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IN PRIMITIVE VERTEBRATES—REPTILES, amphibians, and fish—the forebrain is only a small part of the brain and is devoted mainly to olfactory processing and to the integration of autonomic and endocrine function with the basic behaviors necessary for survival. These basic behaviors include feeding, drinking, sexual reproduction, sleep, and emergency responses. Although we are accustomed to thinking that the forebrain orchestrates most human behaviors, many complex responses, such as feeding—the coordination of chewing, licking, and swallowing—are actually made up of relatively simple, stereotypic motor responses governed by ensembles of neurons in the brain stem.

The importance of this pattern of organization in human behavior is clear from observing infants born without a forebrain (hydranencephaly). Hydranencephalic infants are surprisingly difficult to distinguish from normal babies. They cry, smile, suckle, and move their eyes, face, arms, and legs. As these sad cases illustrate, the brain stem can organize virtually all of the behavior of the newborn.

In this chapter, we describe the functional anatomy of the brain stem, particularly the cranial nerves, as well as the ensembles of local circuit neurons that organize the simple behaviors of the face and head. Finally, we consider the modulatory functions of nuclei in the brain stem that adjust the sensitivity of sensory, motor, and arousal systems.

The brain stem is the rostral continuation of the spinal cord, and its motor and sensory components are similar in structure to those of the spinal cord. But the portions of the brain stem that control the cranial nerves are much more complex than the corresponding parts of the spinal cord that control the spinal nerves because cranial nerves mediate more complex behaviors. The core of the brain stem, the reticular formation, is homologous to the intermediate gray matter of the spinal cord but is also more complex. Like the spinal cord, the reticular formation contains ensembles of local-circuit interneurons that generate motor and autonomic patterns and coordinate reflexes and simple behaviors. In addition, the brain stem contains glutamatergic and GABAergic circuitry that regulates arousal, wake–sleep cycles, breathing, and other vital functions, as well as monoaminergic modulatory neurons that act to optimize the functions of the nervous system.

The Cranial Nerves Are Homologous to the Spinal Nerves

Because the spinal nerves reach only as high as the first cervical vertebra, the cranial nerves provide the somatic and visceral, sensory and motor innervation for the head. Two cranial nerves, the glossopharyngeal and vagus nerves, also supply visceral sensory and motor innervation of the neck, chest, and most of the abdominal organs with the exception of the pelvis. In addition, some cranial nerves are associated with specialized functions, such as vision or hearing, that go beyond the sensory and motor plan of the spinal cord.

Assessment of the cranial nerves is an important part of the neurological examination because abnormalities ...

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