The Medulla Oblongata

The medulla oblongata, or myelencephalon, is the most caudal segment of the brainstem. It extends from the level of the foramen magnum to the pons. The cavity of the medulla consists of a narrow, caudal part, which is the continuation of the central canal of the cervical spinal cord, and a flared, rostral portion, which is the medullary part of the fourth ventricle. The modest size of the medulla (0.5% of total brain weight) belies its importance. All the tracts passing to or from the spinal cord traverse the medulla, and 7 of the 12 cranial nerves (VI to XII) are associated with the medulla or the pons-medullary junction. Also, the medullary reticular formation contains cell groups that influence heart rate and respiration. The blood supply to the medulla arises from branches of the vertebral arteries.

Development

The basic structural plan of the medulla is an elaboration of that seen in the spinal cord. The basal and alar plates give rise to specific nuclei, and the surrounding mantle layer is invaded by axons originating from other levels. Beginning in the medulla, however, the basic derivatives of the primitive neural tube are augmented by the appearance of other structures that characterize each brainstem level.

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 (somatic efferent [SE] cells). 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.

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 (somatic afferent [SA]). 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. An especially prominent bundle of axons collects on the anterior (ventral) surface of the medulla to form the pyramids.

External Features

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). 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. 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 or C₆), 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. 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 (often 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). Rostral to the obex, these trigeminal fibers are located internal to a progressively enlarging restiform body.

Posterior Medulla

At and caudal to the level of the obex, the posterior surface of the medulla is characterized by the gracile and cuneate fasciculi and their respective tubercles. These tubercles are formed by the underlying gracile and cuneate nuclei. Rostrolateral to the gracile and cuneate tubercles and forming a prominent elevation on the posterolateral aspect of the medulla is the restiform body. This structure contains a variety of afferent cerebellar fibers and becomes progressively larger as it extends toward the pons-medulla junction. In the caudal pons, fibers of the restiform body join with a much smaller bundle, the juxtarestiform body, to form the inferior cerebellar peduncle.

Vasculature

In general, the blood supply to the entire medulla and to the choroid plexus of the fourth ventricle arises from branches of the vertebral arteries. The exceptions are the portion of the choroid plexus that extends out of the foramen of Luschka and the adjacent cochlear nuclei; these are served by branches of the anterior inferior cerebellar artery, a branch of the basilar artery. In general, the medial medulla is served by the anterior spinal artery, the anterolateral medulla by small branches from the vertebral artery, and the posterolateral medulla rostral to the obex by the posterior inferior cerebellar artery (PICA). Caudal to the obex, the posterior medulla is served by the posterior spinal artery.

The vascular territory of the anterior spinal artery encompasses the medial lemniscus, hypoglossal root, and corticospinal fibers in the pyramid. Consequently, lesions of this vascular region result in somatosensory and motor deficits reflecting damage to these structures. In similar manner, lesions in the territory of the PICA will damage the anterolateral system and spinal trigeminal tract and nucleus and give rise to deficits reflecting these structures.