It is one of the three regions that make up the brainstem. It is the lowest of the three and it is continued up with the protuberance and below with the spinal cord.
The cord houses the essential ascending and descending nerves, as well as the brainstem nuclei .
External anatomy of the medulla oblongata
The medulla oblongata is conical in shape, decreasing in width, since it extends downwards. It measures approximately 3 cm long and 2 cm wide at its largest point.
The upper margin of the medulla oblongata is found at the junction between the spinal cord and the pons, while the inferior margin is marked by the origin of the first pair of cervical spinal nerves. This occurs just as the marrow leaves the skull through the foramen magnum.
There are several structures visible on the anterior surface of the cord, that is, the three fissures / grooves, the pyramids, the olives and the five cranial nerves.
In the median line of the medulla lies the anterior medial fissure, which is continuous along the length of the spinal cord. However, it is temporarily interrupted by decussation of the pyramids. As we move away from the midline, two grooves are observed: the ventrolateral sulcus and the posterolateral sulcus.
The pyramids are pairs of swellings that lie between the anterior medial fissure and the ventrolateral sulcus. Information about the pyramids can be found here. The olives are another pair of swellings located laterally to the pyramids, between the ventrolateral and posterolateral grooves.
From the junction between the protuberance and the medulla lies the abducens nerve (CN VI). Extending out of the ventrolateral groove is the hypoglossal nerve (CN XII). In the posterolateral sulcus, three other cranial nerves join the medulla (CN IX, CN X and CN XI).
Internal anatomy of the medulla oblongata
The internal structures of the medulla oblongata must be seen in cross section to understand the design. Generally, three levels of the medulla (inferior – superior) are discussed:
- Level of decussation of the pyramids.
- Level of decussation of the medial lemniscus.
- Level of the olives.
The marrow itself is typically divided into two regions: the open marrow and the closed marrow. This distinction is based on whether the cavities containing CSF are surrounded by the marrow (closed medulla) or not (open marrow). The cord opens when the central channel opens to the fourth ventricle.
Some features are seen in the three cross sections. Previously we can see the matched lumps representing the pyramids that are separated by the anterior middle fissure. In the center, you can see the central channel as it rises to form the fourth ventricle in the final cross section.
Level of Decusation of the Pyramids:
This is the main point of decussation of the descending motor fibers. Approximately 75% of the motor fibers lodged within the pyramids cross diagonally and posteriorly, and continue along the spinal column as the lateral corticospinal tracts.
At this level, the central portion of the medulla contains gray matter, while the outer portions are constituted by white matter. The posterior white matter contains the fasiculus gracilis and the more lateral fasiculus cuneatus.
The corresponding portions of the gray matter extend to these regions and are the gracilis nucleus and the cuneatus nucleus, respectively.
Unchanged from the spinal cord, the spinocerebellar tracts (posterior and anterior) are located laterally, with the lateral spinothalamic tract located between them. The nucleus and the large trigeminal tracts can be found later on these tracts. This is a continuation of the gelatinous substance of the spinal cord.
Level of Decusation of the Medial Lemniscus:
This level marks the sensory decussation that occurs in the medial lemniscus. The purple lines have been used to represent the internal arcuate fibers, since they run from the gracilis nucleus and the cuneatus nucleus around and in front of the central gray matter to form the medial lemniscus.
Lateral to the medial lemniscus, the trigeminal nucleus and the spinal tract can be seen again, as well as the spinocerebellar tracts and the lateral spinothalamic tract. In the same way, later structures are very similar at this level.
In the center, the hypoglossal nucleus and the medial longitudinal fasciculus are seen. Moving laterally, you can see the ambiguous nucleus. Between this structure and the pyramids is the lower olive grove.
Level of olives:
This level shows a significant change in the structure both externally and internally compared to previous levels. The central canal has now expanded to the fourth ventricle and, as such, makes this region the open medulla.