brainstem and diencephalon ventral view
The brainstem is the lowermost part of the brain and is located above the spinal cord. It can be seen by viewing the brain from below (see Figure 15A; also Figure OA and Figure OL). This specimen has been obtained by dissecting out the brainstem, and cerebellum, along with the diencephalon; a photographic view of this specimen is shown in the next illustration (Figure 7). The dienceph-alon will be described subsequently (see Figure 11 and Figure 12).
In the human brain, the brainstem is a relatively small mass of brain tissue compared to the large hemispheres, but it is packed with various nuclei and tracts. Among these nuclei are those of 10 of the cranial nerves (CN III to CN XII). Many basic brain activities are located in the brainstem, including key vital functions (control of blood pressure, pulse, and respiration). Some motor functions are found at various brainstem levels, some as part of the reticular formation; the reticular formation is also part of a system that is responsible for consciousness. Most important, the ascending sensory and descending motor tracts/pathways that connect the spinal cord with "higher" areas of the brain pass through the brainstem (described in Section B). In addition, many of the connections to the cerebellum, including pathways and nuclei, are found in the brainstem. Finally, each part of the brainstem has a part of the ventricular system.
The brainstem is divided anatomically into three parts — the narrow midbrain, which is located under the dien-cephalon; the pons, with its ventral bulge; and the medulla, which connects with the spinal cord. Each of the parts has distinctive features that allow for the identification of the parts, both on the gross brain specimen or a microscopic cross-section.
• The midbrain region (mesencephalon) has two large "pillars" anteriorly called the cerebral peduncles, which consist of millions of axons descending from the cerebral cortex to various levels of the brainstem and spinal cord.
• The pons portion is distinguished by its bulge anteriorly, the pons proper, an area that is composed of nuclei (the pontine nuclei) that connect to the cerebellum.
• The medulla has two distinct elevations on either side of the midline, known as the pyramids; the direct voluntary motor pathway from the cortex to the spinal cord, the cortico-spinal tract, is located within the pyramid. Behind each is a prominent bulge, called the olive, the inferior olivary nucleus, which connects with the cerebellum.
Cranial Nerves and Their Attachment
The cranial nerves of the brainstem will be presented in numerical order, starting at the midbrain level.
Midbrain Level
• CN III, the oculomotor nerve, emerges ven-trally between the cerebral peduncles (in the interpeduncular fossa).
• CN IV, the trochlear nerve, which exits posteriorly, is a thin nerve that wraps around the lowermost border of the cerebral peduncle.
Pontine Level
• CN V, the trigeminal nerve, is a massive nerve attached along the middle cerebellar peduncle.
• CN VI, the abducens nerve, is seen exiting anteriorly at the junction between the pons and medulla.
• CN VII, the facial nerve, and CN VIII (the vestibulocochlear nerve), are both attached to the brainstem at the ponto-cerebellar angle.
Medullary Level
• CN IX, the glossopharyngeal, and CN X, the vagus, are attached to the lateral margin of the medulla, behind the inferior olive.
• CN XI, the spinal accessory nerve, from the uppermost region of the spinal cord, enters the skull and then exits from the skull as if it were a cranial nerve; by convention it is included as a cranial nerve.
• CN XII, the hypoglossal nerve, emerges by a series of rootlets between the inferior olive and the pyramid.
Information concerning the function of the cranial nerves will be discussed with Figure 8A and Figure 8B. The nuclei of the brainstem, including the cranial nerve nuclei, will be studied in cross-sections of the brainstem in Section C of this atlas (see Figure 64-Figure 67).
Additional Detail
Structures labeled, such as the flocculus of the cerebellum, the pituitary stalk, and the mammillary bodies (nuclei), will be considered at the appropriate time.
M = Mammillary bodies CP = Cerebral peduncle Po = Pons Py = Pyramid O = Olive FI = Flocculus
FIGURE 6: Brainstem 1 — Ventral View with Cranial Nerves
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