47: The Autonomic Motor System and the Hypothalamus

Introduction

• The Autonomic Motor System Mediates Homeostasis

• The Autonomic System Contains Visceral Motor Neurons That Are Organized into Ganglia

  • Preganglionic Neurons Are Localized in Three Regions Along the Brain Stem and Spinal Cord

  • Sympathetic Ganglia Project to Many Targets Throughout the Body

  • Parasympathetic Ganglia Innervate Single Organs

  • The Enteric Ganglia Regulate the Gastrointestinal Tract

• Both the Pre- and Postsynaptic Neurons of the Autonomic Motor System Use Co-Transmission at Their Synaptic Connections

• Autonomic Behavior Is the Product of Cooperation Between All Three Autonomic Divisions

• Autonomic and Endocrine Functions Are Coordinated by a Central Autonomic Network Centered in the Hypothalamus

• The Hypothalamus Integrates Autonomic, Endocrine, and Behavioral Responses

  • Magnocellular Neuroendocrine Neurons Control the Pituitary Gland Directly

  • Parvicellular Neuroendocrine Neurons Control the Pituitary Gland Indirectly

• An Overall View

When we are frightened our heart races, our breathing becomes rapid and shallow, our mouth becomes dry, our muscles tense, our palms become sweaty, and we may want to run. These bodily changes accompanying fear are mediated by the autonomic motor system, which controls heart muscle, smooth muscle, and exocrine glands. The autonomic motor system is controlled by a central neuronal network that includes the hypothalamus.

As we shall learn in this and the next two chapters, the hypothalamus regulates the autonomic circuits so as to recruit appropriate physiological responses for specific emotions and to coordinate these physiological and emotional responses with other aspects of behavior to insure constancy of the internal environment (homeostasis). The hypothalamus contributes to the maintenance of homeostasis by acting on three major systems: the autonomic motor system, the endocrine system, and an ill-defined neural system concerned with motivation.

The autonomic motor system is distinct from the somatic motor system, which controls skeletal muscle. Nevertheless, to produce behaviors the somatic and autonomic motor systems must work together. Whereas neurons in the somatic motor system regulate contractions of striated muscles (see Chapter 34), the autonomic motor system regulates gland cells as well as smooth and cardiac muscle, maintains constant body temperature, and controls eating, drinking, and sexual behavior.

Although the autonomic motor system is largely involuntary, the behaviors controlled by it are tightly integrated with voluntary movements controlled by the somatic motor system. Running, climbing, and lifting are voluntary actions with metabolic requirements and thermoregulatory consequences that are automatically met by the autonomic system through changes in cardiorespiratory drive, cardiac output, regional blood flow, and ventilation. Autonomic behaviors similarly are linked to emotional arousal, stress, motivation, and defensive reactions. Feelings of fear, anger, happiness, and sadness have characteristic autonomic manifestations.

In this chapter we first examine the peripheral components of the autonomic system and then their role in mediating behaviors. We then explore how these "autonomic behaviors" are orchestrated through a central autonomic network in the brain stem and hypothalamus. We conclude by considering the role of the amygdala and specialized areas of cerebral cortex in coordinating ...