Medulla oblongata: Hypoglossal nerve level (English)

The various nuclei which make up the gray matter are mainly found within the dorsal part of the medulla oblongata, while the tracts, which form the white matter, pass mainly through the ventral part. At the level of the hypoglossal nerve, the most prominent white matter landmarks are the pyramids or pyramidal tracts, the medial lemniscus and medial longitudinal fasciculus. The decussation of the medial lemniscus (sensory decussation) can also be identified at this level, in which axons from second order neurons emerge as internal arcuate fibres from the cuneate and gracile nuclei, and cross over the midline to form the medial lemniscus of the contralateral side. Joining the internal arcuate fibres are the fibres from the olivocerebellar tract, which then travel in a posterosuperior direction to reach the inferior cerebellar peduncle. The most prominent gray matter structure on this level are the coiled olivary nuclei found dorsal and lateral to the pyramids. Moving further dorsally we see the nucleus ambiguus, spinal nucleus of trigeminal nerve, nucleus of hypoglossal nerve and nucleus of solitary tract. We can see how the hypoglossal nerve extends from its nucleus and crosses the central portion of the medulla oblongata to exit at the tectum of the medulla. In the anterior median fissure, on the anteromedial surface of each pyramid is a small nucleus known as the arcuate nucleus. It projects arcuatocerebellar fibers that form the striae medullaris of the fourth ventricle.