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The brain stem includes the medulla and pons, located ventral to the cerebellum. In addition to housing essential ascending and descending tracts, the brain stem contains nuclei that are essential for maintenance of life. As a result of the relatively tight packaging of numerous ascending and descending tracts, as well as nuclei, within the brain stem, even small lesions within it can injure multiple tracts and nuclei within it and thus can produce very significant neurologic deficits. The cerebellum, located just dorsal to the brain stem, plays a major role in motor coordination. Because of its proximity to the brain stem, injuries which cause swelling of the cerebellum can compress the brain stem, and thus can rapidly become life-threatening.

The lower part of the cranial portion of the neural tube (neuraxis) gives rise to the brain stem. The brain stem is divided into the mesencephalon and rhombencephalon (Fig 7–1).

Figure 7–1

Four stages in early development of brain and cranial nerves (times are approximate). A: 3½ weeks. B: 4½ weeks. C: 7 weeks. D: 11 weeks.

The primitive central canal widens into a four-sided pyramid shape with a rhomboid floor (Fig 7–2). This becomes the fourth ventricle, which extends over the future pons and the medulla.

Figure 7–2

Schematic illustration of the widening of the central cavity in the lower brain stem during development.

The neural tube undergoes local enlargement and shows two permanent flexures: the cephalic flexure at the upper end and the cervical flexure at the lower end. The cephalic flexure in an adult brain is the angle between the brain stem and the horizontal plane of the brain (see Fig 1–6).

The central canal in the rostral brain stem becomes the cerebral aqueduct. The roof of the rostral fourth ventricle undergoes intense cellular proliferation, and this lip produces the neurons and glia that will populate both the cerebellum and the inferior olivary nucleus.

The quadrigeminal plate, the midbrain tegmentum, and the cerebral peduncles develop from the mesencephalon (midbrain; see Fig 7–1), and the cerebral aqueduct courses through it. The rhombencephalon (see Fig 7–1A) gives rise to the metencephalon and the myelencephalon. The metencephalon forms the cerebellum and pons; it contains part of the fourth ventricle. The myelencephalon forms the medulla oblongata; the lower part of the fourth ventricle lies within this portion of the brain stem.

As in the spinal cord, the embryonic brain stem has a central gray core with an alar plate (consisting mostly of sensory components) and a basal plate (composed primarily of motor components). The gray columns are not continuous in the brain stem, however, and the development of the fourth ventricle ...

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