Introduction
The function of the autonomic nervous system is to maintain, at a constant level, the internal environment of the body (homeostasis). This system regulates involuntary functions such as blood pressure, heart rate, respiration, body temperature, glandular secretion, digestion, and reproduction. Based on the nature of its functions, the autonomic nervous system is also called the involuntary or vegetative nervous system. The system consists of both afferent and efferent components.
Divisions of the Autonomic Nervous System
The autonomic nervous system is divided into three divisions: sympathetic, parasympathetic, and enteric.
Sympathetic Division
The neurons from which the outflow of the sympathetic division originates (sympathetic preganglionic neurons) are located in the intermediolateral cell column (IML) of the first thoracic to second lumbar (T1-L2) segments of the spinal cord. For this reason, the sympathetic nervous system is sometimes called the thoracolumbar division of the autonomic nervous system. An overview of the anatomical organization of the sympathetic and parasympathetic divisions of the autonomic nervous system is presented in the following sections and in Figures 22-1 and 22-2. A detailed description of autonomic innervation of important organs is provided later in this topic.
Spinal Sympathetic Preganglionic Neurons
In the spinal cord, the distribution of sympathetic pregan-glionic neurons in the thoracolumbar cord exhibits rostral-to-caudal viscerotopy. Thus, the location of the sympathetic preganglionic neurons providing innervation to the eye is most rostral (T1-T2); the location of the neurons innervating the heart and lungs is more caudal; and the location of neurons innervating the gastrointestinal tract (GIT), bladder, and genitals is most caudal.
Paravertebral Ganglia and the Sympathetic Chains
There are two sympathetic chains (also called sympathetic trunks), one located on each side of the vertebral column. Each sympathetic chain consists of a ganglionated nerve trunk. These ganglia are called paravertebral ganglia (25 pairs) and represent one of the sites where the sympathetic postganglionic neurons are located. Axons of the sympathetic preganglionic neurons synapse on many sympathetic postganglionic neurons in the sympathetic chain; the ratio of preganglionic to postganglionic fibers is about 1:30. This divergence facilitates coordinated activity of sympathetic neurons at different levels of the spinal cord. In the autonomic nervous system, preganglionic axons are myelinated, whereas the postganglionic axons are unmy-elinated.
The course of preganglionic and postganglionic sympathetic fibers at different levels of the spinal cord is shown in Figures 22-1 and 22-2. The axons of sympathetic preganglionic neurons located at T1-L2 exit the spinal cord via ventral roots, travel a short distance in the corresponding spinal nerves, and then enter the sympathetic chain via the white rami (they appear white because they contain myelinated axons).
FIGURE 22-1 An overview of the sympathetic and parasympathetic components of the autonomic nervous system. Red = thoracolumbar division; Blue = craniosacral division. C = cervical; T = thoracic; L = lumbar; S = sacral spinal segments.
FIGURE 22-2 Examples of the course of preganglionic and postganglionic sympathetic fibers innervating different organs. (A) Organs in the head. (B) Organs in the chest. (C) Organs in the abdomen. (D) Adrenal gland. Also note that, at each level, the axons of the postganglionic neurons in the paravertebral ganglia re-enter the corresponding spinal nerves through gray rami, travel within or along the spinal nerve, and innervate the blood vessels, sweat glands, and erectile muscle of hair follicles. IML = intermediolateral cell column.
Some of these preganglionic axons synapse on postganglionic neurons located in the paraver-tebral ganglia, while others pass through the paraverte-bral ganglia without synapsing and finally synapse on postganglionic neurons located in the prevertebral ganglia (see next section). The axons of some of the postgan-glionic neurons located in the paravertebral ganglia exit the sympathetic chain via visceral branches and innervate different organs (Figs. 22-1 and 22-2). For example, sympathetic postganglionic fibers from the superior cervical ganglion exit the sympathetic chain and supply the organs located in the head and neck, including the blood vessels of the brain and cranial muscles, eyes, and lacrimal and salivary glands. Sympathetic postganglionic fibers from the middle and inferior cervical ganglia provide sympathetic innervation to the organs located in the chest cavity (e.g., the heart, lungs, and bronchi).
Some postganglionic fibers exit the sympathetic chain via the gray rami (they appear gray because the fibers passing through them are unmyelinated); reenter the corresponding spinal nerve; course within or along the spinal nerve, and then innervate the blood vessels, sweat glands, and erectile muscles of some hair follicles in the tissues supplied by this spinal nerve (Fig. 22-2). It should be noted that gray rami are present at all levels of the sympathetic chain, whereas white rami are present only in the thoraco-lumbar region of the sympathetic chain (T1-L2 level).
Prevertebral Ganglia
As mentioned earlier, the sympathetic nervous system includes two types of ganglia: the paravertebral and the prevertebral ganglia. The sympathetic postganglionic neurons that innervate the abdominal viscera are located in the prevertebral ganglia. Unlike the paravertebral ganglia, the prevertebral ganglia are located distal to the sympathetic chain and closer to the organs that they innervate. The course of sympathetic fibers that innervate different abdominal organs is as follows. The axons of the sympathetic preganglionic neurons located in the IML at the T5-T9 spinal segments exit through the ventral roots, travel a short distance in the spinal nerve, enter the sympathetic chain via the white rami, exit the sympathetic chain without synapsing, and form the greater splanchnic nerve. The axons of sympathetic preganglionic neurons located in the IML at T10—T12 follow a similar course and form the lesser and least splanchnic nerves. The axons of the greater, lesser, and least splanchnic nerves synapse on the postganglionic neurons located in the celiac ganglion.
The postganglionic fibers emerging from the celiac ganglion innervate the smooth muscle and glands in the stomach, small intestine, liver, spleen, pancreas, and kidney. Some sympathetic preganglionic fibers in the greater, lesser, and least splanchnic nerves pass through the celiac ganglion (without synapsing) and make a synapse on the cells in the adrenal medulla that secrete epinephrine and norepinephrine. It should be noted that the adrenal medulla is analogous to sympathetic ganglion, and the secretory cells present in it are functionally analogous to sympathetic postganglionic neurons.
Some sympathetic preganglionic fibers in the greater, lesser, and least splanchnic nerves pass through the celiac ganglion (without synapsing) and synapse on the postgan-glionic neurons located in the superior mesenteric ganglion. Postganglionic neurons located in this ganglion innervate the distal portions of the small and large intestine, including the ascending and transverse colons. Some sympathetic preganglionic fibers from L1-L2 spinal segments pass through the sympathetic chain without synaps-ing, travel in the lumbar splanchnic nerves, and synapse on the postganglionic neurons located in the inferior mesenteric ganglion. The postganglionic fibers from this ganglion then pass through inferior hypogastric (pelvic) plexus to innervate the transverse and distal colon, rectum, urinary bladder, and male and female sex organs. Some preganglionic fibers from L1-L2 spinal segments descend in the sympathetic chain, pass through the para-vertebral ganglia, and synapse on postganglionic neurons in the inferior hypogastric plexus. The postganglionic fibers from these neurons then innervate the urinary bladder and male and female erectile tissue (penis and clitoris, respectively). Other preganglionic fibers from L1-L2 spinal segments descend in the sympathetic chain and synapse on postganglionic neurons in the paravertebral ganglia. Postganglionic fibers from these neurons then re-enter the spinal nerves via the gray rami and innervate the blood vessels, sweat glands, and erectile muscles of some hair follicles in the lower limb and lower part of the trunk.
Functions of the Sympathetic Nervous System
This division of the autonomic nervous system is activated in stressful situations. Thus, activation of the sympathetic nervous system results in an increase in blood flow in the skeletal muscles; an increase in heart rate, blood pressure, and blood sugar level; and pupillary dilation (mydriasis). These effects are widespread because one sympathetic pre-ganglionic axon innervates several postganglionic neurons. All of these responses prepare the individual for "fight" or "flight." For example, in the need for "flight," an increase in blood flow in the skeletal muscles will help in running away from the site of danger. In the need for "fight," an increase in heart rate and blood pressure will help in better perfusion of different organs, an increase in blood sugar will provide energy, and pupillary dilation will provide better vision. The effects of simultaneous activation of the parasympathetic division of the autonomic nervous system (described later) complement the effects of sympathetic stimulation.
Parasympathetic Division
The neurons from which the parasympathetic outflow arises (parasympathetic preganglionic neurons) are located in the brainstem midbrain, pons, and medulla oblongata) and the sacral region of the spinal cord (second, third, and fourth sacral segments [S2-S4]). For this reason, the parasympathetic division is sometimes referred to as the craniosacral division (Fig. 22-1). A description of the parasympathetic preganglionic neurons, located in the brainstem and spinal cord, is provided in the following sections.
Brainstem Parasympathetic Preganglionic Neurons
The brainstem parasympathetic preganglionic neurons are located in the following nuclei.
The Edinger-Westphal nucleus of the oculomotor nerve (cranial nerve [CN] III), located in the midbrain, provides parasympathetic innervation to the constrictor muscles of the iris and circumferential muscles of the ciliary body. The superior salivatory nucleus of the facial nerve (CN VII), located in the lower pons, provides parasympathetic innervation to the lacrimal glands and sublingual and submandibular salivary glands. The inferior salivatory nucleus of the glossopharyngeal nerve (CN IX), located caudal to the superior salivatory nucleus in the upper medulla, provides parasympathetic innervation to the parotid salivary gland. Other nuclei where the para-sympathetic preganglionic neurons are located include the nucleus ambiguus and the dorsal motor nucleus of vagus. The dorsal motor nucleus of vagus is located in the caudal aspect of the dorsal medulla, whereas the nucleus ambiguus is located more ventrally at approximately the same level of the medulla. The compact region of nucleus ambiguus provides innervation to the muscles of the larynx and pharynx. The region surrounding the compact zone of the nucleus ambiguus contains preganglionic neurons that provide parasympathetic innervation to the heart and mediate the decrease in heart rate following parasympa-thetic stimulation. The preganglionic neurons located in the dorsal motor nucleus of vagus provide primarily para-sympathetic innervation to the lungs, pancreas, and GIT and control secretions of the glands located in these organs.
In addition, their activation elicits peristalsis in the GIT. These neurons may also play a role in eliciting cardiac inhibitory actions because they do provide parasympa-thetic innervation to the heart, although to a lesser extent than that provided by the nucleus ambiguus.
Spinal Parasympathetic Preganglionic Neurons
As mentioned earlier, the spinal cord parasympathetic preganglionic neurons are located in the IML of the sacral spinal cord at the S2-S4 level (Fig. 22-1). Their axons exit through the ventral roots, travel through pelvic nerves, and synapse on postganglionic neurons that are located close to or within the organs being innervated. It should be noted that, in the parasympathetic system, the terminal ganglia are located near or within the organs innervated. Thus, the parasympathetic postganglionic fibers are relatively short, whereas the sympathetic postganglionic fibers are relatively long.
