The autonomic nervous system is responsible for regulating smooth muscle, cardiac muscle, and glands in order to maintain homeostasis and coordinate responses to external stimuli. It consists of sensory and motor components, with the motor component being divided into sympathetic and parasympathetic divisions. The enteric nervous system, which controls the gastrointestinal tract, is often considered part of the autonomic nervous system.

Autonomic motor pathways consist of preganglionic and postganglionic neurons. Preganglionic neurons are located in the central nervous system (CNS), while postganglionic neurons are found in peripheral ganglia. Sympathetic preganglionic neurons are situated in the thoracolumbar region of the spinal cord, while sympathetic postganglionic neurons reside in paravertebral and prevertebral ganglia. Parasympathetic preganglionic neurons can be found in cranial nerve nuclei or in the sacral portion of the spinal cord, while parasympathetic postganglionic neurons are located in ganglia near or within the target organs.

The autonomic nervous system also includes afferent fibers that innervate sensory receptors in the viscera. These fibers primarily activate reflexes, but in some cases, their activation can also lead to conscious sensations.

The enteric nervous system, which is part of the autonomic nervous system, is comprised of the myenteric and submucosal plexuses within the gastrointestinal tract. The myenteric plexus regulates motility, while the submucosal plexus controls ion and water transport as well as secretion.

At the synapses of preganglionic neurons in autonomic ganglia, neurotransmitters such as acetylcholine (acting at nicotinic and muscarinic receptors) and various neuropeptides are released. Catecholamines are released by interneurons in the ganglia. Norepinephrine, acting on adrenergic receptors, is the primary neurotransmitter released by sympathetic postganglionic neurons. However, neuropeptides are also released by these neurons. Sweat glands, which are supplied by sympathetic postganglionic neurons, are stimulated by the release of acetylcholine. Parasympathetic postganglionic neurons release acetylcholine, which acts on muscarinic receptors.

Both the sympathetic and parasympathetic nervous systems control the pupil in a reciprocal manner. Sympathetic activity causes pupil dilation (mydriasis), while parasympathetic activity causes pupil constriction (meiosis).

The emptying of the urinary bladder relies on parasympathetic outflow during the micturition reflex. Sympathetic constriction of the internal sphincter of the urethra prevents voiding. The micturition reflex is triggered by stretch receptors and is controlled by a micturition center in the pons in normal adults.

The hypothalamus, which contains various nuclei, plays a crucial role in regulating basic bodily functions such as body temperature, body weight, and fluid intake.

The primary function of the hypothalamus is to maintain homeostasis of critical physiological parameters by acting as a servomechanism. It receives information about specific physiological parameters and uses this information to maintain each parameter at a specific set point. The hypothalamus achieves this through multiple mechanisms.
The autonomic nervous system controls smooth muscle, cardiac muscle, and glands. It helps maintain homeostasis and coordinates responses to external stimuli. It has sensory and motor components, and the motor component consists of sympathetic and parasympathetic divisions. The enteric nervous system is often considered as part of the autonomic nervous system but is concerned specifically with control of the gastrointestinal tract.
2.
Autonomic motor pathways have preganglionic and postganglionic neurons. Preganglionic neurons reside in the CNS, whereas postganglionic neurons lie in peripheral ganglia. Sympathetic preganglionic neurons are located in the thoracolumbar region of the spinal cord, and sympathetic postganglionic neurons are located in paravertebral and prevertebral ganglia. Parasympathetic preganglionic neurons are located in cranial nerve nuclei or in the sacral portion of the spinal cord. Parasympathetic postganglionic neurons reside in ganglia located in or near the target organs.
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Autonomic afferent fibers innervate sensory receptors in the viscera. Most function to activate reflexes; for some, activation also leads to sensations that are experienced consciously.
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The enteric nervous system includes the myenteric and submucosal plexuses in the wall of the gastrointestinal tract. The myenteric plexus regulates motility, and the submucosal plexus regulates ion and water transport and secretion.
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Neurotransmitters at the synapses of preganglionic neurons in autonomic ganglia include acetylcholine (acting at both nicotinic and muscarinic receptors) and a number of neuropeptides. Interneurons in
Nakamura K. Central circuitries for body temperature regulation and fever. Am J Physiol Regul Integr Comp Physiol. 2011;301: R1207-R1228.
the ganglia release catecholamines. Norepinephrine (acting on adrenergic receptors) is the neurotransmitter generally released by sympathetic postganglionic neurons; neuropeptides are also released. Sympathetic postganglionic neurons that supply sweat glands release acetylcholine. Parasympathetic postganglionic neurons release acetylcholine (acting on muscarinic receptors).
6.
The pupil is controlled reciprocally by the sympathetic and parasympathetic nervous systems. Sympathetic activity causes pupillary dilation (mydriasis); parasympathetic activity causes pupillary constriction (meiosis).
7.
Emptying of the urinary bladder depends on parasympathetic outflow during the micturition reflex. Sympathetic constriction of the internal sphincter of the urethra prevents voiding. The micturition reflex is triggered by stretch receptors, and it is controlled in normal adults by a micturition center in the pons.
8.
The hypothalamus contains many nuclei that have a variety of functions related to regulation of basic bodily functions, including body temperature, body weight, and fluid intake.
9.
The goal of hypothalamic function is to maintain homeostasis of critical physiological parameters by acting as a servomechanism. The hypothalamus receives information about specific physiological parameters and uses this information to maintain each of these parameters at a specific set point. It does so via multiple mechanisms. This chapter illustrates how it maintains homeostasis via its control of the autonomic system.
Saper CB, Chou TC, Elmquist JK. The need to feed: homeostatic and hedonic control of eating. Neuron. 2002;36:199-211.
Squire L, Berg D. Fundamental Neuroscience. 4th ed. New York: Academic Press; 2012.
SECTION 3Muscle JAMES M. WATRAS
Upon completion of this chapter, the student should be able to answer the following questions: 1.