The Medulla Oblongata

Basal and Alar Plates

Maturing neurons of the basal plate of the medulla give rise to the hypoglossal nucleus (somatic efferent [SE] cells), the dorsal motor vagal nucleus and the inferior salivatory nucleus (both contain visceral efferent [VE] cells), and the nucleus ambiguus (SE cells) ( Fig. 11.2 ). Caudal to the obex, the hypoglossal and dorsal motor vagal nuclei are small and are found in the central gray surrounding the central canal. Rostral to the obex, all of these nuclei are located medial to the sulcus limitans ( Fig. 11.2 B ). The nucleus of the accessory nerve (SE) is located in the cervical spinal cord but essentially in a caudorostral line with the SE nuclei of the medulla.

The cranial nerve nuclei derived from the alar plate in the medulla and their corresponding functional components include the vestibular and cochlear nuclei (somatic afferent [SA]), the solitary nucleus (visceral afferent [VA]), and the spinal trigeminal nucleus (SA) ( Fig. 11.2 ). Alar plate neuroblasts caudal to the obex give rise to the gracile and cuneate nuclei. Rostral to the obex, some alar plate cells migrate ventromedially to form the nuclei of the inferior olivary complex.

Concurrent with these developmental events, ascending and descending fibers are traversing the medulla. For example, an especially prominent bundle of corticospinal axons collects on the anterior (ventral) surface of the medulla to form the pyramids ( Fig. 11.2 B ).

Anterior Medulla

The anterior (ventral) aspect of the medulla is characterized by an anterior median fissure; two laterally adjacent longitudinal ridges, the pyramids; and the olive ( inferior olivary eminence ) ( Fig. 11.3 ). The pyramids issue from the basilar pons and extend caudally to the motor ( pyramidal ) decussation, where about 90% of their fibers cross the midline. Most of the fibers that form the pyramid arise in the motor cortex as corticospinal fibers; consequently, their crossing is called the motor decussation. Rootlets of the hypoglossal nerve (cranial nerve XII) exit the medulla via the preolivary sulcus, a shallow groove located between the pyramid and the olive. The abducens nerve (cranial nerve VI) emerges at the pons-medullary junction, generally in line with the rootlets of cranial nerve XII.

Lateral Medulla

On the lateral aspect of the medulla, a shallow trough, the postolivary sulcus or retroolivary sulcus, is located between the restiform body and the large eminence formed by the underlying inferior olivary nucleus ( Fig. 11.4 A , B ). Cranial nerves IX ( glossopharyngeal ) and X ( vagus ) emerge from the postolivary sulcus. Caudal rootlets of the vagus have been incorrectly called the medullary, or bulbar, root of the accessory nerve. In actuality, the accessory nerve is made up of axons that arise from cells in the upper levels of the cervical spinal cord (C1 to C5), ascend through the foramen magnum, coalesce to form the accessory nerve, and then exit the skull via the jugular foramen along with the glossopharyngeal and vagus nerves. Jugular foramen syndromes reflect damage to cranial nerves IX, X, and XI as they exit the skull. Large tumors immediately external to this foramen may also involve the hypoglossal nerve root. Structures served by the accessory nerve receive no innervation from the medulla. The facial nerve (VII), along with its intermediate root, and the vestibulocochlear nerve (VIII) emerge from the posterolateral medulla at the pons-medulla interface. The general region of the exit of the facial and vestibulocochlear nerves is clinically regarded as the cerebellopontine angle. Indeed, a vestibular schwannoma (incorrectly referred to as an acoustic neuroma) is a tumor of the vestibular portion of the eighth cranial nerve and is a lesion located at the cerebellopontine angle. On the lateral medullary surface caudal to the level of the obex, fibers of the spinal trigeminal nucleus and tract assume a superficial location and form the trigeminal tubercle ( tuberculum cinereum ) ( Fig. 11.4 B , C ). Rostral to the obex, these trigeminal fibers are located internal to a progressively enlarging restiform body.