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When we are awake, we are constantly aware of sensory input from our external environment, and we consciously plan how to react to it. When we are asleep, the nervous system has a variety of mechanisms to dissociate cortical function from sensory input and somatic motor output. Among these mechanism...

When we are awake, we are constantly aware of sensory input from our external environment, and we consciously plan how to react to it. When we are asleep, the nervous system has a variety of mechanisms to dissociate cortical function from sensory input and somatic motor output. Among these mechanisms are closing the eyes, blocking the transmission of sensory impulses to the cortex as they pass through the thalamus, and effecting a nearly complete paralysis of skeletal muscles during rapid eye movement (REM) sleep to keep us from physically acting out our dreams. The conscious and discontinuous nature of cortical brain function stands in sharp contrast to that of those parts of the nervous system responsible for control of our internal environment. These "autonomic" processes never stop attending to the wide range of metabolic, cardiopulmonary, and other visceral requirements of our body. Autonomic control continues whether we are awake and attentive, preoccupied with other activities, or asleep. While we are awake, we are unaware of most visceral sensory input, and we avoid any conscious effort to act on it unless it induces distress. In most cases, we have no awareness of motor commands to the viscera, and most individuals can exert voluntary control over visceral motor output in only minor ways. Consciousness and memory are frequently considered the most important functions of the human nervous system, but it is the visceral control system---including the **autonomic nervous system (ANS)** ---that makes life and higher cortical function possible. We have a greater understanding of the physiology of the ANS than of many other parts of the nervous system, largely because it is reasonably easy to isolate peripheral neurons and to study them. As a result of its accessibility, the ANS has served as a key model system for the elucidation of many principles of neuronal and synaptic function. **[\ ]** 1. **[The ANS has sympathetic, parasympathetic, and enteric divisions]** Output from the central nervous system (CNS) travels along two anatomically and functionally distinct pathways: the **somatic motor neurons,** which innervate striated skeletal muscle; and the **autonomic motor neurons,** which innervate smooth muscle, cardiac muscle, secretory epithelia, and glands. All viscera are richly supplied by efferent axons from the ANS that constantly adjust organ function. The autonomic nervous system (from the Greek for "self-governing," functioning independently of the will) was first defined by Langley in 1898 as including the local nervous system of the gut and the efferent neurons innervating glands and involuntary muscle. Thus, this definition of the ANS includes only *efferent* neurons and *enteric* neurons. Since that time, it has become clear that the efferent ANS cannot easily be dissociated from visceral *afferents* as well as from those parts of the CNS that control the output to the ANS and those that receive interoceptive input.  This larger visceral control system monitors afferents from the viscera and the rest of the body, compares this input with current and anticipated needs, and controls output to the body\'s organ systems. The ANS has three divisions: sympathetic, parasympathetic, and enteric. The **sympathetic** and **parasympathetic divisions** of the ANS are the two major efferent pathways controlling targets other than skeletal muscle ( [Fig. 14-1](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0010) ). Each innervates target tissue by a two-synapse pathway. The cell bodies of the first neurons lie within the CNS. These **preganglionic neurons** are found in columns of cells in the brainstem and spinal cord and send axons out of the CNS to make synapses with **postganglionic neurons** in peripheral ganglia interposed between the CNS and their target cells. Axons from these postganglionic neurons then project to their targets. The sympathetic and parasympathetic divisions can act independently of each other. However, in general, they work synergistically to control visceral activity and often act in opposite ways, like an accelerator and brake to regulate visceral function. An increase in output of the sympathetic division occurs under conditions such as stress, anxiety, physical activity, fear, or excitement, whereas parasympathetic output increases during sedentary activity, eating, or other "vegetative" behavior. The **enteric division** of the ANS is a collection of afferent neurons, interneurons, and motor neurons that form networks of neurons called plexuses (from the Latin "to braid") that surround the gastrointestinal (GI) tract. It can function as a separate and independent nervous system, but it is normally controlled by the CNS through sympathetic and parasympathetic fibers. **[Sympathetic preganglionic neurons originate from spinal segments T1 to L3 and synapse with postganglionic neurons in paravertebral or prevertebral ganglia]** ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------- **Preganglionic Neurons** ------------------------- The cell bodies of preganglionic sympathetic motor neurons are located in the thoracic and upper lumbar spinal cord between levels T1 and L3. At these spinal levels, autonomic neurons lie in the **intermediolateral cell column,** or lateral horn, between the dorsal and ventral horns ( [Fig. 14-2](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0015) ). Axons from preganglionic sympathetic neurons exit the spinal cord through the ventral roots along with axons from *somatic* motor neurons. After entering the spinal nerves, sympathetic efferents diverge from somatic motor axons to enter the white **rami communicantes.** These rami, or branches, are white because most preganglionic sympathetic axons are myelinated.  Figure 14-2 Anatomy of the sympathetic division of the ANS. The figure shows a cross section of the thoracic spinal cord and the nearby paravertebral ganglia as well as a prevertebral ganglion. Sympathetic preganglionic neurons are shown in red and postganglionic neurons in dark blue-violet. Afferent (sensory) pathways are in blue. Interneurons are shown in black. **Paravertebral Ganglia** ------------------------- Axons from preganglionic neurons enter the nearest sympathetic paravertebral ganglion through a white ramus. These ganglia lie adjacent to the vertebral column. Although preganglionic sympathetic fibers emerge only from levels T1 to L3, the chain of sympathetic ganglia extends all the way from the upper part of the neck to the coccyx, where the left and right sympathetic chains merge in the midline and form the coccygeal ganglion. In general, one ganglion is positioned at the level of each spinal root, but adjacent ganglia are fused in some cases. The most rostral ganglion, the **superior cervical ganglion,** arises from fusion of C1 to C4 and supplies the head and neck. The next two ganglia are the **middle cervical ganglion,** which arises from fusion of C5 and C6, and the **inferior cervical ganglion** (C7 and C8), which is usually fused with the first thoracic ganglion to form the **stellate ganglion.** Together, the middle cervical and stellate ganglia, along with the upper thoracic ganglia, innervate the heart, lungs, and bronchi. The remaining paravertebral ganglia supply organs and portions of the body wall in a segmental fashion. After entering a paravertebral ganglion, a preganglionic sympathetic axon has one or more of three fates. It may (1) synapse within that segmental paravertebral ganglion, (2) travel up or down the sympathetic chain to synapse within a neighboring paravertebral ganglion, or (3) enter the greater or lesser splanchnic nerve to synapse within one of the ganglia of the *pre* vertebral plexus. **Prevertebral Ganglia** ------------------------ The **prevertebral plexus** lies in front of the aorta and along its major arterial branches and includes the prevertebral ganglia and interconnected fibers ( [Fig. 14-3](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0020) ). The major prevertebral ganglia are named according to the arteries that they are adjacent to and include the celiac, superior mesenteric, aorticorenal, and inferior mesenteric ganglia. Portions of the prevertebral plexus extend down the major arteries and contain other named and unnamed ganglia and plexuses of nerve fibers, which altogether make up a dense and extensive network of sympathetic neuron cell bodies and nerve fibers. Each *pre* ganglionic sympathetic fiber synapses on many *post* ganglionic sympathetic neurons that are located within one or several nearby paravertebral or prevertebral ganglia. It has been estimated that each preganglionic sympathetic neuron branches and synapses on as many as 200 postganglionic neurons, which enables the sympathetic output to have more widespread effects. However, any impulse arriving at its target end organ has only crossed a single synapse between the preganglionic and postganglionic sympathetic neurons. **Postganglionic Neurons** -------------------------- The cell bodies of postganglionic sympathetic neurons that are located within *para* vertebral ganglia send out their axons through the nearest **gray rami communicantes,** which rejoin the spinal nerves (see [Fig. 14-2](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0015) ). These rami are gray because most postganglionic axons are unmyelinated. Because preganglionic sympathetic neurons are located only in the thoracic and upper lumbar spinal segments (T1 to L3), *white* rami are found only at these levels ( [Fig. 14-4](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0025) , left panel). However, because *each* sympathetic ganglion sends out postganglionic axons, *gray* rami are present at all spinal levels from C2 or C3 to the coccyx. Postganglionic sympathetic axons from paravertebral and prevertebral ganglia travel to their target organs within other nerves or by traveling along blood vessels. Because the paravertebral and prevertebral sympathetic ganglia lie near the spinal cord and thus relatively far from their target organs, the postganglionic axons of the sympathetic division tend to be long. On their way to reach their targets, some postganglionic sympathetic axons travel through *para* sympathetic terminal ganglia or cranial nerve ganglia without synapsing.  Figure 14-4 Organization of the sympathetic and parasympathetic divisions of the ANS. The left panel shows the sympathetic division. The cell bodies of sympathetic preganglionic neurons (red) are in the intermediolateral column of the thoracic and lumbar spinal cord (T1--L3). Their axons project to paravertebral ganglia (the sympathetic chain) and prevertebral ganglia. Postganglionic neurons (blue) therefore have long projections to their targets. The right panel shows the parasympathetic division. The cell bodies of parasympathetic preganglionic neurons (orange) are either in the brain (midbrain, pons, medulla) or in the sacral spinal cord (S2--S4). Their axons project to ganglia very near (or even inside) the end organs. Postganglionic neurons (green) therefore have short projections to their targets. **N14-2** **Tracing of Nerve Tracts Using Pseudorabies Virus** The CNS neuroanatomy of autonomic control has been difficult to define experimentally. However, a technique developed by Arthur Loewy and his colleagues that traces nerve tracts with the pseudorabies virus has helped to define more clearly the central pathways for autonomic control. For example, if axons of preganglionic sympathetic neurons are exposed to pseudorabies virus, the virus is transported back into the cell bodies, where it replicates. After a delay of several days, neurons that make synapses with these preganglionic neurons (i.e., "premotor" neurons) become infected and the virus is transported to their cell bodies. After longer periods of incubation, neurons farther upstream are also infected. Histological staining can then be used at different time points to visualize neurons that contain the virus at each level upstream.