The Autonomic Nervous System

The autonomic nervous system is a part of the peripheral nervous system that functions to regulate the basic visceral (organ) processes needed for the maintenance of normal bodily functions. It operates independently of voluntary control, although certain events, such as emotional stress, fear, sexual excitement, and alterations in the sleep-wakefulness cycle, change the level of autonomic activity.

The autonomic system is usually defined as a motor system that innervates three major types of tissue: cardiac muscle, smooth muscle, and glands. However, this definition needs to be expanded to encompass the fact that it also relays visceral sensory information into the central nervous system and processes it in such a way as to make alterations in the activity of specific autonomic motor outflows, such as those that control the heart, blood vessels, and other visceral organs. It also causes the release of certain hormones involved in energy metabolism (e.g., insulin, glucagon, epinephrine) or cardiovascular functions (e.g., renin, vasopressin). These integrated responses maintain the normal internal environment of the body in an equilibrium state called homeostasis.

The autonomic system consists of two major divisions: the sympathetic nervous system and the parasympathetic nervous system. These often function in antagonistic ways. The motor outflow of both systems is formed by two serially connected sets of neurons. The first set, called preganglionic neurons, originates in the brain stem or the spinal cord, and the second set, called ganglion cells or postganglionic neurons, lies outside the central nervous system in collections of nerve cells called autonomic ganglia. Parasympathetic ganglia tend to lie close to or within the organs or tissues that their neurons innervate, whereas sympathetic ganglia lie at a more distant site from their target organs. Both systems have associated sensory fibers that send feedback information into the central nervous system regarding the functional condition of target tissues. To view a figure depicting the difference in function between the sympathetic and parasympathetic nervous system click here.

A third division of the autonomic system, termed the enteric nervous system, consists of a collection of neurons embedded within the wall of the entire gastrointestinal tract and its derivatives. This system controls gastrointestinal motility and secretions.

Sympathetic nervous system

Sympathetic preganglionic neurons originate in the lateral horns of the 12 thoracic and the first 2 or 3 lumbar segments of the spinal cord. (For this reason the sympathetic system is sometimes referred to as the thoracolumbar outflow. The diagram to the left depicts this.) The axons of these neurons exit the spinal cord in the ventral roots and then synapse on either sympathetic ganglion cells or specialized cells in the adrenal gland called chromaffin cells.

Sympathetic ganglia

Sympathetic ganglia can be divided into two major groups, paravertebral and prevertebral (or preaortic), on the basis of their location within the body. Paravertebral ganglia generally lie on each side of the vertebrae and are connected to form the sympathetic chain or trunk. There are usually 21 or 22 pairs of these ganglia: 3 in the cervical region, 10 to 11 in the thoracic region, 4 in the lumbar region, 4 in the sacral region, and a single, unpaired ganglion lying in front of the coccyx called the ganglion impar. The three cervical sympathetic ganglia are the superior cervical ganglion, the middle cervical ganglion, and the cervicothoracic ganglion (also called the stellate ganglion). The superior ganglion innervates viscera of the head; the middle and stellate ganglia innervate viscera of the neck, thorax (i.e., the bronchi and heart), and upper limb. The thoracic sympathetic ganglia innervate the trunk region, and the lumbar and sacral sympathetic ganglia innervate the pelvic floor and lower limb. All the paravertebral ganglia provide sympathetic innervation to blood vessels in muscle and skin, arrector pili muscles attached to hairs, and sweat glands.

The three preaortic ganglia are the celiac, superior mesenteric, and inferior mesenteric. Lying on the anterior surface of the aorta, they provide axons that are distributed with the three major gastrointestinal arteries arising from the aorta. The three ganglia retain a pattern of innervation that originates in the embryo. Thus, the celiac ganglion innervates structures derived from the embryonic foregut, including the stomach, liver, pancreas, duodenum, and the first part of the small intestine; the superior mesenteric ganglion innervates the small intestine, which is derived from the embryonic midgut; and the inferior mesenteric ganglion innervates embryonic hindgut derivatives, which include the descending colon, sigmoid colon, rectum, urinary bladder, and sexual organs.

Neurotransmitters and receptors

Upon reaching their target organs by traveling with the blood vessels that supply them, sympathetic fibers terminate as a series of varicosities close to the end organ. Because of this anatomical arrangement, autonomic transmission takes place across a junction rather than a synapse. “Presynaptic” sites can be identified because they contain aggregations of synaptic vesicles and membrane thickenings; postjunctional membranes, on the other hand, rarely possess morphological specializations, but they do contain specific receptors for various neurotransmitters. The distance between pre- and postsynaptic elements can be quite large as compared to typical synapses. For instance, the gap between cell membranes of a typical chemical synapse is 30–50 nanometres, while in blood vessels the distance is often greater than 100 nanometres and, in some cases, 1–2 micrometres (1,000–2,000 nanometres). Owing to these relatively large gaps between autonomic nerve terminals and their effector cells, transmitters tend to act slowly; they become inactivated rather slowly as well. To compensate for this apparent inefficiency, many effector cells, such as those in smooth and cardiac muscle, are connected by low-resistance pathways that allow for electrotonic coupling of the cells. In this way, if only one cell is activated, multiple cells will respond and work as a group.

At a first approximation, chemical transmission in the sympathetic system appears simple: preganglionic neurons use acetylcholine as a neurotransmitter, whereas most postganglionic neurons utilize norepinephrine (noradrenaline)—with the major exception that postganglionic neurons innervating sweat glands use acetylcholine. On closer inspection, however, neurotransmission is seen to be more complex, because multiple chemicals are released, and each functions as a specific chemical code affecting different receptors on the target cell. In addition, these chemical codes are self-regulatory, in that they act on presynaptic receptors located on their own axon terminals.

The chemical codes are specific to certain tissues. For example, most sympathetic neurons that innervate blood vessels secrete both norepinephrine and neuropeptide Y, sympathetic neurons that innervate the submucosal neural plexus of the gut contain both norepinephrine and somatostatin, and sympathetic neurons that innervate sweat glands contain calcitonin gene-related peptide, vasoactive intestinal polypeptide, and acetylcholine. In addition, other chemicals besides the neuropeptides mentioned above are released from autonomic neurons along with the so-called classical neurotransmitters, norepinephrine and acetylcholine. For instance, some neurons synthesize a gas, nitric oxide, that functions as a novel type of neuronal messenger molecule. Thus, neural transmission in the autonomic nervous systems involves the release of combinations of different neuroactive agents that affect both pre- and postsynaptic receptors.

Neurotransmitters released from nerve terminals bind to specific receptors, which are specialized macromolecules embedded in the cell membrane. The binding action initiates a series of specific biochemical reactions in the target cell that produce a physiological response. These effects can be modified by various drugs that act as agonists or antagonists. In the sympathetic nervous system, for example, there are five types of adrenergic receptors (receptors binding epinephrine): a 1, a 2, b 1, b 2, and b 3. These are found in different combinations in various cells throughout the body. Activation of a 1 receptors in arterioles causes blood-vessel constriction, whereas stimulation of a 2 autoreceptors (receptors located in sympathetic presynaptic nerve endings) function to inhibit the release of norepinephrine. Other types of tissue have unique adrenergic receptors. Heart rate and myocardial contractility, for example, is controlled by b 1 receptors, bronchial smooth muscle relaxation is mediated by b 2 receptors, and lipolysis is controlled by b 3 receptors.

Cholinergic receptors (receptors binding acetylcholine) also are found in the sympathetic system (as well as the parasympathetic system). Nicotinic cholinergic receptors cause sympathetic postganglionic neurons, adrenal chromaffin cells, and parasympathetic postganglionic neurons to fire and release their chemicals. Muscarinic receptors are associated mainly with parasympathetic functions and are located in peripheral tissues (e.g., glands, smooth muscle). Peptidergic receptors exist in target cells as well.

The length of time that each type of chemical acts on its target cell is variable. As a rule, peptides cause slowly developing, long-lasting effects (one or more minutes), whereas the classical transmitters produce short-term effects (about 25 milliseconds).

The sympathetic nervous system normally functions to produce localized adjustments (such as sweating) and reflex adjustments of the cardiovascular system. Under conditions of stress, however, the entire sympathetic nervous system is activated, producing an immediate, widespread response that has been called the “fight or flight” response. This is characterized by the release of large quantities of epinephrine from the adrenal gland, an increase in heart rate, an increase in cardiac output, skeletal muscle vasodilation, cutaneous and gastrointestinal vasoconstriction, pupillary dilation, bronchial dilation, and piloerection. The overall effect is to prepare the individual for imminent danger.

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