Материал: Kaplan USMLE-1 (2013) - Anatomy

MEDULLA

In the caudal medulla, 2 of the neural systems-the corticospinal and dorsal column-medial lemniscal pathways-send axons across the midline. The nucleus gracilis and nucleus cuneatus give rise to axons that decussate in the caudal me­ dulla (the crossing axons are the internal arcuate fibers), which then form and ascend in the medial lemniscus.

The corticospinal (pyramidal) tracts, which are contained in the pyramids, course ventromedially through the medulla. Most of these fibers decussate in the caudal medulla just below the crossing of axons from the dorsal column nuclei, and then travel down the spinal cord as the (lateral) corticospinal tract.

The olives are located lateral to the pyramids in the rostral two-thirds ofthe medul­ la. The olives contain the convoluted inferior olivary nuclei. The olivary nuclei send climbing (olivocerebellar) fibers into the cerebellum through the inferior cerebellar peduncle. The olives are a key distinguishing feature ofthe medulla.

The spinothalamic tract and the descending hypothalamic fibers course together in the lateral part of the medulla below the inferior cerebellar peduncle and near the spinal nucleus and tract of CN V.

Cranial Nerve Nuclei

Spinal nucleus ofV

The spinal nucleus of the trigeminal nerve (CN V) is located in a position analo­ gous to the dorsal horn of the spinal cord. The spinal tract ofthe trigeminal nerve lies just lateral to this nucleus and extends from the upper cervical cord (C2) to the point of entry of the fifth cranial nerve in the pons. Central processes from cells in the trigeminal ganglion conveying pain and temperature sensations from the face enter the brain stem in the rostral pons but descend in the spinal tract of CN V and synapse on cells in the spinal nucleus (Figure IV-5-3).

Solitary nucleus

The solitary nucleus receives the axons of all general and special visceral afferent fibers carried into the CNS by CN VII, IX, and X. These include taste, cardiore­ spiratory, and gastrointestinal sensations carried by these cranial nerves. Taste and visceral sensory neurons allhave their cell bodies in ganglia associated with CN VII, IX, and X outside the CNS.

Nucleus ambiguus

The nucleus ambiguus is a column of large motoneurons situated dorsal to the inferior olive. Axons arising from cells in this nucleus course in the ninth and tenth cranial nerves. The component to the ninth nerve is insignificant. In the tenth nerve, these fibers supply muscles of the soft palate, larynx, pharynx, and upper esophagus. A unilateral lesion will produce ipsilateral paralysis of the soft palate causing the uvula to deviate away from the lesioned nerve and nasal regur­ gitation of liquids, weakness of laryngeal muscles causing hoarseness, and pha­ ryngeal weakness resulting in difficulty in swallowing.

Chapter 5 • The Brain Stem

MEDICAL 391