Somatosensory Tracts II

Question 1

What is the tract of the ALS?

Answer 1

1. Sensory fibers enter the SC via lateral division of posterior root.
2. Fibers then travel in Lissauer’s tract (posterolateral fasciculus), then bifurcate into ascending and decsending branches.
3. These fibers synapse in lamina I, II and V of posteriorr horn.
4. Axons cross the midline at the anterior white commissure.
5. Axons then ascend in the contralateral ALS.
6. 2nd order neurons synapse with 3rd order neurons in VPL.
7. 3rd order neurons travel to the post-central gyrus.

Question 2

What is the indirect spinothalamic pathway?

Answer 2

Pathway that carries info to the reticular formation.

1. Fibers ascend/descend in the posterolateral fasciculus (Lissauer’s tract) and synapse in laminae II and III and sense fibers acroos the AWC to the ALS.
2. The spinoreticular fibers terminate in the reticular formation.
3. Third order neurons in RF project to the medial thalamic nuclei.

Question 3

Spinothlamaic fibers

Answer 3

ALS fibers that ascend to the VPL (or VPI).

Question 4

Spinohypothalamic fibers

Answer 4

Fibers from posterior horns that ascend directly to hypothalamus.

Question 5

Spinomesencephalic fibers

Answer 5

Fibers that terminate in midbrain reticular fomration

Question 6

Somatotopic arrangement of ALS (medial to lateral)

Answer 6

1. Upper cervical
2. Lower cervical
3. Thoracic
4. Lumbar
5. Sacral and coccygeal

Question 7

BS to the ALS

Answer 7

Arterial vasocorona and sulcal branches of anterior spinal a.

Question 8

Occlusion of blood supply to the ALS results in:

Answer 8

Patchy loss of pain, thermal and touch over CL side of body beginning approx 2 segments below.

Question 9

Anterolateral cordotomy results in:

Answer 9

Complete loss of ALS-like sensation.

Question 10

What happens with both ALS and PCML sensation when the SC is hemisected?

Answer 10

CL loss of ALS-like sensation beginning approx 2 segments below level of lesion.
IL loss of PCML-like sensation at and below lesion.

Question 11

Syringomyelia

Answer 11

Cystic cavitation in gray matter in SC that can impinge AWC and ALS fibers.
Occurs often at C4-C5.
Produces BL loss of ALS-like sensation. that begins several segments below lesion.

Question 12

Dissociated sensory loss

Answer 12

Due to lesion of one of the tracts (ALS or PCML)
CL loss of discriminative touch and vibration (because the PCML fibers have decussated) but pain/thermal sensation is norma OR vice versa.
.

Question 13

Where do afferents from trigeminal n. enter the brainstem? Where do they turn?

Answer 13

Pons, caudally within the spinal trigeminal tract.

Question 14

Where do the 2nd order neurons originate from the spinal trigeminal tract?

Answer 14

Pars caudalis

Question 15

What are the orientations of pars caudalis, pars oralis and pars interpolaris?

Answer 15

Pars caudalis is most caudal.
Pars interpolaris is in between.
Pars oralis is rostral.

Question 16

When the spinal trigeminal tract is lesioned, what sensory loss pattern is noticed? Why?

Answer 16

Onion-peel, due to the overlapping spinal fibers and trigeminal fibers ending in the cervical cord.

Question 17

What is the overall pathway of the trigeminal nerve for pain/temperature?

Answer 17

1. 1st order neurons enter at the pons and descend in the spinal trigeminal tract.
2. They synapse in the pars caudalis and the second order neurons decussate and ascend in the anterior trigeminothalamic tract.
3. 2nd order neurons terminate in the VPM and extend into the posterior limb of the internal capsule to the primary somatosensory cortex.

Question 18

Where is the pars oralis and what does it receive?

Answer 18

From pons to the superior medulla.

Receives tactile info from central region of face.

Question 19

Where is pars interpolaris and what does it receive?

Answer 19

From superior medulla to obex.

Receives info form peripheral face.

Question 20

What is the blood supply to trigeminal structures in medulla?

Answer 20

PICA and posterior spinal a.

Question 21

Lateral medullary syndrome

Answer 21

Vascular lesions of PICA, which supplies the ALS and spinal trigeminal nucleus/tract.
Causes CL loss of pain and temperature over the body with ipsilateral loss of both over the face.