{"id":254,"date":"2016-08-19T15:35:13","date_gmt":"2016-08-19T15:35:13","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/?post_type=chapter&#038;p=254"},"modified":"2016-08-19T15:35:14","modified_gmt":"2016-08-19T15:35:14","slug":"the-peripheral-nervous-system","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/chapter\/the-peripheral-nervous-system\/","title":{"raw":"The Peripheral Nervous System","rendered":"The Peripheral Nervous System"},"content":{"raw":"<div title=\"13.4.&#xA0;The Peripheral Nervous System\" xml:lang=\"en\">\n<div>\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<div>\n<div>\n<ul><li>Describe the structures found in the PNS<\/li>\n\t<li>Distinguish between somatic and autonomic structures, including the special peripheral structures of the enteric nervous system<\/li>\n\t<li>Name the twelve cranial nerves and explain the functions associated with each<\/li>\n\t<li>Describe the sensory and motor components of spinal nerves and the plexuses that they pass through<\/li>\n<\/ul><\/div>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<ul><li><a href=\"#m46553-fs-id2582754\">Ganglia<\/a><\/li>\n\t<li><a href=\"#m46553-fs-id1942417\">Nerves<\/a>\n<ul><li><a href=\"#m46553-fs-id1335222\">Cranial Nerves<\/a><\/li>\n\t<li><a href=\"#m46553-fs-id1502532\">Spinal Nerves<\/a><\/li>\n<\/ul><\/li>\n<\/ul><\/div>\nThe PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the\u00a0<strong><em>enteric nervous system<\/em><\/strong><a id=\"id745728\"\/>\u00a0and are a special subset of the PNS.\n<div title=\"Ganglia\">\n<div>\n<h3 id=\"m46553-fs-id2582754\">Ganglia<\/h3>\n<\/div>\nA ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a\u00a0<strong><em>dorsal (posterior) root ganglion<\/em><\/strong><a id=\"id745757\"\/>. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root (<a title=\"Figure&#xA0;13.19.&#xA0;Dorsal Root Ganglion\" href=\"#m46553-fig-ch13_04_01\">Figure\u00a013.19<\/a>). The cells of the dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding\u2014as if they were orbiting\u2014the neuron cell bodies.\n<div id=\"m46553-fig-ch13_04_01\" title=\"Figure&#xA0;13.19.&#xA0;Dorsal Root Ganglion\">\n<div>\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152012\/1318b_Dorsal_Root_Ganglion.jpg\" alt=\"This micrograph shows the structure of the dorsal root ganglion. The cell bodies of the neurons and the axon bundles are also labeled.\" width=\"580\"\/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.19.\u00a0Dorsal Root Ganglion<\/strong><\/address><address>The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address><address>\u00a0<\/address><address>\u00a0<\/address><\/div>\n<div id=\"m46553-fig-ch13_04_02\" title=\"Figure&#xA0;13.20.&#xA0;Spinal Cord and Root Ganglion\">\n<div>\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152016\/1318b_DRG.jpg\" alt=\"This micrograph shows a magnified view of the dorsal root ganglion, showing the satellite cells and the cell bodies of sensory neurons.\" width=\"580\"\/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.20.\u00a0Spinal Cord and Root Ganglion<\/strong><\/address><address>The slide includes both a cross-section of the lumbar spinal cord and a section of the dorsal root ganglion (see also\u00a0<a title=\"Figure&#xA0;13.19.&#xA0;Dorsal Root Ganglion\" href=\"#m46553-fig-ch13_04_01\">Figure\u00a013.19<\/a>) (tissue source: canine). LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address><\/div>\n<div id=\"m46553-fs-id2127824\">\n<div\/>\n<div>\n<div class=\"bcc-box bcc-info\">\n<h3>Interactive Link<\/h3>\nView the University of Michigan WebScope at\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/spinalroot\" target=\"_blank\">http:\/\/virtualslides.med.umich.edu\/Histology\/Basic%20Tissues\/Nervous%20Tissue\/065-2_HISTO_40X.svs\/view.apml<\/a>\u00a0to explore the tissue sample in greater detail. If you zoom in on the dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?\n\n<\/div>\nAnother type of sensory ganglion is a\u00a0<strong><em>cranial nerve ganglion<\/em><\/strong><a id=\"id745929\"\/>. This is analogous to the dorsal root ganglion, except that it is associated with a\u00a0<strong><em>cranial nerve<\/em><\/strong><a id=\"id745943\"\/>\u00a0instead of a\u00a0<strong><em>spinal nerve<\/em><\/strong><a id=\"id745956\"\/>. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the\u00a0<strong><em>trigeminal ganglion<\/em><\/strong><a id=\"id745970\"\/>\u00a0is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells. The other major category of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The\u00a0<strong><em>sympathetic chain ganglia<\/em><\/strong><a id=\"id745994\"\/>\u00a0constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. Superior to the chain ganglia are three\u00a0<strong><em>paravertebral ganglia<\/em><\/strong><a id=\"id746009\"\/>\u00a0in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the\u00a0<strong><em>prevertebral ganglia<\/em><\/strong><a id=\"id746023\"\/>, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms. Another group of autonomic ganglia are the\u00a0<strong><em>terminal ganglia<\/em><\/strong><a id=\"id746048\"\/>\u00a0that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms. These two sets of ganglia, sympathetic and parasympathetic, often project to the same organs\u2014one input from the chain ganglia and one input from a terminal ganglion\u2014to regulate the overall function of an organ. For example, the heart receives two inputs such as these; one increases heart rate, and the other decreases it. The terminal ganglia that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities. Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a\u00a0<strong><em>plexus<\/em><\/strong><a id=\"id746084\"\/>. A plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or structures containing blood vessels (such as a choroid plexus). For example, the\u00a0<strong><em>enteric plexus<\/em><\/strong><a id=\"id746099\"\/>\u00a0is the extensive network of axons and neurons in the wall of the small and large intestines. The enteric plexus is actually part of the enteric nervous system, along with the\u00a0<strong><em>gastric plexuses<\/em><\/strong><a id=\"id746113\"\/>\u00a0and the\u00a0<strong><em>esophageal plexus<\/em><\/strong><a id=\"id746127\"\/>. Though the enteric nervous system receives input originating from central neurons of the autonomic nervous system, it does not require CNS input to function. In fact, it operates independently to regulate the digestive system.\n\n<\/div>\n<\/div>\n<\/div>\n<div title=\"Nerves\">\n<div>\n<h3 id=\"m46553-fs-id1942417\">Nerves<\/h3>\n<\/div>\nBundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the\u00a0<strong><em>epineurium<\/em><\/strong><a id=\"id746160\"\/>. Within the nerve, axons are further bundled into\u00a0<strong><em>fascicles<\/em><\/strong><a id=\"id746173\"\/>, which are each surrounded by their own layer of fibrous connective tissue called\u00a0<strong><em>perineurium<\/em><\/strong><a id=\"id746187\"\/>. Finally, individual axons are surrounded by loose connective tissue called the\u00a0<strong><em>endoneurium<\/em><\/strong><a id=\"id746201\"\/>\u00a0(<a title=\"Figure&#xA0;13.21.&#xA0;Nerve Structure\" href=\"#m46553-fig-ch13_04_03\">Figure\u00a013.21<\/a>). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.\n<div id=\"m46553-fig-ch13_04_03\" title=\"Figure&#xA0;13.21.&#xA0;Nerve Structure\">\n<div>\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152020\/1319_Nerve_StructureN.jpg\" alt=\"This figure shows the structure of a nerve. The top panel shows the cross section of a spinal nerve and the major parts are labeled. The bottom panel shows a micrograph of the cross-section of a spinal nerve.\" width=\"480\"\/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.21.\u00a0Nerve Structure<\/strong><\/address><address>The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian). LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address><address>\u00a0<\/address><address>\u00a0<\/address><address>\u00a0<\/address><\/div>\n<div id=\"m46553-fig-ch13_04_04\" title=\"Figure&#xA0;13.22.&#xA0;Close-Up of Nerve Trunk\">\n<div>\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152024\/1319B_Nerve_Mag.jpg\" alt=\"This micrograph shows a magnified view of the nerve. The perineurium and the endoneurium are labeled.\" width=\"580\"\/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.22.\u00a0Close-Up of Nerve Trunk<\/strong><\/address><address>Zoom in on this slide of a nerve trunk to examine the endoneurium, perineurium, and epineurium in greater detail (tissue source: simian). LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address><\/div>\n<div id=\"m46553-fs-id534298\">\n<div\/>\n<div\/>\n<div>\n<div class=\"bcc-box bcc-info\">\n<h3>Interactive Link<\/h3>\nView the University of Michigan WebScope at\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/nervetrunk\" target=\"_blank\">http:\/\/virtualslides.med.umich.edu\/Histology\/Basic%20Tissues\/Nervous%20Tissue\/068_HISTO_40X.svs\/view.apml<\/a>\u00a0to explore the tissue sample in greater detail. With what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?\n\n<\/div>\n<\/div>\n<\/div>\n<div title=\"Cranial Nerves\">\n<div>\n<h4 id=\"m46553-fs-id1335222\">Cranial Nerves<\/h4>\n<\/div>\nThe nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for \u201cCranial Nerve,\u201d using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves. Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, \u201cOn Old Olympus\u2019 Towering Tops\/A Finn And German Viewed Some Hops,\u201d in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in\u00a0<a title=\"Table&#xA0;13.3.&#xA0;\" href=\"#m46553-tbl-ch13_03\">Table\u00a013.3<\/a>\u00a0along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). They are listed here with a brief explanation of each nerve (<a title=\"Figure&#xA0;13.23.&#xA0;The Cranial Nerves\" href=\"#m46553-fig-ch13_04_05\">Figure\u00a013.23<\/a>). The\u00a0<strong><em>olfactory nerve<\/em><\/strong><a id=\"id746429\"\/>\u00a0and<strong>\u00a0<em>optic nerve<\/em><\/strong><a id=\"id746443\"\/>\u00a0are responsible for the sense of smell and vision, respectively. The\u00a0<strong><em>oculomotor nerve<\/em><\/strong><a id=\"id746457\"\/>\u00a0is responsible for eye movements by controlling four of the\u00a0<strong><em>extraocular muscles<\/em><\/strong><a id=\"id746471\"\/>. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The<strong>\u00a0<em>trochlear nerve<\/em><\/strong><a id=\"id746485\"\/>\u00a0and the\u00a0<strong><em>abducens nerve<\/em><\/strong><a id=\"id746498\"\/>\u00a0are both responsible for eye movement, but do so by controlling different extraocular muscles. The\u00a0<strong><em>trigeminal nerve<\/em><\/strong><a id=\"id746513\"\/>\u00a0is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The\u00a0<strong><em>facial nerve<\/em><\/strong><a id=\"id746527\"\/>\u00a0is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The\u00a0<strong><em>vestibulocochlear nerve<\/em><\/strong><a id=\"id746542\"\/>\u00a0is responsible for the senses of hearing and balance. The\u00a0<strong><em>glossopharyngeal nerve<\/em><\/strong><a id=\"id746555\"\/>\u00a0is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The\u00a0<strong><em>vagus nerve<\/em><\/strong><a id=\"id746570\"\/>\u00a0is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The\u00a0<strong><em>spinal accessory nerve<\/em><\/strong><a id=\"id746585\"\/>\u00a0is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The\u00a0<strong><em>hypoglossal nerve<\/em><\/strong><a id=\"id746599\"\/>\u00a0is responsible for controlling the muscles of the lower throat and tongue.\n<div id=\"m46553-fig-ch13_04_05\" title=\"Figure&#xA0;13.23.&#xA0;The Cranial Nerves\">\n<div>\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152027\/1320_The_Cranial_Nerves.jpg\" alt=\"This diagrams shows the brain and the main nerves in the brain are labeled.\" width=\"480\"\/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.23.\u00a0The Cranial Nerves<\/strong><\/address><address>The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.<\/address><address>\u00a0<\/address><address>\u00a0<\/address><\/div>\nThree of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities.\n<div id=\"m46553-fs-id1492164\">\n<div class=\"bcc-box bcc-info\">\n<h3>Interactive Link<\/h3>\nVisit this\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/NYTmeningitis\" target=\"_blank\">site<\/a>\u00a0to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?\n\n<\/div>\n<\/div>\nAnother important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see\u00a0<a title=\"Table&#xA0;13.3.&#xA0;\" href=\"#m46553-tbl-ch13_03\">Table\u00a013.3<\/a>). The sentence, \u201cSome Say Marry Money But My Brother Says Brains Beauty Matter More,\u201d corresponds to the basic function of each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The three eye-movement nerves are all motor: the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVII), glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other concerns the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.\n\n<a href=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/11\/20090404\/Screen-Shot-2014-11-13-at-2.54.45-PM.png\"><img class=\"alignnone wp-image-2499 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152030\/Screen-Shot-2014-11-13-at-2.54.45-PM.png\" alt=\"Screen Shot 2014-11-13 at 2.54.45 PM\" width=\"578\" height=\"682\"\/><\/a>\n<div id=\"m46553-tbl-ch13_03\"\/>\n<\/div>\n<div title=\"Spinal Nerves\">\n<div>\n<h4 id=\"m46553-fs-id1502532\">Spinal Nerves<\/h4>\n<\/div>\nThe nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve. There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to L5, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra. Spinal nerves extend outward from the vertebral column to enervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together into a\u00a0<strong><em>systemic nerve<\/em><\/strong><a id=\"id747186\"\/>. This occurs at four places along the length of the vertebral column, each identified as a\u00a0<strong><em>nerve plexus<\/em><\/strong><a id=\"id747200\"\/>, whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies. Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level (<a title=\"Figure&#xA0;13.24.&#xA0;Nerve Plexuses of the Body\" href=\"#m46553-fig-ch13_04_06\">Figure\u00a013.24<\/a>).\n\nThe\u00a0<strong><em>cervical plexus<\/em><\/strong><a id=\"id747233\"\/>\u00a0is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the\u00a0<strong><em>phrenic nerve<\/em><\/strong><a id=\"id747247\"\/>, which connects to the diaphragm at the base of the thoracic cavity. The other plexus from the cervical level is the\u00a0<strong><em>brachial plexus<\/em><\/strong><a id=\"id747261\"\/>. Spinal nerves C4 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. A large nerve from this plexus is the<strong>\u00a0<em>radial nerve<\/em><\/strong><a id=\"id747276\"\/>\u00a0from which the\u00a0<strong><em>axillary nerve<\/em><\/strong><a id=\"id747289\"\/>\u00a0branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the\u00a0<strong><em>ulnar nerve<\/em><\/strong><a id=\"id747303\"\/>\u00a0and the\u00a0<strong><em>median nerve<\/em><\/strong><a id=\"id747316\"\/>. The\u00a0<strong><em>lumbar plexus<\/em><\/strong><a id=\"id747329\"\/>\u00a0arises from all the lumbar spinal nerves and gives rise to nerves enervating the pelvic region and the anterior leg. The\u00a0<strong><em>femoral nerve<\/em><\/strong><a id=\"id747344\"\/>\u00a0is one of the major nerves from this plexus, which gives rise to the\u00a0<strong><em>saphenous nerve<\/em><\/strong><a id=\"id747358\"\/>\u00a0as a branch that extends through the anterior lower leg. The\u00a0<strong><em>sacral plexus<\/em><\/strong><a id=\"id747372\"\/>\u00a0comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the\u00a0<strong><em>sciatic nerve<\/em><\/strong><a id=\"id747387\"\/>, which is a combination of the\u00a0<strong><em>tibial nerve<\/em><\/strong><a id=\"id747401\"\/>\u00a0and the\u00a0<strong><em>fibular nerve<\/em><\/strong><a id=\"id747414\"\/>. The sciatic nerve extends across the hip joint and is most commonly associated with the condition\u00a0<strong><em>sciatica<\/em><\/strong><a id=\"id747428\"\/>, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it. These plexuses are described as arising from spinal nerves and giving rise to certain systemic nerves, but they contain fibers that serve sensory functions or fibers that serve motor functions. This means that some fibers extend from cutaneous or other peripheral sensory surfaces and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. Other fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions. For example, the radial nerve contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm. Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the<strong>\u00a0<em>intercostal nerves<\/em><\/strong><a id=\"id747464\"\/>\u00a0found between the ribs, which articulate with the vertebrae surrounding the spinal nerve.\n<div id=\"m46553-fig-ch13_04_06\" title=\"Figure&#xA0;13.24.&#xA0;Nerve Plexuses of the Body\">\n<div>\n<div><img src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152033\/1321_Spinal_Nerve_Plexuses.jpg\" alt=\"This figure shows a torso of a human body. The spinal cord is shown in the body and the main nerves along the spinal cord are labeled.\" width=\"440\"\/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.24.\u00a0Nerve Plexuses of the Body<\/strong><\/address><address>There are four main nerve plexuses in the human body. The cervical plexus supplies nerves to the posterior head and neck, as well as to the diaphragm. The brachial plexus supplies nerves to the arm. The lumbar plexus supplies nerves to the anterior leg. The sacral plexus supplies nerves to the posterior leg.<\/address><address>\u00a0<\/address><\/div>\n<div id=\"m46553-fs-id886679\">\n<div>\n<div class=\"bcc-box bcc-success\">\n<h3>Aging and the Nervous System<\/h3>\n<div>\n<p title=\"Nervous System\">Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them. Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity. Anosmia results in a loss of the enjoyment of food. As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure.<\/p>\n\n<\/div>\n<\/div>\n\u00a0\n\n<\/div>\n<div\/>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"id748220\" title=\"Glossary\"\/>","rendered":"<div title=\"13.4.&#xa0;The Peripheral Nervous System\" xml:lang=\"en\">\n<div>\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<div>\n<div>\n<ul>\n<li>Describe the structures found in the PNS<\/li>\n<li>Distinguish between somatic and autonomic structures, including the special peripheral structures of the enteric nervous system<\/li>\n<li>Name the twelve cranial nerves and explain the functions associated with each<\/li>\n<li>Describe the sensory and motor components of spinal nerves and the plexuses that they pass through<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div>\n<ul>\n<li><a href=\"#m46553-fs-id2582754\">Ganglia<\/a><\/li>\n<li><a href=\"#m46553-fs-id1942417\">Nerves<\/a>\n<ul>\n<li><a href=\"#m46553-fs-id1335222\">Cranial Nerves<\/a><\/li>\n<li><a href=\"#m46553-fs-id1502532\">Spinal Nerves<\/a><\/li>\n<\/ul>\n<\/li>\n<\/ul>\n<\/div>\n<p>The PNS is not as contained as the CNS because it is defined as everything that is not the CNS. Some peripheral structures are incorporated into the other organs of the body. In describing the anatomy of the PNS, it is necessary to describe the common structures, the nerves and the ganglia, as they are found in various parts of the body. Many of the neural structures that are incorporated into other organs are features of the digestive system; these structures are known as the\u00a0<strong><em>enteric nervous system<\/em><\/strong><a id=\"id745728\">\u00a0and are a special subset of the PNS.<\/p>\n<div title=\"Ganglia\">\n<div>\n<h3 id=\"m46553-fs-id2582754\">Ganglia<\/h3>\n<\/div>\n<p>A ganglion is a group of neuron cell bodies in the periphery. Ganglia can be categorized, for the most part, as either sensory ganglia or autonomic ganglia, referring to their primary functions. The most common type of sensory ganglion is a\u00a0<strong><em>dorsal (posterior) root ganglion<\/em><\/strong><\/div>\n<p><\/a><a id=\"id745757\">. These ganglia are the cell bodies of neurons with axons that are sensory endings in the periphery, such as in the skin, and that extend into the CNS through the dorsal nerve root. The ganglion is an enlargement of the nerve root. Under microscopic inspection, it can be seen to include the cell bodies of the neurons, as well as bundles of fibers that are the posterior nerve root (<\/a><a title=\"Figure&#xa0;13.19.&#xa0;Dorsal Root Ganglion\" href=\"#m46553-fig-ch13_04_01\">Figure\u00a013.19<\/a>). The cells of the dorsal root ganglion are unipolar cells, classifying them by shape. Also, the small round nuclei of satellite cells can be seen surrounding\u2014as if they were orbiting\u2014the neuron cell bodies.<\/p>\n<div id=\"m46553-fig-ch13_04_01\" title=\"Figure&#xa0;13.19.&#xa0;Dorsal Root Ganglion\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152012\/1318b_Dorsal_Root_Ganglion.jpg\" alt=\"This micrograph shows the structure of the dorsal root ganglion. The cell bodies of the neurons and the axon bundles are also labeled.\" width=\"580\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.19.\u00a0Dorsal Root Ganglion<\/strong><\/address>\n<address>The cell bodies of sensory neurons, which are unipolar neurons by shape, are seen in this photomicrograph. Also, the fibrous region is composed of the axons of these neurons that are passing through the ganglion to be part of the dorsal nerve root (tissue source: canine). LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address>\n<address>\u00a0<\/address>\n<address>\u00a0<\/address>\n<\/div>\n<div id=\"m46553-fig-ch13_04_02\" title=\"Figure&#xa0;13.20.&#xa0;Spinal Cord and Root Ganglion\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152016\/1318b_DRG.jpg\" alt=\"This micrograph shows a magnified view of the dorsal root ganglion, showing the satellite cells and the cell bodies of sensory neurons.\" width=\"580\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.20.\u00a0Spinal Cord and Root Ganglion<\/strong><\/address>\n<address>The slide includes both a cross-section of the lumbar spinal cord and a section of the dorsal root ganglion (see also\u00a0<a title=\"Figure&#xa0;13.19.&#xa0;Dorsal Root Ganglion\" href=\"#m46553-fig-ch13_04_01\">Figure\u00a013.19<\/a>) (tissue source: canine). LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address>\n<\/div>\n<div id=\"m46553-fs-id2127824\">\n<div>\n<div>\n<div class=\"bcc-box bcc-info\">\n<h3>Interactive Link<\/h3>\n<p>View the University of Michigan WebScope at\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/spinalroot\" target=\"_blank\">http:\/\/virtualslides.med.umich.edu\/Histology\/Basic%20Tissues\/Nervous%20Tissue\/065-2_HISTO_40X.svs\/view.apml<\/a>\u00a0to explore the tissue sample in greater detail. If you zoom in on the dorsal root ganglion, you can see smaller satellite glial cells surrounding the large cell bodies of the sensory neurons. From what structure do satellite cells derive during embryologic development?<\/p>\n<\/div>\n<p>Another type of sensory ganglion is a\u00a0<strong><em>cranial nerve ganglion<\/em><\/strong><a id=\"id745929\">. This is analogous to the dorsal root ganglion, except that it is associated with a\u00a0<strong><em>cranial nerve<\/em><\/strong><\/a><a id=\"id745943\">\u00a0instead of a\u00a0<strong><em>spinal nerve<\/em><\/strong><\/a><a id=\"id745956\">. The roots of cranial nerves are within the cranium, whereas the ganglia are outside the skull. For example, the\u00a0<strong><em>trigeminal ganglion<\/em><\/strong><\/a><a id=\"id745970\">\u00a0is superficial to the temporal bone whereas its associated nerve is attached to the mid-pons region of the brain stem. The neurons of cranial nerve ganglia are also unipolar in shape with associated satellite cells. The other major category of ganglia are those of the autonomic nervous system, which is divided into the sympathetic and parasympathetic nervous systems. The\u00a0<strong><em>sympathetic chain ganglia<\/em><\/strong><\/a><a id=\"id745994\">\u00a0constitute a row of ganglia along the vertebral column that receive central input from the lateral horn of the thoracic and upper lumbar spinal cord. Superior to the chain ganglia are three\u00a0<strong><em>paravertebral ganglia<\/em><\/strong><\/a><a id=\"id746009\">\u00a0in the cervical region. Three other autonomic ganglia that are related to the sympathetic chain are the\u00a0<strong><em>prevertebral ganglia<\/em><\/strong><\/a><a id=\"id746023\">, which are located outside of the chain but have similar functions. They are referred to as prevertebral because they are anterior to the vertebral column. The neurons of these autonomic ganglia are multipolar in shape, with dendrites radiating out around the cell body where synapses from the spinal cord neurons are made. The neurons of the chain, paravertebral, and prevertebral ganglia then project to organs in the head and neck, thoracic, abdominal, and pelvic cavities to regulate the sympathetic aspect of homeostatic mechanisms. Another group of autonomic ganglia are the\u00a0<strong><em>terminal ganglia<\/em><\/strong><\/a><a id=\"id746048\">\u00a0that receive input from cranial nerves or sacral spinal nerves and are responsible for regulating the parasympathetic aspect of homeostatic mechanisms. These two sets of ganglia, sympathetic and parasympathetic, often project to the same organs\u2014one input from the chain ganglia and one input from a terminal ganglion\u2014to regulate the overall function of an organ. For example, the heart receives two inputs such as these; one increases heart rate, and the other decreases it. The terminal ganglia that receive input from cranial nerves are found in the head and neck, as well as the thoracic and upper abdominal cavities, whereas the terminal ganglia that receive sacral input are in the lower abdominal and pelvic cavities. Terminal ganglia below the head and neck are often incorporated into the wall of the target organ as a\u00a0<strong><em>plexus<\/em><\/strong><\/a><a id=\"id746084\">. A plexus, in a general sense, is a network of fibers or vessels. This can apply to nervous tissue (as in this instance) or structures containing blood vessels (such as a choroid plexus). For example, the\u00a0<strong><em>enteric plexus<\/em><\/strong><\/a><a id=\"id746099\">\u00a0is the extensive network of axons and neurons in the wall of the small and large intestines. The enteric plexus is actually part of the enteric nervous system, along with the\u00a0<strong><em>gastric plexuses<\/em><\/strong><\/a><a id=\"id746113\">\u00a0and the\u00a0<strong><em>esophageal plexus<\/em><\/strong><\/a><a id=\"id746127\">. Though the enteric nervous system receives input originating from central neurons of the autonomic nervous system, it does not require CNS input to function. In fact, it operates independently to regulate the digestive system.<\/p>\n<p><\/a><\/div>\n<\/div>\n<\/div>\n<div title=\"Nerves\">\n<div>\n<h3 id=\"m46553-fs-id1942417\">Nerves<\/h3>\n<\/div>\n<p>Bundles of axons in the PNS are referred to as nerves. These structures in the periphery are different than the central counterpart, called a tract. Nerves are composed of more than just nervous tissue. They have connective tissues invested in their structure, as well as blood vessels supplying the tissues with nourishment. The outer surface of a nerve is a surrounding layer of fibrous connective tissue called the\u00a0<strong><em>epineurium<\/em><\/strong><a id=\"id746160\">. Within the nerve, axons are further bundled into\u00a0<strong><em>fascicles<\/em><\/strong><\/a><a id=\"id746173\">, which are each surrounded by their own layer of fibrous connective tissue called\u00a0<strong><em>perineurium<\/em><\/strong><\/a><a id=\"id746187\">. Finally, individual axons are surrounded by loose connective tissue called the\u00a0<strong><em>endoneurium<\/em><\/strong><\/a><a id=\"id746201\">\u00a0(<\/a><a title=\"Figure&#xa0;13.21.&#xa0;Nerve Structure\" href=\"#m46553-fig-ch13_04_03\">Figure\u00a013.21<\/a>). These three layers are similar to the connective tissue sheaths for muscles. Nerves are associated with the region of the CNS to which they are connected, either as cranial nerves connected to the brain or spinal nerves connected to the spinal cord.<\/p>\n<div id=\"m46553-fig-ch13_04_03\" title=\"Figure&#xa0;13.21.&#xa0;Nerve Structure\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152020\/1319_Nerve_StructureN.jpg\" alt=\"This figure shows the structure of a nerve. The top panel shows the cross section of a spinal nerve and the major parts are labeled. The bottom panel shows a micrograph of the cross-section of a spinal nerve.\" width=\"480\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.21.\u00a0Nerve Structure<\/strong><\/address>\n<address>The structure of a nerve is organized by the layers of connective tissue on the outside, around each fascicle, and surrounding the individual nerve fibers (tissue source: simian). LM \u00d7 40. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address>\n<address>\u00a0<\/address>\n<address>\u00a0<\/address>\n<address>\u00a0<\/address>\n<\/div>\n<div id=\"m46553-fig-ch13_04_04\" title=\"Figure&#xa0;13.22.&#xa0;Close-Up of Nerve Trunk\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152024\/1319B_Nerve_Mag.jpg\" alt=\"This micrograph shows a magnified view of the nerve. The perineurium and the endoneurium are labeled.\" width=\"580\" \/><\/div>\n<\/div>\n<address><strong>Figure\u00a013.22.\u00a0Close-Up of Nerve Trunk<\/strong><\/address>\n<address>Zoom in on this slide of a nerve trunk to examine the endoneurium, perineurium, and epineurium in greater detail (tissue source: simian). LM \u00d7 1600. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/address>\n<\/div>\n<div id=\"m46553-fs-id534298\">\n<div>\n<div>\n<div>\n<div class=\"bcc-box bcc-info\">\n<h3>Interactive Link<\/h3>\n<p>View the University of Michigan WebScope at\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/nervetrunk\" target=\"_blank\">http:\/\/virtualslides.med.umich.edu\/Histology\/Basic%20Tissues\/Nervous%20Tissue\/068_HISTO_40X.svs\/view.apml<\/a>\u00a0to explore the tissue sample in greater detail. With what structures in a skeletal muscle are the endoneurium, perineurium, and epineurium comparable?<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div title=\"Cranial Nerves\">\n<div>\n<h4 id=\"m46553-fs-id1335222\">Cranial Nerves<\/h4>\n<\/div>\n<p>The nerves attached to the brain are the cranial nerves, which are primarily responsible for the sensory and motor functions of the head and neck (one of these nerves targets organs in the thoracic and abdominal cavities as part of the parasympathetic nervous system). There are twelve cranial nerves, which are designated CNI through CNXII for \u201cCranial Nerve,\u201d using Roman numerals for 1 through 12. They can be classified as sensory nerves, motor nerves, or a combination of both, meaning that the axons in these nerves originate out of sensory ganglia external to the cranium or motor nuclei within the brain stem. Sensory axons enter the brain to synapse in a nucleus. Motor axons connect to skeletal muscles of the head or neck. Three of the nerves are solely composed of sensory fibers; five are strictly motor; and the remaining four are mixed nerves. Learning the cranial nerves is a tradition in anatomy courses, and students have always used mnemonic devices to remember the nerve names. A traditional mnemonic is the rhyming couplet, \u201cOn Old Olympus\u2019 Towering Tops\/A Finn And German Viewed Some Hops,\u201d in which the initial letter of each word corresponds to the initial letter in the name of each nerve. The names of the nerves have changed over the years to reflect current usage and more accurate naming. An exercise to help learn this sort of information is to generate a mnemonic using words that have personal significance. The names of the cranial nerves are listed in\u00a0<a title=\"Table&#xa0;13.3.&#xa0;\" href=\"#m46553-tbl-ch13_03\">Table\u00a013.3<\/a>\u00a0along with a brief description of their function, their source (sensory ganglion or motor nucleus), and their target (sensory nucleus or skeletal muscle). They are listed here with a brief explanation of each nerve (<a title=\"Figure&#xa0;13.23.&#xa0;The Cranial Nerves\" href=\"#m46553-fig-ch13_04_05\">Figure\u00a013.23<\/a>). The\u00a0<strong><em>olfactory nerve<\/em><\/strong><a id=\"id746429\">\u00a0and<strong>\u00a0<em>optic nerve<\/em><\/strong><\/a><a id=\"id746443\">\u00a0are responsible for the sense of smell and vision, respectively. The\u00a0<strong><em>oculomotor nerve<\/em><\/strong><\/a><a id=\"id746457\">\u00a0is responsible for eye movements by controlling four of the\u00a0<strong><em>extraocular muscles<\/em><\/strong><\/a><a id=\"id746471\">. It is also responsible for lifting the upper eyelid when the eyes point up, and for pupillary constriction. The<strong>\u00a0<em>trochlear nerve<\/em><\/strong><\/a><a id=\"id746485\">\u00a0and the\u00a0<strong><em>abducens nerve<\/em><\/strong><\/a><a id=\"id746498\">\u00a0are both responsible for eye movement, but do so by controlling different extraocular muscles. The\u00a0<strong><em>trigeminal nerve<\/em><\/strong><\/a><a id=\"id746513\">\u00a0is responsible for cutaneous sensations of the face and controlling the muscles of mastication. The\u00a0<strong><em>facial nerve<\/em><\/strong><\/a><a id=\"id746527\">\u00a0is responsible for the muscles involved in facial expressions, as well as part of the sense of taste and the production of saliva. The\u00a0<strong><em>vestibulocochlear nerve<\/em><\/strong><\/a><a id=\"id746542\">\u00a0is responsible for the senses of hearing and balance. The\u00a0<strong><em>glossopharyngeal nerve<\/em><\/strong><\/a><a id=\"id746555\">\u00a0is responsible for controlling muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva. The\u00a0<strong><em>vagus nerve<\/em><\/strong><\/a><a id=\"id746570\">\u00a0is responsible for contributing to homeostatic control of the organs of the thoracic and upper abdominal cavities. The\u00a0<strong><em>spinal accessory nerve<\/em><\/strong><\/a><a id=\"id746585\">\u00a0is responsible for controlling the muscles of the neck, along with cervical spinal nerves. The\u00a0<strong><em>hypoglossal nerve<\/em><\/strong><\/a><a id=\"id746599\">\u00a0is responsible for controlling the muscles of the lower throat and tongue.<\/p>\n<div id=\"m46553-fig-ch13_04_05\" title=\"Figure&#xa0;13.23.&#xa0;The Cranial Nerves\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152027\/1320_The_Cranial_Nerves.jpg\" alt=\"This diagrams shows the brain and the main nerves in the brain are labeled.\" width=\"480\" \/><\/div>\n<\/div>\n<\/div>\n<p><\/a><\/p>\n<address><strong>Figure\u00a013.23.\u00a0The Cranial Nerves<\/strong><\/address>\n<address>The anatomical arrangement of the roots of the cranial nerves observed from an inferior view of the brain.<\/address>\n<address>\u00a0<\/address>\n<address>\u00a0<\/address>\n<\/div>\n<p>Three of the cranial nerves also contain autonomic fibers, and a fourth is almost purely a component of the autonomic system. The oculomotor, facial, and glossopharyngeal nerves contain fibers that contact autonomic ganglia. The oculomotor fibers initiate pupillary constriction, whereas the facial and glossopharyngeal fibers both initiate salivation. The vagus nerve primarily targets autonomic ganglia in the thoracic and upper abdominal cavities.<\/p>\n<div id=\"m46553-fs-id1492164\">\n<div class=\"bcc-box bcc-info\">\n<h3>Interactive Link<\/h3>\n<p>Visit this\u00a0<a href=\"http:\/\/openstaxcollege.org\/l\/NYTmeningitis\" target=\"_blank\">site<\/a>\u00a0to read about a man who wakes with a headache and a loss of vision. His regular doctor sent him to an ophthalmologist to address the vision loss. The ophthalmologist recognizes a greater problem and immediately sends him to the emergency room. Once there, the patient undergoes a large battery of tests, but a definite cause cannot be found. A specialist recognizes the problem as meningitis, but the question is what caused it originally. How can that be cured? The loss of vision comes from swelling around the optic nerve, which probably presented as a bulge on the inside of the eye. Why is swelling related to meningitis going to push on the optic nerve?<\/p>\n<\/div>\n<\/div>\n<p>Another important aspect of the cranial nerves that lends itself to a mnemonic is the functional role each nerve plays. The nerves fall into one of three basic groups. They are sensory, motor, or both (see\u00a0<a title=\"Table&#xa0;13.3.&#xa0;\" href=\"#m46553-tbl-ch13_03\">Table\u00a013.3<\/a>). The sentence, \u201cSome Say Marry Money But My Brother Says Brains Beauty Matter More,\u201d corresponds to the basic function of each nerve. The first, second, and eighth nerves are purely sensory: the olfactory (CNI), optic (CNII), and vestibulocochlear (CNVIII) nerves. The three eye-movement nerves are all motor: the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI). The spinal accessory (CNXI) and hypoglossal (CNXII) nerves are also strictly motor. The remainder of the nerves contain both sensory and motor fibers. They are the trigeminal (CNV), facial (CNVII), glossopharyngeal (CNIX), and vagus (CNX) nerves. The nerves that convey both are often related to each other. The trigeminal and facial nerves both concern the face; one concerns the sensations and the other concerns the muscle movements. The facial and glossopharyngeal nerves are both responsible for conveying gustatory, or taste, sensations as well as controlling salivary glands. The vagus nerve is involved in visceral responses to taste, namely the gag reflex. This is not an exhaustive list of what these combination nerves do, but there is a thread of relation between them.<\/p>\n<p><a href=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images-archive-read-only\/wp-content\/uploads\/sites\/198\/2014\/11\/20090404\/Screen-Shot-2014-11-13-at-2.54.45-PM.png\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-2499 size-full\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152030\/Screen-Shot-2014-11-13-at-2.54.45-PM.png\" alt=\"Screen Shot 2014-11-13 at 2.54.45 PM\" width=\"578\" height=\"682\" \/><\/a><\/p>\n<div id=\"m46553-tbl-ch13_03\">\n<\/div>\n<div title=\"Spinal Nerves\">\n<div>\n<h4 id=\"m46553-fs-id1502532\">Spinal Nerves<\/h4>\n<\/div>\n<p>The nerves connected to the spinal cord are the spinal nerves. The arrangement of these nerves is much more regular than that of the cranial nerves. All of the spinal nerves are combined sensory and motor axons that separate into two nerve roots. The sensory axons enter the spinal cord as the dorsal nerve root. The motor fibers, both somatic and autonomic, emerge as the ventral nerve root. The dorsal root ganglion for each nerve is an enlargement of the spinal nerve. There are 31 spinal nerves, named for the level of the spinal cord at which each one emerges. There are eight pairs of cervical nerves designated C1 to C8, twelve thoracic nerves designated T1 to T12, five pairs of lumbar nerves designated L1 to L5, five pairs of sacral nerves designated S1 to S5, and one pair of coccygeal nerves. The nerves are numbered from the superior to inferior positions, and each emerges from the vertebral column through the intervertebral foramen at its level. The first nerve, C1, emerges between the first cervical vertebra and the occipital bone. The second nerve, C2, emerges between the first and second cervical vertebrae. The same occurs for C3 to C7, but C8 emerges between the seventh cervical vertebra and the first thoracic vertebra. For the thoracic and lumbar nerves, each one emerges between the vertebra that has the same designation and the next vertebra in the column. The sacral nerves emerge from the sacral foramina along the length of that unique vertebra. Spinal nerves extend outward from the vertebral column to enervate the periphery. The nerves in the periphery are not straight continuations of the spinal nerves, but rather the reorganization of the axons in those nerves to follow different courses. Axons from different spinal nerves will come together into a\u00a0<strong><em>systemic nerve<\/em><\/strong><a id=\"id747186\">. This occurs at four places along the length of the vertebral column, each identified as a\u00a0<strong><em>nerve plexus<\/em><\/strong><\/a><a id=\"id747200\">, whereas the other spinal nerves directly correspond to nerves at their respective levels. In this instance, the word plexus is used to describe networks of nerve fibers with no associated cell bodies. Of the four nerve plexuses, two are found at the cervical level, one at the lumbar level, and one at the sacral level (<\/a><a title=\"Figure&#xa0;13.24.&#xa0;Nerve Plexuses of the Body\" href=\"#m46553-fig-ch13_04_06\">Figure\u00a013.24<\/a>).<\/p>\n<p>The\u00a0<strong><em>cervical plexus<\/em><\/strong><a id=\"id747233\">\u00a0is composed of axons from spinal nerves C1 through C5 and branches into nerves in the posterior neck and head, as well as the\u00a0<strong><em>phrenic nerve<\/em><\/strong><\/a><a id=\"id747247\">, which connects to the diaphragm at the base of the thoracic cavity. The other plexus from the cervical level is the\u00a0<strong><em>brachial plexus<\/em><\/strong><\/a><a id=\"id747261\">. Spinal nerves C4 through T1 reorganize through this plexus to give rise to the nerves of the arms, as the name brachial suggests. A large nerve from this plexus is the<strong>\u00a0<em>radial nerve<\/em><\/strong><\/a><a id=\"id747276\">\u00a0from which the\u00a0<strong><em>axillary nerve<\/em><\/strong><\/a><a id=\"id747289\">\u00a0branches to go to the armpit region. The radial nerve continues through the arm and is paralleled by the\u00a0<strong><em>ulnar nerve<\/em><\/strong><\/a><a id=\"id747303\">\u00a0and the\u00a0<strong><em>median nerve<\/em><\/strong><\/a><a id=\"id747316\">. The\u00a0<strong><em>lumbar plexus<\/em><\/strong><\/a><a id=\"id747329\">\u00a0arises from all the lumbar spinal nerves and gives rise to nerves enervating the pelvic region and the anterior leg. The\u00a0<strong><em>femoral nerve<\/em><\/strong><\/a><a id=\"id747344\">\u00a0is one of the major nerves from this plexus, which gives rise to the\u00a0<strong><em>saphenous nerve<\/em><\/strong><\/a><a id=\"id747358\">\u00a0as a branch that extends through the anterior lower leg. The\u00a0<strong><em>sacral plexus<\/em><\/strong><\/a><a id=\"id747372\">\u00a0comes from the lower lumbar nerves L4 and L5 and the sacral nerves S1 to S4. The most significant systemic nerve to come from this plexus is the\u00a0<strong><em>sciatic nerve<\/em><\/strong><\/a><a id=\"id747387\">, which is a combination of the\u00a0<strong><em>tibial nerve<\/em><\/strong><\/a><a id=\"id747401\">\u00a0and the\u00a0<strong><em>fibular nerve<\/em><\/strong><\/a><a id=\"id747414\">. The sciatic nerve extends across the hip joint and is most commonly associated with the condition\u00a0<strong><em>sciatica<\/em><\/strong><\/a><a id=\"id747428\">, which is the result of compression or irritation of the nerve or any of the spinal nerves giving rise to it. These plexuses are described as arising from spinal nerves and giving rise to certain systemic nerves, but they contain fibers that serve sensory functions or fibers that serve motor functions. This means that some fibers extend from cutaneous or other peripheral sensory surfaces and send action potentials into the CNS. Those are axons of sensory neurons in the dorsal root ganglia that enter the spinal cord through the dorsal nerve root. Other fibers are the axons of motor neurons of the anterior horn of the spinal cord, which emerge in the ventral nerve root and send action potentials to cause skeletal muscles to contract in their target regions. For example, the radial nerve contains fibers of cutaneous sensation in the arm, as well as motor fibers that move muscles in the arm. Spinal nerves of the thoracic region, T2 through T11, are not part of the plexuses but rather emerge and give rise to the<strong>\u00a0<em>intercostal nerves<\/em><\/strong><\/a><a id=\"id747464\">\u00a0found between the ribs, which articulate with the vertebrae surrounding the spinal nerve.<\/p>\n<div id=\"m46553-fig-ch13_04_06\" title=\"Figure&#xa0;13.24.&#xa0;Nerve Plexuses of the Body\">\n<div>\n<div><img decoding=\"async\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/511\/2016\/08\/19152033\/1321_Spinal_Nerve_Plexuses.jpg\" alt=\"This figure shows a torso of a human body. The spinal cord is shown in the body and the main nerves along the spinal cord are labeled.\" width=\"440\" \/><\/div>\n<\/div>\n<\/div>\n<p><\/a><\/p>\n<address><strong>Figure\u00a013.24.\u00a0Nerve Plexuses of the Body<\/strong><\/address>\n<address>There are four main nerve plexuses in the human body. The cervical plexus supplies nerves to the posterior head and neck, as well as to the diaphragm. The brachial plexus supplies nerves to the arm. The lumbar plexus supplies nerves to the anterior leg. The sacral plexus supplies nerves to the posterior leg.<\/address>\n<address>\u00a0<\/address>\n<\/div>\n<div id=\"m46553-fs-id886679\">\n<div>\n<div class=\"bcc-box bcc-success\">\n<h3>Aging and the Nervous System<\/h3>\n<div>\n<p title=\"Nervous System\">Anosmia is the loss of the sense of smell. It is often the result of the olfactory nerve being severed, usually because of blunt force trauma to the head. The sensory neurons of the olfactory epithelium have a limited lifespan of approximately one to four months, and new ones are made on a regular basis. The new neurons extend their axons into the CNS by growing along the existing fibers of the olfactory nerve. The ability of these neurons to be replaced is lost with age. Age-related anosmia is not the result of impact trauma to the head, but rather a slow loss of the sensory neurons with no new neurons born to replace them. Smell is an important sense, especially for the enjoyment of food. There are only five tastes sensed by the tongue, and two of them are generally thought of as unpleasant tastes (sour and bitter). The rich sensory experience of food is the result of odor molecules associated with the food, both as food is moved into the mouth, and therefore passes under the nose, and when it is chewed and molecules are released to move up the pharynx into the posterior nasal cavity. Anosmia results in a loss of the enjoyment of food. As the replacement of olfactory neurons declines with age, anosmia can set in. Without the sense of smell, many sufferers complain of food tasting bland. Often, the only way to enjoy food is to add seasoning that can be sensed on the tongue, which usually means adding table salt. The problem with this solution, however, is that this increases sodium intake, which can lead to cardiovascular problems through water retention and the associated increase in blood pressure.<\/p>\n<\/div>\n<\/div>\n<p>\u00a0<\/p>\n<\/div>\n<div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"id748220\" title=\"Glossary\"><\/div>\n<\/div>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-254\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Anatomy &amp; Physiology. <strong>Authored by<\/strong>: OpenStax. <strong>Provided by<\/strong>: OpenStax CNX. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25\">http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Download for free at http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25.<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":15,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Anatomy & Physiology\",\"author\":\"OpenStax\",\"organization\":\"OpenStax CNX\",\"url\":\"http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Download for free at http:\/\/cnx.org\/contents\/14fb4ad7-39a1-4eee-ab6e-3ef2482e3e22@8.25.\"}]","CANDELA_OUTCOMES_GUID":"","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-254","chapter","type-chapter","status-publish","hentry"],"part":23,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/pressbooks\/v2\/chapters\/254","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":1,"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/pressbooks\/v2\/chapters\/254\/revisions"}],"predecessor-version":[{"id":357,"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/pressbooks\/v2\/chapters\/254\/revisions\/357"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/pressbooks\/v2\/parts\/23"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/pressbooks\/v2\/chapters\/254\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/wp\/v2\/media?parent=254"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/pressbooks\/v2\/chapter-type?post=254"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/wp\/v2\/contributor?post=254"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/umd-publichealthbio\/wp-json\/wp\/v2\/license?post=254"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}