Parasympathetic Nervous System PDF

[3. Parasympathetic preganglionic neurons originate from the brainstem and sacral spinal cord and synapse with postganglionic neurons in ganglia located near target organs] The cell bodies of preganglionic parasympathetic neurons are located in the medulla, pons, and midbrain and in the S2 through...

[3. Parasympathetic preganglionic neurons originate from the brainstem and sacral spinal cord and synapse with postganglionic neurons in ganglia located near target organs] The cell bodies of preganglionic parasympathetic neurons are located in the medulla, pons, and midbrain and in the S2 through S4 levels of the spinal cord (see [[Fig. 14-4]](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0025) , right panel). Thus, unlike the sympathetic---or **thoracolumbar** ---division, whose preganglionic cell bodies are in the thoracic and lumbar spinal cord, the parasympathetic---or **craniosacral** ---division\'s preganglionic cell bodies are cranial and sacral. The preganglionic parasympathetic fibers originating in the brain distribute with four cranial nerves: the oculomotor nerve (CN III), the facial nerve (CN VII), the glossopharyngeal nerve (CN IX), and the vagus nerve (CN X). The preganglionic parasympathetic fibers originating in S2 through S4 distribute with the pelvic splanchnic nerves. Postganglionic parasympathetic neurons are located in **terminal ganglia** that are more peripherally located and more widely distributed than are the sympathetic ganglia. Terminal ganglia often lie within the walls of their target organs. Thus, in contrast to the sympathetic division, postganglionic fibers of the parasympathetic division are short. In some cases, individual postganglionic parasympathetic neurons are found in isolation or in scattered cell groups rather than in encapsulated ganglia. **Cranial Nerves III, VII, and IX** ----------------------------------- The preganglionic parasympathetic neurons that are distributed with CN III, CN VII, and CN IX originate in three groups of nuclei. 1. The **Edinger-Westphal nucleus **is a subnucleus of the oculomotor complex in the midbrain ( [[Fig. 14-5 ]](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0030)). Parasympathetic neurons in this nucleus travel in the **oculomotor nerve **(CN III) and synapse onto postganglionic neurons in the ciliary ganglion (see [[Fig. 14-4 ]](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0025), right panel). The postganglionic fibers project to two smooth muscles of the eye: the constrictor muscle of the pupil and the ciliary muscle, which controls the shape of the lens. Figure 14.5 Supraspinal nuclei containing neurons that are part of the ANS. These nuclei contain the cell bodies of the preganglionic parasympathetic neurons (i.e., efferent). The Edinger-Westphal nucleus contains cell bodies of preganglionic fibers that travel with CN III to the ciliary ganglion. The superior salivatory nucleus contains cell bodies of preganglionic fibers that travel with CN VII to the pterygopalatine and submandibular ganglia. The inferior salivatory nucleus contains cell bodies of preganglionic fibers that travel with CN IX to the otic ganglion. The rostral portion of the nucleus ambiguus contains preganglionic cell bodies that distribute with CN IX; the rest of the nucleus ambiguus and the dorsal motor nucleus of the vagus contain cell bodies of preganglionic fibers that travel with CN X to a host of terminal ganglia in the viscera of the thorax and abdomen. The NTS, which is not part of the ANS, receives visceral afferents and is part of the larger visceral control system. The figure also illustrates other cranial nerves that are not involved in controlling the ANS (gray labels). 2. The ***superior *salivatory nucleus **is in the rostral medulla (see [Fig. 14-5 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0030)) and contains parasympathetic neurons that project, through a branch of the **facial nerve **(CN VII), to the pterygopalatine ganglion (see [Fig. 14-4 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0025), right panel). The postganglionic fibers supply the lacrimal glands, which produce tears. Another branch of the facial nerve carries preganglionic fibers to the submandibular ganglion. The postganglionic fibers supply two salivary glands, the submandibular and sublingual glands. 3. The ***inferior *salivatory nucleus **and the rostral part of the **nucleus ambiguus **in the rostral medulla (see [Fig. 14-5 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0030)) contain parasympathetic neurons that project through the **glossopharyngeal nerve **(CN IX) to the otic ganglion (see [Fig. 14-4 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0025), right panel). The postganglionic fibers supply a third salivary gland, the parotid gland. **Cranial Nerve X** ------------------- Most parasympathetic output occurs through the **vagus nerve** (CN X). Cell bodies of vagal preganglionic parasympathetic neurons are found in the medulla within the **nucleus ambiguus** and the **dorsal motor nucleus of the vagus** (see [Fig. 14-5](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0030) ). This nerve supplies parasympathetic innervation to all the viscera of the thorax and abdomen, including the GI tract between the pharynx and distal end of the colon (see [Fig. 14-4](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0025) , right panel). Among other effects, electrical stimulation of the nucleus ambiguus results in contraction of striated muscle in the pharynx, larynx, and upper esophagus due to activation of somatic motor neurons (not autonomic), as well as slowing of the heart due to activation of vagal preganglionic parasympathetic neurons. Stimulation of the dorsal motor nucleus of the vagus induces many effects in the viscera, including initiation of secretion of gastric acid, insulin, and glucagon. Preganglionic parasympathetic fibers of the vagus nerve join the esophageal, pulmonary, and cardiac plexuses and travel to terminal ganglia that are located within their target organs. **\ ** **Sacral Nerves** ----------------- The cell bodies of preganglionic parasympathetic neurons in the sacral spinal cord (S2 to S4) are located in a position similar to that of the preganglionic *sympathetic* neurons---although they do not form a distinct intermediolateral column. Their axons leave through ventral roots and travel with the pelvic splanchnic nerves to their terminal ganglia in the descending colon and rectum (see [p. 862 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000410?scrollTo=%23p0400)), as well as to the bladder (see [pp. 736--737 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000331?scrollTo=%23s0150)) and the reproductive organs of the male (see [p. 1104 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000549?scrollTo=%23p0580)) and female (see [p. 1127 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000550?scrollTo=%23ulist0040)). **The visceral control system also has an important afferent limb** ------------------------------------------------------------------- All internal organs are densely innervated by visceral afferents. Some of these receptors monitor nociceptive (painful) input. Others are sensitive to a variety of mechanical and chemical (physiological) stimuli, including stretch of the heart, blood vessels, and hollow viscera, as well as  PCO2, PO2 , pH, blood glucose, and temperature of the skin and internal organs. Many visceral nociceptive fibers travel in sympathetic nerves (blue projections in [Fig. 14-2](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0015) ). Most axons from physiological receptors travel with parasympathetic fibers. As is the case with somatic afferents (see [p. 271 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000100?scrollTo=%23p0560)), the cell bodies of visceral afferent fibers are located within the dorsal root ganglia or cranial nerve ganglia (e.g., nodose and petrosal ganglia). Ninety percent of these visceral afferents are unmyelinated. The largest concentration of visceral afferent axons can be found in the **vagus nerve,** which carries non-nociceptive afferent input to the CNS from all viscera of the thorax and abdomen. Most fibers in the vagus nerve are *afferents,* even though all parasympathetic preganglionic output (i.e., *efferents* ) to the abdominal and thoracic viscera also travels in the vagus nerve. Vagal afferents, whose cell bodies are located in the nodose ganglion, carry information about the distention of hollow organs (e.g., blood vessels, cardiac chambers, stomach, bronchioles), blood gases (e.g., PCO2 ,  PO2, pH from the aortic bodies), and body chemistry (e.g., glucose concentration) to the medulla. Internal organs also have nociceptive receptors that are sensitive to excessive stretch, noxious chemical irritants, and very large decreases in pH. In the CNS, this visceral pain input is mapped (see [pp. 400--401 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000161?scrollTo=%23p0250)) *viscerotopically* at the level of the spinal cord because most visceral nociceptive fibers travel with the sympathetic fibers and enter the spinal cord at a specific segmental level along with a spinal nerve (see [Fig. 14-2](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0015) ). This viscerotopic mapping is also present in the brainstem but not at the level of the cerebral cortex. Thus, awareness of visceral pain is not usually localized to a specific organ but is instead "referred" to the dermatome (see [p. 273 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000100?scrollTo=%23p0620)) that is innervated by the same spinal nerve. This **referred pain** results from lack of precision in the central organization of visceral pain pathways. Thus, you know that the pain is associated with a particular spinal nerve, but you do not know where the pain is coming from (i.e., from the skin or a visceral organ). For example, nociceptive input from the left ventricle of the heart is referred to the left T1 to T5 dermatomes and leads to discomfort in the left arm and left side of the chest, whereas nociceptive input from the diaphragm is referred to the C3 to C5 dermatomes and is interpreted as pain in the shoulder. This visceral pain is often felt as a vague burning or pressure sensation. **The enteric division is a self-contained nervous system of the GI tract and receives sympathetic and parasympathetic input** ------------------------------------------------------------------------------------------------------------------------------ The enteric nervous system (ENS) is a collection of nerve plexuses that surround the GI tract, including the pancreas and biliary system. Although it is entirely peripheral, the ENS receives input from the sympathetic and parasympathetic divisions of the ANS. The ENS is estimated to contain \>100 million neurons, including afferent neurons, interneurons, and efferent postganglionic parasympathetic neurons. Enteric neurons contain many different neurotransmitters and neuromodulators. Thus, not only does the total number of neurons in the enteric division exceed that of the spinal cord, but the neurochemical complexity of the ENS also approaches that of the CNS. The anatomy of the ENS as well as its role in controlling GI function is discussed in [Chapter 41 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000410?scrollTo=%23c00041). The plexuses of the ENS are a system of ganglia sandwiched between the layers of the gut and connected by a dense meshwork of nerve fibers. The **myenteric** or Auerbach\'s plexus ( [Fig. 14-6](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/f0035) ) lies between the outer longitudinal and the inner circular layers of smooth muscle, whereas the **submucosal** or Meissner\'s plexus lies between the inner circular layer of smooth muscle and the most internal layer of smooth muscle, the muscularis mucosae (see [Fig. 41-3 ](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/#!/content/3-s2.0-B9781455743773000410?scrollTo=%23f0020)). In the intestinal wall, the myenteric plexus is involved primarily in the control of motility, whereas the submucosal plexus is involved in the control of ion and fluid transport. Both the myenteric and the submucosal plexuses receive *pre* ganglionic *parasympathetic* innervation from the vagus nerve (or sacral nerves in the case of the distal portion of colon and rectum). Thus, in one sense, the enteric division is homologous to a large and complex parasympathetic terminal ganglion. The other major input to the ENS is from *post* ganglionic *sympathetic* neurons. Thus, the ENS can be thought of as "postganglionic" or as a "terminal organ" with respect to the parasympathetic division and "post-postganglionic" with respect to the sympathetic division. Input from both the sympathetic and parasympathetic divisions modulates the activity of the ENS, but the ENS can by and large function normally without extrinsic input. The isolated ENS can respond appropriately to local stimuli and control most aspects of gut function, including initiating peristaltic activity in response to gastric distention, controlling secretory and absorptive functions, and triggering biliary contractions ( [Box 14-1](https://www-clinicalkey-com.utrechtuniversity.idm.oclc.org/b0020) ). Figure 14-6 The myenteric (Auerbach\'s) plexus. The image is a scanning electron micrograph of the myenteric plexus of the mouse large intestine. The external longitudinal muscle of the intestine was removed so that the view is of the plexus (the highly interconnected meshwork of neuron cell bodies, axons, and dendrites on the surface) spreading over the deeper circular layer of muscle