Medulla oblongata: Tracts and nuclei

Overview

For clarity, the term nucleus (pl. nuclei) is used in reference to a collection of neuronal cell bodies within a particular area (e.g. nucleus of the CN XII). Tracts, on the other hand, refer to a collection of nerve fibers running to/from a particular target (e.g. spinocerebellar tracts running from the spinal cord to the cerebellum). The tracts and nuclei are 3D structures that can span multiple levels. Therefore, it is not uncommon for the same tract to be found in cross-section at the decussation of the pyramids and at the decussation of the medial lemnisci.

The tracts can either be ascending tracts – sensory (afferent) fibers going from the spinal cord to higher brain centres; or descending tracts – motor (efferent) fibers coming from higher brain centres to the spinal cord. Remembering all the tracts can be a daunting task. A simple trick to remember where the tracts are running is to break the composite name into its respective parts (i.e. spino [spinal] – cerebellar [cerebellum]). The prefix tells where the tracts arise and the suffix tells where they will terminate or transition into another set of fibers.

Cross section at the decussation of the pyramids

The gracile and cuneate nuclei are located in the posterior aspect of the medulla; each being just anterior to their respective fasciculi and bilateral to the posterior median sulcus.

The fasciculi are continuations of the dorsal column of the spinal cord. As a result, they carry information regarding joint proprioception, tactile discrimination, and vibration sense. The nuclei house the second-order cell bodies connected to the fibers of the dorsal column-medial lemniscus pathway

The spinal nucleus of the trigeminal nerve and its associated tract are found posterolaterally in the medulla; with the tract being more posterior (i.e. closer to the surface of the medulla) than the nucleus. The tract carries nociceptive, light touch sensation and thermal information from the face via first-order general somatic afferent (GSA) neurons of CN V, CN VII, CN IX and CN X.

This information is then conveyed to the spinal trigeminal nucleus, which happens to be a cranial continuation of the substantia gelatinosa (Rexed II) of the posterior grey horn of the spinal cord. The spinal trigeminal nucleus then gives off second-order neurons that decussate and form the ventral trigeminothalamic tract. These fibers eventually synapse at the ventral posteromedial nucleus (VPMN) of the thalamus.

The corticospinal fibers coming from the giant cells of Betz in Brodmann areas 6 (premotor cortex), 4 (precentral motor cortex), and 3-1-2 (post-central sensory cortex), give rise to the pyramids. Along the caudal route, a large amount of these fibers cross the midline to form the pyramidal decussation. Each bundle of fibers forms the contralateral lateral corticospinal tract in the posterolateral aspect of the lateral funiculus of the spinal cord. These fibers have the responsibility of regulating contralateral voluntary motor activity and as a result, the pyramidal decussation is also known as the great motor decussation.

The medial longitudinal fasciculus (MLF) is located laterally to the lateral corticospinal tract prior to its decussation. Similarly, the medial accessory olivary nucleus is also laterally to the lateral corticospinal tract at the point of decussation. The anterior and posterior spinocerebellar (conveying instinctive proprioception to the cerebellum) and the lateral spinothalamic tracts (nociception and thermal sensation to the ventral posterolateral nucleus of the thalamus) also continue ipsilaterally from the spinal cord through the medulla.

Cross section at the decussation of the lemnisci

The medial lemniscus pathway acts as a bridge between the dorsal column and the ventral posterolateral nucleus of the thalamus. This tract is formed when the gracile and cuneate nuclei give off their internal arcuate fibers from their anterior surface.

The internal arcuate fibers from the opposite side travel anteriorly – lateral to the central canal, hypoglossal nucleus and the medial longitudinal fasciculus. Then, they course medially (forming a geometric arc) and decussate as the medial lemnisci. The decussation occur posterior to the pyramid and medial to the inferior olivary nucleus.

Also lateral to the decussation of the lemnisci are the spinotectal tracts and the anterior and lateral spinothalamic tracts. Due to their proximity to each other, these fibers are referred to as the spinal lemniscus and are responsible for carrying pain, crude touch and temperature sensation. The spinal trigeminal nuclei and tracts, and posterior and anterior spinocerebellar tracts maintain their position as previously described.

Cross section at the level of the olives

At the level of the olives, a few structures previously described persist in different locations, while additional structures are encountered. Since this area is closer to the base of the medulla, it is wider than the previous sections. Additionally, the central canal has journeyed posteriorly and widened into the fourth ventricle (which is covered here by the inferior medullary velum).

The hypoglossal and dorsal vagal nuclei are located posteromedially (with the former being the most medial structure) in the medulla. The most medial structures (from posterior to anterior) are the medial longitudinal fasciculus, the tectospinal tract and the medial lemniscus. The reticular formation is placed deep in the medulla, anterior to the hypoglossal and dorsal vagal nuclei, lateral to the medial longitudinal fasciculus and tectospinal tracts and overlapping the nucleus ambiguous.

Lateral to the dorsal vagal nucleus is the nucleus of tractus solitarius (solitary nucleus) and the medial vestibular nucleus. The inferior vestibular nucleus and the posterior cochlear nucleus are lateral to the medial vestibular and the solitary nuclei, but medial to the body of the inferior cerebellar peduncle.

The spinal trigeminal nuclei and tracts are medial to the inferior cerebellar peduncle while the anterior cochlear nucleus is anterior to it. The anterior spinocerebellar tract is laterally placed and located superficially, anterior to the exiting fibers of the CN X (between the inferior cerebellar peduncle and olive). The lateral spinothalamic tract is just medial to the anterior spinocerebellar tract, and lateral to the dorsal accessory olivary nucleus.

The inferior olivary nucleus is a convoluted, C-shaped body that is deep to the olive. Its olivocerebellar fibers travel through the inferior cerebellar peduncle to enter in the cerebellar cortex and nuclei of the opposite side. It also acts as a relay station for the rubrocerebellar fibers (fibers from the red nucleus of the cerebellum).

In the anterior medial 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.

Cross section at the pontomedullary junction

There are no significant changes in the arrangement of tracts and nuclei at this level. One noteworthy point, however, is that the posterior cochlear nucleus can be observed posterior to the inferior cerebellar peduncle.