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Medulla oblongata: Tracts and nuclei: want to learn more about it?

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Medulla oblongata: Tracts and nuclei

In continuing the discussion on the medulla oblongata, this article will focus on the nuclei and tracts that are found within it at different cross-sectional levels.

Some clinical manifestation associated with damage to these tracts or their respective nuclei will also be addressed.


Posterior nucleus of vagus nerve (cross section)

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.

Gracile nucleus (cross section)

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

Cuneate nucleus (cross section)

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.

Spinal nucleus and tract of trigeminal nerve (posterior view)

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.

Pyramidal decussation (cross section)

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.

Medial accessory olivary nucleus (cross section)

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.

Internal arcuate fibers (cross section)

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.

Hypoglossal nucleus (cross section)

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.

Spinothalamic tract (cross section)

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).

Reticular formation (cross section)

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.

Nucleus ambiguus (cross section)

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.

Medial vestibular nucleus (cross section)

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.

Anterior cochlear nucleus (posterior view)

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).

Inferior olivary nucleus (cross section)

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.

Arcuate nucleus (cross section)

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.

Clinical aspects

Lateral medullary syndrome of Wallenberg

Clinical vignettes regarding vascular lesions of the medulla was introduced in a different article. The pathophysiology behind the symptoms will be discussed further. Vascular insults to the medulla can give rise to the condition known as Lateral Medullary Syndrome of Wallenberg. In this case, obstruction of the vertebral artery or the posterior inferior cerebellar artery (PICA) (which supplies the dorsolateral part of the medulla) results in several symptoms, including:

  • Nystagmus (random eye movement), vomiting, nausea, and vertigo (dizziness)
  • Insensitivity of pain and temperature of the ipsilateral face
  • Dysarthria – inability to talk
  • Dysphagia – inability to swallow
  • And decreased or absent voluntary coordination of movement with respect to gait and movement on the same side

These symptoms arise because with vascular compromise to the dorsolateral medulla, the nucleus ambiguous (motor supply to laryngeal and palatal muscles), the spinal trigeminal nucleus and tract (sensory from the face), the vestibular nucleus (balance and coordination) and the inferior cerebellar peduncle (numerous tracts to and from the cerebellum) would be compromised.

Medial medullary syndrome 

Vascular compromise of the anterior spinal artery or the vertebral artery can also affect blood flow through the direct medullary arteries that supply the medial medulla. Consequently, the patient can present with Medial Medullary Syndrome, whose clinical presentation includes:

  • Weakness of the trunk and limbs of the side opposite the lesion would present since the corticospinal tract would be affected
  • The medial lemniscus pathway is also a midline structure that would be compromised by the above stated vascular insult. The patient would therefore present with absent proprioception, loss of axial and limb vibration sense and decreased two-point tactile sensation, all on the contralateral side
  • And flaccid paralysis of the muscles of the tongue on the same side as the lesion due to damage of the CN XII nerve rootlets.

Medulla oblongata: Tracts and nuclei: want to learn more about it?

Our engaging videos, interactive quizzes, in-depth articles and HD atlas are here to get you top results faster.

What do you prefer to learn with?

“I would honestly say that Kenhub cut my study time in half.” – Read more. Kim Bengochea Kim Bengochea, Regis University, Denver

Show references


  • Fix, J. (2008). Neuroanatomy. 4th ed. Philadelphia: Wolters Kluwer/Lippincott Williams & Wilkins.
  • Snell, R. (2010). Clinical Neuroanatomy. 7th ed. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins, pp.198-206, 217-218.


  • Posterior nucleus of vagus nerve (cross section) - Paul Kim
  • Gracile nucleus (cross section) - Paul Kim
  • Spinal nucleus and tract of trigeminal nerve (posterior view) - Paul Kim
  • Cuneate nucleus (cross section) - Paul Kim
  • Pyramidal decussation (cross section) - Paul Kim
  • Medial accessory olivary nucleus (cross section) - Paul Kim
  • Internal arcuate fibers (cross section) - Paul Kim
  • Hypoglossal nucleus (cross section) - Paul Kim
  • Spinothalamic tract (cross section) - Paul Kim
  • Reticular formation (cross section) - Paul Kim
  • Nucleus ambiguus (cross section) - Paul Kim
  • Medial vestibular nucleus (cross section) - Paul Kim
  • Anterior cochlear nucleus (posterior view) - Paul Kim
  • Inferior olivary nucleus (cross section) - Paul Kim
  • Arcuate nucleus (cross section) - Paul Kim
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