The spinal cord becomes the medulla oblongata, which also contains white and gray matter, but the arrangement changes, due to the embryonic expansion of the central canal to form the hindbrain vesicle, which will become the fourth ventricle. Development of the ventricle pushes dorsally situated structures more dorsolaterally. The transition is clearly seen in transverse section. The spinal cord becomes the medulla, which initially resembles the upper cervical segments. The substantia gelatinosa is now much larger in size and has become the spinal nucleus of the trigeminal nerve. In transverse section, descending fibers of the spinal trigeminal tract can be seen immediately dorsolateral to the nucleus. There is an increase in the amount of gray matter surrounding the central canal.
At low medullary level (A; see Figure opposite), the most prominent sign of transition to medulla is the appearance of the decussation at the pyramids. This is where the descending corticospinal motor tracts cross over. These fibers cross ventral (anterior) to the central gray matter and project dorsolaterally across the base of the ventral horn of the medulla. The pyramidal decussation almost eliminates the spinal anterior median fissure. (In the human, approximately 90% of the descending corticospinal fibers decussate and descend the cord in the lateral corti-cospinal tract, while about 10% do not cross, and descend in the uncrossed lateral and ventral corticospinal tracts.) The de-cussation explains the contralateral control of body movements by the motor cortex. At this level can also be seen the tracts of the gracile and cuneate fasciculi, which are the CNS projections of the cells of the spinal ganglia, and the lower ends of the gracile and cuneate nuclei where they terminate. At this level are also the cut fibers of the ascending ventral (ante rior) and lateral spinocerebellar tracts, which carry information from the sense organs in tendons and muscle spindles, the inferior olivary nucleus, and the spinal root of the accessory nerve.
Transaction at a higher level of the medulla (B) reveals another prominent de-cussation, that of the medial lemniscus. This is where fiber tracts from the ascending gracile and cuneate nuclei cross the midline of the medulla on their way up to higher centers. The nuclei are complex and arranged to correspond topographically with the body areas from which the ascending fibers come. Ascending fibers from the nuclei curve round the central gray matter and decussate to form the medial lemniscus. At this level, the spinal nucleus of the trigeminal nerve, which innervates the head region, is prominent, and immediately dorsolateral to it are the fibers of the descending trigeminal nerve. At both levels, the ascending spinocere-bellar and spinothalamic tracts are both visible, and in B the medial accessory olivary nucleus lies medial to these tracts.
In summary, the transition from spinal cord to medulla is marked by (i) the expansion of the central canal; (ii) decussa-tion at the pyramids; (iii) formation of the medial lemniscus through the decussation of ascending fibers arising from the cuneate and gracile nuclei; (iv) dorso-lateral displacement of the dorsal horn of gray matter; (v) appearance of cranial nerve nuclei and various relay nuclei projecting to the cerebellum.
A. TS medulla at level of decussation of pyramids
A I after Snell, 1986)
anterior spinocerebellar tract hypoglossal nerve fasciculus cuneatis internal arcuate fibers posterior spinocerebellar tract pinal root of accessory nerve lateral spinothalamic tract anterior spinocerebellar tract hypoglossal nerve medial longitudinal fasciculus
B. TS medulla at level of decussation of medial lemnisci
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