Somatosensory Tracts 1: Proprioception and Tactile Pathways

Question 1

What are some important somatosensory pathways?

Answer 1

PCMLS (touch/vibration and proprioception)
Trigeminothalamic pathway (pain and temp)
Spinocerebellar pathway (proprioception)
ALS (pain and temp)

Question 2

What is the PCMLS involved with?

Answer 2

Tactile information (3D recognition), two-point discrimination, and conscious awareness of body/limb position

Question 3

How do primary fibers enter the spinal cord in the PCMLS?

Answer 3

Via medial division of posterior root

Question 4

Where are deficits located in SC lesions?

Answer 4

Ipsilateral side = reduction/loss of discriminative, positional, and vibratory tactile sensations at/below level of injury

Question 5

What is sensory ataxia?

Answer 5

Loss of DTRs and proprioceptive losses from extremities. Wide-based stance seen.

Question 6

Where are second order cell bodies located in the PCMLS?

Answer 6

In gracile nucleus or cuneate nucleus in the medulla

Question 7

What do second order neurons do in the PCMLS?

Answer 7

Synapse with ipsilateral first order neurons in the gracile/cuneate nuclei and then arc and cross at midline (sensory decussation) to medial lemniscus. Go up medial lemniscus to contralateral thalamus

Question 8

What happens to the fibers as they run up the ML in the PCMLS?

Answer 8

They rotate laterally, UE fibers lie medially, LE fibers lie latearlly

Question 9

Where does the ML terminate?

Answer 9

Ventral posterolateral nucleus (VPL) of the thalamus

Question 10

What do third order neurons do in the PCMLS?

Answer 10

Synapse with second order neurons in the VPL and then go through the posterior limb of the internal capsule to the primary/secondary somatosensory cortices

Question 11

What are the VPL and VPM called?

Answer 11

The ventral posterior nuclei

Question 12

What are the Brodmann areas of the primary somatosensory cortex?

Answer 12

3a, 3b, 1, 2 (higher integration of sensory info)

Question 13

What is the blood supply to the S1?

Answer 13

ACA and MCA

Question 14

What is agnosia?

Answer 14

Don’t know what your limb is doing but still have sensation

Question 15

What are the trigeminal nuclei?

Answer 15

Mesencephalic nucleus (TMJ, oral cavity, extraocular muscles)
Principle/chief sensory nucleus (discriminative touch, pressure, and proprioception)
Trigeminal motor nucleus
Spinal nucleus (pain and temperature)

Question 16

What are the two divisions of the trigeminal principle sensory nucleus?

Answer 16

Dorsomedial division (oral cavity)
Ventrolateral division (major - V1, V2, V3)

Question 17

How does the trigeminal principle sensory nucleus work?

Answer 17

Primary afferent cell bodies located in trigeminal ganglion. Enter laterally at mid pons to synpase in the principle sensory nucleus with secondary fibers. Secondary fiber cell bodies located in principle sensory nucleus, go either to ventrolateral/anterior (contralateral) division or dorsomedial/posterior (ipsilateral) division. Synapse with third order neurons in VPM of thalamus which travel via the posterior limb of the internal capsule to the primary somatosensory cortex (face region)

Question 18

What is the blood supply to the trigeminothalamic tract in the midbrain?

Answer 18

PICA/PSA

Question 19

What does the trigeminal mesencephalic nucleus/tract do?

Answer 19

Unconscious proprioceptive and pressure info from jaw/TMJ and eyes. Also forms circuit for jaw-jerk reflex.

Question 20

How does the jaw-jerk reflex work?

Answer 20

Mesencephalic trigeminal neuron (afferent) innervates masseter muscle. Synapses with trigeminal motor neuron (efferent), causes muscles to contract bilaterally

Question 21

What is the blood supply to the mesencephalic and principle sensory nuclei?

Answer 21

Located in midbrain and mid-pons:
Long circumferential branches of basilar a
Branches of AICA
Branches of superior cerebellar a