1. Overview of the Nervous System
Part 3 of 4

Nachum Dafny, Ph.D.
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The Brain Stem

The brain stem consists of mesencephalon (midbrain), metencephalon, and myelencephalon. The metencephalon and myelencephalon together compose the rhombencephalon (hindbrain), which divides into pons, and medulla oblongata (Figures 1.11 and 1.12).

Mesencephalon (midbrain) is continuous with the diencephalon rostrally and with the pons caudally. The midbrain is the smallest part of the brain stem, being about 2 cm in length. It consists of a tectum posteriorly, a tegmentum inferiorly, and a base anteriorly. The tectum forms the roof of the cerebral aqueduct, which connects the third ventricle with the fourth ventricle and the tegmentum its floor. The base of the midbrain consists of the cerebral peduncle, which contain nerve fibers descending from the cerebral cortex. The nuclei of the 3rd (oculomotor), the 4th (trochlear) and part of the 5th (trigeminal) are located in the midbrain tegmentum. The red nucleus and the substantia nigra, two prominent nuclei, are also found in the midbrain tegmentum. The midbrain tectum is formed by two pairs of rounded structures: the superior and inferior colliculi. The superior and inferior colliculi (Figure 1.12) are involved in visual and auditory functions respectively.

Pons is continuous with the midbrain and is composed of two parts, the pontine tegmentum (located internally) and the basilar pons. At the level of the pons, the cerebral aqueduct has expanded to form the fourth ventricle (Figure 1.12). The cerebellum is situated posterior to the pons and forms part of the roof (tectum) of the forth ventricle. The pons contains nuclei that receive axons from various cortical areas. Projections from the axons of these pontine neurons form large transverse fiber bundles that traverse the pons and ascend to the contralateral cerebellum via the middle cerebellar peduncles. Also, within the pons base and tegmentum are longitudinally ascending and descending fibers. The nuclei of the 5th (trigeminal), 6th (abducens), 7th (facial) and the 8th (vestibulocochlear) nerves are located in the pons tegmentum.

Medulla Oblongata (myelencephalon is also known as the medulla). The medulla lies between the pons rostrally and the spinal cord caudally. It is continuous with the spinal cord just above to foramen magnum and the first spinal nerve. The posterior surface of the medulla forms the caudal half of the fourth ventricle floor and the cerebellum, its roof (Figure 1.12). The base of the medulla is formed by the pyramidal-descending fibers from the cerebral cortex. The medulla tegmentum contains ascending and descending fibers and nuclei from the 9th (glossopharyngeal), 10th (vagus), 11th (accessory) and the 12th (hypoglossal) nerves. The corticospinal fibers (pyramid) are alongside the anterior median fissure, and decussate (cross the midline) to the contralateral side on their way to the spinal cord. Other prominent structures in the medulla are the inferior olive, and the inferior cerebellar peduncle. The medulla contains nuclei which regulate respiration, swallowing, sweating, gastric secretion, cardiac, and vasomotor activity.

The arterial blood supply to the brain is derived from two arterial systems: the carotid system and the vertebrobasilar system. A series of an anastomotic channels lying at the base of the brain, known as the circle of Willis, permits communication between these two systems (Figure 1.13).

The arterial blood supply to the spinal cord is derived from two branches of vertebral artery, the anterior and two posterior spinal arteries which run the length of the spinal cord and form an irregular plexus around it (Figure 1.14).

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