The medulla seems significantly smaller in size at this level, approaching the size of the spinal cord below. The section is still easily recognized as medullary because of the presence of the pyramids anteriorly (the cortico-spinal tract) and the adjacent inferior olivary nucleus.
The tegmentum contains the cranial nerve nuclei, the reticular formation and the other tracts. The nuclei of CN X and CN XII, as well as the descending nucleus and tract of V, are present as before (as in the mid-medullary section, see Figure 67B). The MLF and anterolateral fibers are also in the same position. The solitary tract and nucleus are still found in the same location. The internal arcuate fibers are present at this level; these are the fibers from the nuclei gracilis and cuneatus, which cross (decussate) to form the medial lemniscus (see below). These fibers usually obscure visualization of the nucleus ambiguus. Finally, the reticular formation is still present.
The dorsal aspect of the medullary tegmentum is occupied by two large nuclei: the nucleus cuneatus (cuneate nucleus) laterally, and the nucleus gracilis (gracile nucleus) more medially. These are found on the dorsal aspect of the medulla (see Figure 9B and Figure 40). These nuclei are the synaptic stations of the tracts of the same name that have ascended the spinal cord in the dorsal column (see Figure 33, Figure 68, and Figure 69). The gracilis is mainly for the upper limb and upper body; the cuneatus carries information from the lower body and lower limb. The fibers relay in these nuclei and then move through the medulla anteriorly as the internal arcuate fibers, cross (decussate), and form the medial lemniscus on the opposite side (see Figure 40). At this level, the medial lemniscus is situated between the olivary nuclei and dorsal to the pyramids, and is oriented anteroposteri-orly.
Posteriorly, the fourth ventricle is tapering down in size, giving a "V-shaped" appearance to the dorsal aspect of the medulla (see Figure 20B). It is common for the ventricle roof to be absent at this level. This is likely accounted for by the presence of the foramen of Magendie, where the CSF escapes from the ventricular system into the subarachnoid space (see Figure 21). Posterior to this area is the cerebello-medullary cistern, otherwise known as the cisterna magna (see Figure 2, Figure 18, and Figure 21).
One special nucleus is found in the "floor" of the ventricle at this level, the area postrema. This forms a little bulge that can be appreciated on some sections. The nucleus is part of the system that controls vomiting, and it is often referred to as the vomiting "center." It is interesting to note that this region lacks a blood-brain barrier, allowing this particular nucleus to be "exposed" directly to whatever is circulating in the blood stream. It likely connects with the nuclei of the vagus nerve, which are involved in the act of vomiting.
The accessory cuneate nucleus is found at this level, as well as at the mid-medullary level. This nucleus is a relay for some of the cerebellar afferents from the upper extremity (see Figure 55). The fibers then go to the cerebellum via the inferior cerebellar peduncle. The inferior cerebellar peduncle has not yet been formed at this level.
Cross-sections through the lowermost part of the medulla may include the decussating cortico-spinal fibers, i.e., the pyramidal decussation (see Figure 40); this would therefore alter significantly the appearance of the structures in the actual section.
Gracilis t. Gracilis n.
Cuneatus t. Cuneatus n.
Dorsal motor n. Hypoglossal n.
Vagus nerve (CN X) Internal arcuate fibers
Anterolateral system Medial lemniscus
Cortico-spinal fibers Hypoglossal nerve (CN XII)
Foramen of Magendie 4th ventricle
Area postrema Accessory cuneate n.
Solitary n. Solitary t.
N. ambiguus Reticular formation
Inferior olivary n.
FIGURE 67C: Brainstem Histology - Lower Medulla
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