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The Cranial Nerves Are Homologous to the Spinal Nerves
Cranial Nerve Nuclei in the Brain Stem Are Organized on the Same Basic Plan As Are Sensory and Motor Regions of the Spinal Cord
Adult Cranial Nerve Nuclei Have a Columnar Organization
Embryonic Cranial Nerve Nuclei Have a Segmental Organization
The Organization of the Brain Stem and Spinal Cord Differs in Three Important Ways
Neuronal Ensembles in the Brain Stem Reticular Formation Coordinate Reflexes and Simple Behaviors Necessary for Homeostasis and Survival
Cranial Nerve Reflexes Involve Mono- and Polysynaptic Brain Stem Relays
Pattern Generator Neurons Coordinate Stereotypic and Autonomic Behaviors
A Complex Pattern Generator Regulates Breathing
An Overall View
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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 human behavior originates mainly in the forebrain, 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.
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The importance of this pattern of organization in human behavior is clear from observing infants born without a forebrain (hydrancephaly). Hydrancephalic 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.
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In this chapter we examine the role of the brain stem in reflex behavior. We also review the cranial nerves, their origin in the brain stem, as well as the ensembles of local circuit neurons that organize the simple behaviors of the face and head.
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The brain stem is the rostral continuation of the spinal cord and its motor and sensory components are similar in structure to that 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, it contains clusters of dopaminergic, noradrenergic, and other modulatory neurons that act to optimize the functions of the nervous system. The modulatory actions of these nuclei ...