3 - Somatosensory System: Dorsal Column/Medial Lemniscus

Question 1

What types of sensations does the dorsal column/medical lemniscus system transmit?

Answer 1

1. Light touch: 2 pt disc., stereognosis (depth), graphesthesia (writing on skin).
2. Pressure
3. Vibration
4. Proprioception/limb position
5. Kinesthesia (motion sense)
6. Tactile extinction - paying attention to a stimulus on one side and not the other

Question 2

What are the components of the DC/ML system? How many neurons are there?

Answer 2

1st neuron: in sensory ganglion (afferent)
2nd neuron: in brainstem nuclei (or spinal cord) - decussates
3rd neuron: in thalamus

Question 3

Describe the location and projection of the 1st neuron of the DC/ML system? What is it’s function?

Answer 3

Primary afferent neuron with peripheral process that transmits info from mechanoreceptors.

Cell body in DRG, central process transmits info into dorsal root and ascends within ipsilateral dorsal columns.

Terminates in dorsal column nuclei.

Question 4

Describe the location and projection of the 2nd neuron of the DC/ML system?

Answer 4

Cell body in nucleus gracilis or nucleus cuneatus.

Axon decussates to contralateral side and ascends as medial lemniscus.

Terminates in ventral posterior lateral (VPN) nucleus of thalamus.

Question 5

Describe the location and projection of the 3rd neuron of the DC/ML system?

Answer 5

Cell body in the VPL of the thalamus.

Axons pass through the posterior limb of internal capsule.

Terminates in S1 cortex (postcentral gyrus, primary somatosensory cortex).

Question 6

What are the two dorsal columns that the 1st neuron travels through? Where is each located?

Answer 6

Fasciculus gracilis: located medially; axons from sp cd levels caudal to T6. Lower limbs “graceful ballet dancer”.

Fasciculus cuneatus: laterally; axons from T6 and rostral. Upper limbs (“cunningly rubs hands”).

Question 7

Where is each levels dermatome located in the 1st neuron of the spinal cord? What is the purpose of this placement?

Answer 7

Sacral dermatomes medial.

Lumbar, thoracic, and cervical dermatomes are located progressively more laterally.

Helps preserve info about location and nature of stimulus.

Question 8

What does a unilateral spinal cord lesion of the 1st neuron result in?

Answer 8

Loss of light touch, pressure, vibration, and proprioception on the SAME SIDE of the lesion from dermatomes just below the level of the lesion.

Loss of other sensory and motor functions.

Question 9

What does a bilateral spinal cord lesion of the 1st neuron result in?

Answer 9

Loss of light touch, pressure, vibration, and proprioception from dermatomes just below the level of the lesion.

Loss of other sensory and motor functions.

Question 10

What is posterior cord syndrome of the first neuron?

Answer 10

Loss of light touch, pressure, vibration, and proprioception from dermatomes below the level of the lesion (whole body except head).

Other sensory and motor functions intact.

Question 11

What is the result of a large central cord lesion of the 1st neuron?

Answer 11

Loss of light touch, pressure, vibration, and proprioception from dermatomes below the level of the lesion.

*Sacral region spared because those regions are pushed more laterally to the back of the cord.

Loss of other sensory and motor functions.

Question 12

What is the path of the 2nd neuron of the DC/ML system?

Answer 12

Cell bodies in nucleus gracilis and nucleus cuneatus of the caudal medulla.

Axons decussate as internal arcuate fibers.

Axons from medial lemniscus through rostral medulla, pons, and midbrain.

Terminate in the VPL.

Question 13

What is the somatotopy of the 2nd neuron at the level of the rostral medulla and the pons?

Answer 13

Rostral medulla level: “headless hemi-man standing on the pyramids”

Pons level: fibers from face are added via trigeminal nerve.

Question 14

What does a lesion of the medial lemniscus of the 2nd neuron result in? What is this called?

Answer 14

Loss of light touch, pressure, vibration, and proprioception from dermatomes below the level of the lesion on the CONTRALATERAL side of the body.

Medical medullary syndrome.

Question 15

Where is the cell body of the 3rd neuron in the DC/ML system located? Where do axons travel and terminate?

Answer 15

Cell body in the VPL of thalamus.

Axons pass through posterior limb of internal capsule.

Terminate in S1 cortex (postcentral gyrus, primary somatosensory cortex).

Question 16

What are the functions of the VPL and VPM of the thalamus?

Answer 16

VPL: sensation from the body

VPM: sensation from the face

Question 17

What is the path of the third neuron. What is the somatotopy in this region?

Answer 17

Third order neuron cell bodies in the thalamus send axons to form the internal capsule and make contact with the cerebral cortex.

VDL concerned with sensation from the body.

Also a few fibers from face in VDM.

Question 18

What do the axons of the 3rd neuron pass through? What does this tell us about injuries to this region?

Answer 18

The posterior limb of the internal capsule.

More distal lesions in the radiating fibers have less of an impact because the fibers are like holding a hand of cards; lower lesions have a greater impact and impact more fibers (“cards”).

Question 19

What do the axons of the 3rd neuron fan out as? Where do fibers from VPL and VPM pass?

Answer 19

Fan out as corona radiata.

Fibers from VPL and VPM pass to the S1 cortex.

Question 20

Sensation from which body parts are located more laterally in the S1 cortex? What body parts transmit sensation more medially?

Answer 20

Lower limb sensation occurs medially.

Torso, upper limb, and face have a huge representation in the more lateral regions of the cortex.

Question 21

What results from lesions of the thalamus or S1 cortex?

Answer 21

Loss of sensation from CONTRALATERAL half of the body.

Question 22

The S1 cortex contains which Broadmann areas? What is in each of these areas?

Answer 22

3a, 3b, 1, and 2

In each there’s a complete map of the skin.

Question 23

What are Brodmann area 3a and 3b of the S1 cortex involved in?

Answer 23

3a - limb movement

3b - basic tactile info (edges, texture)

Question 24

What are Brodmann area 1 and 2 of the S1 cortex involved in?

Answer 24

Area 1 - motion and direction of movement of objects

Area 2- limb position and shape of objects.

Question 25

Where do S1 neurons project? Where are these located?

Answer 25

S1 neurons project to the S2 cortex - located alone the upper border of sylvian fissure and insular cortex.

Question 26

Where are the parietal association cortices located? What is the function of this?

Answer 26

Along the intraparietal sulcus.

Receives sensory info and project to motor cortex.

Question 27

How would you know that you’re looking at a section of the cervical spinal cord?

Answer 27

You would see the fusciculus gracilis and the fasciculis cuneatus.

Question 28

What dermatome levels are located where in the spinal cord?

Answer 28

Sacral regions (that for dermatomes of the lower limbs) are more medial in the spinal cord (fasciculus gracilis)

Thoracic and cervical regions (for dermatomes of the upper limbs) are located more laterally in the spinal cord (fasiculus cuneatus).

Question 29

What association cortex is in the parietal lobe? What brodmanns areas are these? What would a lesion here cause?

Answer 29

Somatosensory association in the superior parietal lobule.

Areas 5 and 7.

Lesion causes astereognosis - telling what something in your hand is.

Question 30

What association cortex is in the occipital lobe? What broadmanns area is this?

Answer 30

Visual association - ie seeing something red and round and knowing its an apple.

Areas 18 and 19.

Question 31

What association cortex is in the temporal lobe? What brodmanns area is this?

Answer 31

Auditory association.

Area 42.

Question 32

What are multimodal association cortices? What do multimodal lesions in these cortices cause?

Answer 32

Cortices that combine sensation with motivation, attention, and relevance.

Contralateral neglect (usually happens on right hemisphere, causing left hemineglect).