The medulla is the longest portion of the brainstem, measuring approximately 3 cm in length (eFig. 9.32A). At the rostral connection of the medulla to the pons, it has a broad conical shape that tapers caudally before connecting with the spinal cord at the level of the foramen magnum. Along the midline of the anterior surface is the anterior median fissure, which continues on to the anterior surface of the spinal cord (eFig. 9.33B). Lateral to the fissure are the medullary pyramids composed of motor fibers descending from the cerebral cortex (eFig. 9.33B). Continuing caudally, the pyramids eventually give way to the decussation of the pyramids where the majority of motor fibers decussate to the opposing side. Lateral to the pyramids are the olives, which represent the underlying inferior olivary nuclei. It is at the junction of the pyramid and the olive that the rootlets of cranial nerve XII, the hypoglossal nerve, emerge (eFig. 9.33D). Oriented posterior to the olives are the inferior cerebellar peduncles, which form a connection between the cerebellum and medulla (eFig. 9.33D). At the junction of the olive and inferior cerebellar peduncles from rostral to caudal, rootlets of cranial nerves IX (glossopharyngeal), X (vagus), and XI (spinal accessory) emerge. Like the pons, the posterior surface of the medulla forms the floor of the fourth ventricle (eFig. 9.32A). Coursing down the midline of the posterior medulla is the posterior median sulcus, which continues into the spinal cord. On either side of the sulcus are the gracile and cuneate tubercles, formed by the underlying gracile nucleus and cuneate nuclei (eFig. 9.32A).
Internal structures of the brainstem can be identified by their general location in the tectum (Latin for “roof”), tegmentum (Latin for “covering”), or basis (eFig. 9.34). To identify the structures present in each level of the brainstem, it is best to view serial sections stained for myelin. This way, nuclear groups and myelinated axons can be more easily distinguished from one another.
The tectum is the most obvious landmark of the midbrain, as it consists of the prominent superior colliculi, inferior colliculi, and underlying cerebral aqueduct. Given the short length of the midbrain, stained serial sections typically include either the superior colliculi, which are located rostrally, or the inferior colliculi, which are caudal. In addition to the superior colliculi, within a rostral section of the midbrain prominent nuclei such as the oculomotor nuclei, Edinger–Westphal nuclei, red nuclei, mesencephalic nuclei of cranial nerve V, and substantia nigra can be seen (eFig. 9.35). In caudal sections of the midbrain the inferior colliculus, trochlear nucleus, mesencephalic nuclei of cranial nerve V, and substantia nigra are present (eFig. 9.36).
Sections from the rostral pons are significant for pontine nuclei and multiple transversely oriented pontocerebellar fibers en route to the contralateral cerebellum. Interspersed among these horizontally running axon are longitudinally running corticospinal fibers (eFig. 9.37). Dorsal to this collection of fiber bundles, the medial lemniscus is oriented horizontally, forming a borderlike structure between the basal and tegmental portion of the pons (eFig. 9.37). At the lateral edge of the medial lemniscus, the spinothalamic tract can be seen, because it neighbors the superior cerebellar peduncle (eFig. 9.37). Near the midline, the medial longitudinal fasciculus resides just ventral to the periaqueductal gray matter.