Somatosensory Tracts #1

Question 1

This tract is involved with discriminative touch, 2-point localization, vibration, pressure, and proprioception of the limbs.

Answer 1

Posterior Column - Medial Lemniscal (PCML)

Question 2

Areas of skin innervated by somatic afferent fibers that have small receptive fields have (LOW/HIGH) receptor density. Areas with large receptive fields have (LOW/HIGH) receptor density.

Answer 2

High
Low

***If it’s a small receptive field, you need more to cover more space. If it’s a large receptive field, you don’t need as many because they cover more space. Homunculus follows this rule.

Question 3

In the PCML, a spinal cord lesion will result in (IPSILATERAL/CONTRALATERAL) reduction or loss of discriminative, positional, and vibratory tactile sensations at and below the segmental level of the injury.

Answer 3

Ipsilateral

Question 4

This is a loss of muscle stretch (tendon) reflexes, and proprioceptive losses from the extremities due to lack of sensory input. Patient may also have a wide-based stance and may place the feet to the floor with force, in an effort to create the missing proprioceptive input.

Answer 4

Sensory ataxia

Question 5

These are in the posterior medulla and contain 2nd order neurons of the PCMLS. They also receive input from 1st order neurons (primary afferents) from the ipsilateral DRG.

Answer 5

Gracile nucleus
Cuneate nucleus

***Remember T6 and up is cuneate, T6 and below is gracile

Question 6

The 2nd order neurons of the PCMLS send axons to the (IPSILATERAL/CONTRALATERAL) thalamus via the Internal Arcuate Fibers.

Answer 6

Contralateral

Question 7

In the PCMLS, once the 2nd order neurons decussate via Internal Arcuate Fibers across the medulla, they ascend as the ________ ________.

Answer 7

Medial Lemniscus

Question 8

In the Medial Lemniscus (ML), it rotates laterally in the pons, making the UE fibers lie (MEDIALLY/LATERALLY) and the LE fibers lie (MEDIALLY/LATERALLY).

Answer 8

Medially

Laterally

Question 9

The ML terminates in what nucleus within the thalamus?

Answer 9

VPL (Ventral Posterolateral)

Question 10

In the PCMLS, the 3rd order neurons leave the VPL and traverse what to get to the primary somatosensory cortex (in post central gyrus)?

Answer 10

Posterior limb of internal capsule

Question 11

The primary somatosensory cortex is subdivided anterior to posterior by what?

Answer 11

Brodmann areas – 3a, 3b, 1, and 2

Question 12

Blood supply to the primary somatosensory cortex is by the…

Answer 12

Anterior Cerebral As.

Middle Cerebral As.

Question 13

Lesions in this artery produces tactile loss over the contralateral upper body and face.

Answer 13

MCA

Question 14

Lesions in this artery affect the contralateral lower limb.

Answer 14

ACA

Question 15

For somatosensory to the face, the Trigeminal nerve is used. For pain, temperature, and crude touch the ________ tract is used. For discriminative touch, 2-point localization, vibration, and proprioception the ________ ________ tract is used.

Answer 15

Spinal

Principal Sensory

Question 16

In the Trigeminal - Principal Sensory Tract, what nucleus do the 1st order neurons synapse in that are involved with discriminative touch, 2-point localization, and vibration?

Answer 16

Principal Sensory Nucleus

Question 17

In the Trigeminal - Principal Sensory Tract, what nucleus does the 1st order neurons synapse in that are involved with proprioception?

Answer 17

Mesencephalic Nucleus

Question 18

What nucleus does the 1st order neurons synapse in for the Trigeminal - Spinal Tract?

Answer 18

Spinal Trigeminal Nucleus

***Remember, involved with pain, temperature, and crude touch

Question 19

For the sensory to the face, what is the nucleus that the 2nd order neurons synapse in (for all tracts)?

Answer 19

VPM nucleus (ventral posteromedial)

Question 20

Some axons from the Mesencephalic Nucleus terminate in the trigeminal motor nucleus. This forms the circuit for the…

Answer 20

Jaw-Jerk reflex

Question 21

The Principal Sensory Nucleus can project axons dorsomedially called the _________ ________ tract, which are ipsilateral and go to the oral cavity. Or, it can project axons ventrolaterally called the ________ ________ tracts, which are contralateral and go to V1, V2, and V3 areas.

Answer 21

Posterior Trigeminothalamic

Anterior Trigeminothalamic

Question 22

What is the general pathway of the PCMLS for the upper extremity?

Answer 22

UE – Fasciculus Cuneatus – Nucleus Cuneatus – Medial Leminiscus (contralateral) – VPL – Primary Somatosensory Cortex

Question 23

What is the general pathway of the PCMLS for the lower extremity?

Answer 23

LE – Fasciculus Gracilis – Nucleus Gracilis – Medial Leminiscus (contralateral) – VPL – Primary Somatosensory Cortex

Question 24

What is the general pathway of the Trigeminal Principal Sensory Tract to the oral cavity?

Answer 24

Head/Neck – Principal Sensory Nucleus (dorsomedial) – Posterior Trigeminothalamic Tract – Ipsilateral VPM – Primary Somatosensory Cortex

Question 25

What is the general pathway of the Trigeminal Principal Sensory Tract to the areas of V1, V2, and V3?

Answer 25

Head/Neck – Principal Sensory Nucleus (ventrolateral) – Anterior Trigeminothalamic Tract – Contralateral VPM – Primary Somatosensory Cortex

Question 26

A lesion of the R posterior column(s) at L1 produces what impairment(s)?

Answer 26

– Loss of light touch, vibration, and proprioception of the R leg

***Only Fasciculus Gracilis exists below T6

Question 27

A lesion of the R fasciculus cuneatus at C3 produces what impairment(s)?

Answer 27

– Loss of light touch, vibration, and proprioception of R arm and trunk

Question 28

A complete transection of the posterior columns in the cervical region would produce what impairment(s)?

Answer 28

• • Damage to fasciculi cuneatus and gracilis

- - Absence of light touch, vibration, proprioception bilaterally, from neck down (below lesion level)