripheral neuropathies?

Answer 22

Vibration

Question 23

What is Stereognosis ?

Answer 23

Ability to perceive the properties of a coherent object, like its size, shape, and texture, by holding it in the hand (tested with eyes closed)

Question 24

What is Graphesthesia ?

Answer 24

Ability to distinguish letters drawn on the skin (tested with eyes closed)

Question 25

What is Tabes Dorsalis?

Answer 25

consequence of a syphilitic infection.

Destruction of dorsal root ganglion cells with large diameter myelinated axons

Question 26

What senses are lost in Tabes Dorsalis?

Answer 26

Deficit in touch and proprioception.

Nociception and temperature sense remain almost unaffected.

Question 27

What causes phantom limb sensations

Answer 27

reorganization of the cortical maps