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Histology : Brain Stem

Closed medulla at the level of sensory decussation

1) Outline : still the same as2) Closed medulla with central canal that has moved somewhat posteriorly

Characteristic features of this level :

y Appearance of the pyramids anteriorly.y Presence of sensory decussation of internal arcuate fibre in between the central canal and the

pyramids.

Tracts and nuclei.

1. Most of ascending and descending tracts are present at this levelThe gracile and cuneate

y The gracile and cuneate are largey While their tracts are small (because the fibres of the tracts relay in the nerve cells of their

nuclei)

y The internal arcuate fibre arises from the nerve cell present in the two nuclei on each side,y They cross the opposite side and interdigitate forming the sensory decussation.y These crossed fibres ascend on the opposite side of the brain stemy And they continue as the Medial Lemniscus.

Corticospinal fibres of the pyramidal tracts are present at this level.

y They appear in the anterior part of the medulla at both sides of the anterior fissure

Medial accessory olivary nuclei

y Appear above the pyramids.

Arcuate nuclei

y Appear medial to the pyramidy These nuclei acts as relay station for the propioceptive pathway coming from the spinal cord to

the cerebellum.

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y From gracile and cuneate nuclei other fibres called external arcuate fibres arise and relay inarcuate nuclei

y Then they enter the cerebellum through the inferior cerebral peduncles.

Lateral cuneate nuclei

y Appear lateral to cuneate nuclei

Cranial nerve of the hypoglossal (12th

) and of the vagus (10th

dorsal motor nucleus)

y Appear in the gray mater around the central canal at this level

Transverse section passing through the sensory decussation.

1. Anterior median fissure. 2. Posterior median sulcus. 3, 3. Head and base of anterior column (in red). 4.

Hypoglossal nerve. 5. Bases of posterior column. 6. Gracile nucleus. 7. Cuneate nucleus. 8, 8. Lemniscus. 9. Sensory

decussation. 10. Cerebrospinal fasciculus.

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Lesion in the ventral part of the medulla (medial medullary syndrome):

This lesion involves the pyramidal tract and the emerging fibers of the hypoglossal nerve. It

results in the following:

* Hemiplegia: on the contralateral side (UMNL).

* Paralysis of the tongue on the same side of the lesion (LMNL). The tongue deviates towardsthe side of the lesion on protrusion. This is called alternating hypoglossal hemiplegia.

* Involvement of the medial lemniscus leads to loss of proprioceptive sensation together withloss of sense of localization and fine touch on the opposite side of the lesion.

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Histology : Brain Stem

References

1. Human Histology for Medical Students part 2, University of Alexandria.2. http://www.anatomyatlases.org/MicroscopicAnatomy/Section17/Plate17328.shtml3. http://en.wikipedia.org/wiki/Sensory_decussation