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Lab 9 (ƒ 10) - Cranial Nerve Nuclei and Brain Stem Circulation

Cranial Nerve X - Review

This section is at the level of the rostral medulla where the 4th ventricle is open and the hypoglossal nucleus (axons exit brain as Cranial Nerve XII) forms a bump called the hypoglossal trigone on the floor of the fourth ventricle. Immediately lateral to the hypoglossal nucleus, the dorsal vagal motor nucleus (axons exit brain in Cranial Nerve X) forms another bump called the vagal trigone.

The medial and inferior vestibular nuclei (vestibular afferents in Cranial Nerve VIII) are located lateral to the sulcus limitans. The spinal trigeminal nucleus and tract (somatosensory afferents of Cranial Nerve V, VII, IX, X) are located immediately medial to the inferior cerebellar peduncle and anterior to the vestibular nuclei. At this level of the brainstem, the spinal trigeminal nucleus axons form the ventral trigeminal lemniscus and are dispersed beside the medial lemniscus. The solitary nucleus and tract (viscerosensory afferents of Cranial Nerves X and IX) lie beneath the vestibular nuclei.

Recall that at this level of the medulla the four sensory nuclei are: 1. Medial vestibular nucleus 2. Inferior vestibular nucleus 3. Solitary nucleus 4. Spinal trigeminal nucleus.

The nucleus ambiguus (axons exit the brain in Cranial Nerves XI, X and IX) is a narrow column of cells in the reticular formation. The nucleus ambiguus, as its name implies, is difficult to see in cross section; however, its location can be determined as half way between the spinal trigeminal nucleus and inferior olivary nucleus. To find it, look at the midpoint of an imaginary line connecting the spinal trigeminal nucleus with the posterior part of the inferior olive. The nucleus ambiguus extends longitudinally from the caudal border of the decussation of the medial lemniscus up to the level of the striae medullares. Reticular formation axons relaying bilateral corticobulbar inputs reach the nucleus ambiguus and mediate the voluntary control of the pharynx and larynx in swallowing and phonation.

Blood Supply

Branches of the anterior spinal artery continue to supply the medial areas of the medulla which include the pyramids, medial lemniscus, tectospinal tract, medial longitudinal fasciculus, hypoglossal nucleus and nerve - as well as the medial region of the inferior olivary nucleus. Branches of the vertebral artery also continue to supply more lateral areas of the brain stem, which include the lateral part of the inferior olivary nucleus, the reticular formation, which surrounds the ambiguus nucleus, and the dorsal vagal nucleus. At this level of the medulla, branches of the posterior inferior cerebellar artery (PICA) are beginning to replace those of the posterior spinal artery to supply the posterolateral areas of the rostral “open” medulla. The branches of the posterior spinal artery continue to supply the medial and inferior vestibular nuclei and posterior parts of the inferior cerebellar peduncle. The areas receiving blood via the PICA include the solitary tract and nucleus, lateral parts of the reticular formation, the inferior cerebellar peduncle, spinal trigeminal nucleus and tract, spinothalamic tract, anterior spinocerebellar tract, rubrospinal tract, medullary reticulospinal tract.