The medulla oblongata, or simply the medulla, is a cone-shaped structure located at the base of the brainstem that is approximately 3 cm long and 2 cm wide at its largest point. It is located at the junction between the medulla and pons at the superior margin and at the origin of the first pair of cervical spinal nerves at the inferior margin, just as the medulla exits the skull through the foramen magnum.
The anterior surface of the medulla contains several structures of note, including three fissures or sulci, pyramids, olives, and five cranial nerves. The anterior median fissure, a midline feature that is continuous along the length of the spinal cord, is interrupted at its midpoint by the decussation of the pyramids. On either side of the fissure are the ventrolateral and posterolateral sulci. The pyramids, a pair of swellings between the fissure and the ventrolateral sulcus, are adjacent to the olives, another pair of swellings between the ventrolateral and posterolateral sulci. Emerging from the junction between the medulla and pons is the abducens nerve (CN VI), which extends from the ventrolateral sulcus. Meanwhile, the hypoglossal nerve (CN XII) is found in the posterolateral sulcus, along with three other cranial nerves (CN IX, CN X, and CN XI).
The posterior surface of the medulla is largely obstructed from view due to the presence of the cerebellum; however, it does have a midline feature – the posterior median sulcus – that is continuous below as the posterior median sulcus of the spinal cord and ends above at the point of junction between the fourth ventricle and the medulla.
The internal structures of the medulla must be viewed in cross-section to understand their layout, and three levels of the medulla are typically discussed (inferior to superior): the level of decussation of the pyramids, the level of decussation of the medial lemnisci, and the level of the olives. The medulla can be divided into two regions – the open and the closed medulla – based on whether the central cavities of the medulla are surrounded by the medulla (closed medulla) or not (open medulla). This distinction occurs when the central canal opens into the fourth ventricle.
At each of the three cross sections, several features are shared. Anteriorly, two lumps representing the pyramids can be seen, separated by the anterior median fissure. Centrally, the central canal rises to form the fourth ventricle. More specifically, at the level of decussation of the pyramids, the central portion of the medulla contains gray matter while the outer portions consist of white matter. The posterior white matter contains the fasiculus gracilis and the fasiculus cuneatus more laterally, with corresponding gray matter nuclei of the nucleus gracilis and nucleus cuneatus, respectively. The spinocerebellar tracts (posterior and anterior) are located laterally, with the lateral spinothalamic tract situated between them, and the large trigeminal nucleus and tracts can be found posterior to these tracts.
The level of decussation of the pyramids is the major decussation point of the descending motor fibres. Roughly 75% of motor fibres housed within the pyramids cross diagonally and posteriorly, and continue down the spinal column as the lateral corticospinal tracts. The sensory decussation of the medial lemniscus also occurs at this level. Purple lines, representing the internal arcuate fibres, are used to show the path of the fibres from the nucleus gracilis and nucleus cuneatus around and anterior to the central gray matter to form the medial lemniscus. Lateral to the medial lemniscus are the trigeminal nucleus and spinal tract, the spinocerebellar tracts, and the lateral spinothalamic tract.
The level of the olives shows significant changes in structure both externally and internally when compared to the previous levels. This level marks the major decussation of the ipsilateral spinothalamic tract. Its anterior margin forms an inverted 'U', which corresponds to the nucleus of the trapezoid body. The trapezoid body houses the cochlear fibers that enter from the superior cochlear nucleus.
The internal structure of the medulla oblongata is complex, with diverse anatomical features along the anterior and posterior surfaces and at the three levels of the medulla. It is essential to understanding its anatomy in order to properly appreciate its vital role in the human body.
The medulla oblongata (medulla) is one of the three regions that make up the brainstem. It is the most inferior of the three and is continuous above with the pons and below with the spinal cord. The medulla houses essential ascending and descending nerve tracts as well as brainstem nuclei. In this article, we shall look at the anatomy of the medulla - its external features, internal anatomy, and blood supply.
The medulla is conical in shape, decreasing in width as it extends inferiorly. It is approximately 3cm long and 2cm wide at its largest point. The superior margin of the medulla is located at the junction between the medulla and pons, while the inferior margin is marked by the origin of the first pair of cervical spinal nerves. This occurs just as the medulla exits the skull through the foramen magnum.
Anterior Surface
There are several structures visible on the anterior surface of the medulla - namely the three fissures/sulci, the pyramids, the olives, and five cranial nerves. In the midline of the medulla is the anterior median fissure, which is continuous along the length of the spinal cord. However, it is interrupted temporarily by the decussation of the pyramids (see below). As we move away from the midline, two sulci are visible - the ventrolateral sulcus and the posterolateral sulcus.
The pyramids are paired swellings found between the anterior median fissure and the ventrolateral sulcus. The olives are another pair of swellings located laterally to the pyramids - between the ventrolateral and posterolateral sulci. Arising from the junction between the pons and medulla is the abducens nerve (CN VI). Extending out of the ventrolateral sulcus is the hypoglossal nerve (CN XII). In the posteriolateral sulcus, three more cranial nerves join the medulla (CN IX, CN X, and CN XI).
The internal structures of the medulla must be viewed in cross section to understand the layout. The central canal has now expanded into the fourth ventricle and as such makes this region the open medulla. The large inferior olivary nucleus is responsible for the external expansion of the olives. The related medial and dorsal accessory olivary nuclei can be seen medial and posterior to this structure respectively. The large inferior cerebellar peduncles come into view and are surrounded by multiple nuclei. The two vestibular nuclei (medial and inferior) are both found towards the midline while the two cochlear nuclei are found somewhat above and below the peduncles. Now a much smaller structure, the trigeminal tract and nucleus is seen adjacent to the peduncle. The nucleus ambiguous remains as it was previously, while the hypoglossal nucleus has migrated with the central canal posteriorly, joined by the medial longitudinal fasciulus. An additional cranial nucleus comes into view lateral to the hypoglossal – the dorsal vagal nucleus. Moving further lateral, the nucleus of tractus solitarius comes into view. Centrally, the medial lemniscus hugs the midline posterior to the pyramids, as does the tectospinal tract. Between the peduncle and the olivary nuclei resides the lateral spinothalamic tract and the more lateral anterior spinocerebellar tract.
The vasculature of the medulla is complex and is dependent on the level being viewed (Fig. 7). The following attempts to simplify this complexity. Despite this it may suffice the reader to know that the vessels that supply the medulla include- the anterior spinal, the posterior spinal, the posterior inferior cerebellar, the anterior inferior cerebellar, and vertebral arteries. Throughout the medulla, the anterior spinal artery supplies a region beginning at the central canal (or anterior border of the fourth ventricle), and fans out to encompass the pyramids. Below the level of the olives the posterior half of the medulla is supplied by the posterior spinal artery. No other regions are supplied by this vessel. The remaining portions are supplied by the posterior inferior cerebellar and vertebral arteries. In cross section through the olives both the posterior inferior cerebellar and vertebral arteries take on greater territories posterolaterally and anterolaterally respectively. They continue to do so as the medulla ascends.
The blood supply of the medulla is important to keep in mind, due to its close proximity to vital brain and spinal fluid structures. The medulla is provided with an abundant supply of blood from four main arteries – the anterior spinal, posterior spinal, posterior inferior cerebellar, and vertebral arteries. The anterior spinal artery is important for supplying the anterior third of the medulla, while the posterior spinal artery is important for supplying the posterior two-thirds of the medulla. The posterior inferior cerebellar and vertebral arteries take on greater territory in the regions of the olives and the posterior portion of the medulla respectively. Together they provide the medulla with an abundant supply of blood and oxygen in order to support its vital functions.
The medulla oblongata is one of the three main regions of the brainstem – along with the pons and midbrain. It is the most inferior of the three and is continuous above with the pons and below with the spinal cord. The medulla houses essential ascending and descending nerve tracts as well as brainstem nuclei. We have examined the anatomy of the medulla – its external and internal features, as well as its extensive vasculature. We hope this article has provided some insight into this important region of the brain.
The medulla is typically divided into two regions - the open and closed medulla - based on whether the CSF-containing cavities are surrounded by the medulla (closed medulla) or not (open medulla). The medulla becomes open when the central canal opens into the fourth ventricle. Generally, three levels are discussed when discussing the medulla: the level of decussation of the pyramids, level of decussation of the medial lemnisci, and the level of the olives.
At this level, the central portion of the medulla contains gray matter, while the outer portions consist of white matter. Anteriorly, the paired lumps representing the pyramids can be seen, separated by the anterior median fissure. It is at this level that the major decussation point of the descending motor fibres takes place - roughly 75% of motor fibres housed within the pyramids cross diagonally and posteriorly, and continue down the spinal column as the lateral corticospinal tracts. The posterior white matter contains the fasiculus gracilis and the more lateral fasiculus cuneatus, while corresponding portions of gray matter extend to these regions and are the nucleus gracilis and nucleus cuneatus respectively. Unchanged from the spinal cord, the spinocerebellar tracts (both posterior and anterior) are located laterally, with the lateral spinothalamic tract situated between them. The large trigeminal nucleus and tracts can be found posterior to these tracts, a continuation of the substantia gelatinosa of the spinal cord.
At this level, purple lines can be seen to represent the internal arcuate fibres as they run from the nucleus gracilis and nucleus cuneatus around and anterior to the central gray matter to form the medial lemniscus. This is the level of sensory decussation of the medial lemniscus (Fig. 5). Lateral to the medial lemniscus, the trigeminal nucleus and spinal tract can once again be seen, as can the spinocerebellar tracts and the lateral spinothalamic tract. Similarly, the posterior structures are much the same at this level. The hypoglossal nucleus and medial longitudinal fasciculus are seen in the central area, with the nucleus ambiguous situated laterally. Between this structure and the pyramids is the inferior olivary nucleus.
This level shows significant change in structure both externally and internally when compared with previous levels. This is in part because the central canal has now expanded into the fourth ventricle, making this region the open medulla. The large inferior olivary nucleus is responsible for the external expansion of the olives, with the related medial and dorsal accessory olivary nuclei found medial and posterior to this structure respectively. The large inferior cerebellar peduncles come into view and are surrounded by multiple nuclei, including the two vestibular nuclei (medial and inferior) found towards the midline and the two cochlear nuclei - somewhat above and below the peduncles. Now a much smaller structure, the trigeminal tract and nucleus is seen adjacent to the peduncle. The nucleus ambiguous remains as it was previously, while the hypoglossal nucleus and medial longitudinal fasciulus have migrated posteriorly with the central canal, joined by an additional cranial nucleus, the dorsal vagal nucleus, found lateral to the hypoglossal. Further laterally is the nucleus of tractus solitarius. Centrally, the medial lemniscus hugs the midline posterior to the pyramids, and is joined by the tectospinal tract. Between the peduncle and the olivary nuclei resides the lateral spinothalamic tract, and the more lateral anterior spinocerebellar tract.
The vasculature of the medulla is complex and depends on the level being viewed. Generally, the vessels that supply the medulla include the anterior spinal, the posterior spinal, the posterior inferior cerebellar, the anterior inferior cerebellar, and the vertebral arteries.
The blood supply to the medulla is provided by several vessels. As the anterior spinal artery is the main vessel supplying the medulla, it begins at the central canal (or anterior border of the fourth ventricle) and extends to the pyramids in the anterior region. Below the olives, the posterior half of the medulla is supplied by the posterior spinal artery, while the remaining portions are supplied by the posterior inferior cerebellar and vertebral arteries.
The anterior inferior cerebellar artery supplies the outermost portion of the posterior region. This happens at the highest point in the medulla. Its supply is limited to this part of the medulla, as no other regions are supplied by this vessel.
The anterior spinal artery is the primary vessel supplying the medulla through its expansive network, which begins at the central canal and extends down to various points in the medulla. This supply reaches the pyramids and encompasses the anterior part of the medulla.
As the supply from the anterior spinal artery is limited to the anterior part of the medulla, the posterior half of the medulla is mainly supplied by the posterior spinal artery. The posterior spinal artery originates from the posterior medulla at the olives and extends throughout the posterior medulla to provide adequate circulation.
In addition to the anterior and posterior spinal arteries, the posterior inferior cerebellar and vertebral arteries also provide blood supply to the medulla. The posterior inferior cerebellar artery supplies the outermost part of the posterior region at the highest point in the medulla, while the vertebral artery provides additional blood supply both to the anterior and posterior regions.
The different vessels supplying blood to the medulla create a network of vessels that provide circulation throughout the organ. The anterior spinal artery provides the most comprehensive supply, as it supplies the anterior portion of the medulla. The posterior spinal artery extends past the olives and supplies the posterior region of the medulla. Finally, the posterior inferior cerebellar and vertebral arteries provide additional supply to the outermost part of the posterior region.
