{"id":2219,"date":"2014-10-31T15:33:30","date_gmt":"2014-10-31T15:33:30","guid":{"rendered":"https:\/\/courses.candelalearning.com\/apvccs\/?post_type=chapter&p=2219"},"modified":"2019-11-24T23:52:53","modified_gmt":"2019-11-24T23:52:53","slug":"divisions-of-the-autonomic-nervous-system","status":"web-only","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-dutchess-anatomy-physiology\/chapter\/divisions-of-the-autonomic-nervous-system\/","title":{"raw":"Divisions of the Autonomic Nervous System","rendered":"Divisions of the Autonomic Nervous System"},"content":{"raw":"
\r\n\r\n \r\n\r\n<\/div>\r\nThe nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.\r\n\r\nThe autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In ition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the\u00a0sympathetic division<\/strong>\u00a0and the\u00a0parasympathetic division<\/strong>. The sympathetic system is associated with the\u00a0fight-or-flight response<\/strong>, and parasympathetic activity is referred to by the epithet of\u00a0rest and digest<\/strong>. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.\r\n
\r\n\r\nWatch this\u00a0video\u00a0to learn more about adrenaline and the fight-or-flight response.\r\n\r\nhttps:\/\/youtu.be\/m2GywoS77qc\r\n\r\nWhen someone is said to have a rush of adrenaline, the image of bungee jumpers or skydivers usually comes to mind. But adrenaline, also known as epinephrine, is an important chemical in coordinating the body\u2019s fight-or-flight response. In this video, you look inside the physiology of the fight-or-flight response, as envisioned for a firefighter. His body\u2019s reaction is the result of the sympathetic division of the autonomic nervous system causing system-wide changes as it prepares for extreme responses. What two changes does adrenaline bring about to help the skeletal muscle response?\r\n\r\n<\/div>\r\n

Sympathetic Division of the Autonomic Nervous System<\/h2>\r\nTo respond to a threat\u2014to fight or to run away\u2014the sympathetic system causes divergent effects as many different effector organs are activated together for a common purpose. More oxygen needs to be inhaled and delivered to skeletal muscle. The respiratory, cardiovascular, and musculoskeletal systems are all activated together. itionally, sweating keeps the excess heat that comes from muscle contraction from causing the body to overheat. The digestive system shuts down so that blood is not absorbing nutrients when it should be delivering oxygen to skeletal muscles. To coordinate all these responses, the connections in the sympathetic system diverge from a limited region of the central nervous system (CNS) to a wide array of ganglia that project to the many effector organs simultaneously. The complex set of structures that compose the output of the sympathetic system make it possible for these disparate effectors to come together in a coordinated, systemic change.\r\n\r\nThe sympathetic division of the autonomic nervous system influences the various organ systems of the body through connections emerging from the thoracic and upper lumbar spinal cord. It is referred to as the\u00a0thoraco-lumbar system<\/strong>\u00a0to reflect this anatomical basis. A\u00a0preganglionic neuron<\/strong>\u00a0in the lateral horn of any of these spinal regions projects to ganglia adjacent to the vertebral column through the ventral spinal roots.\r\n\r\nThe majority of ganglia of the sympathetic system belong to a network of\u00a0sympathetic chain ganglia<\/strong>\u00a0that runs alongside the vertebral column. The ganglia appear as a series of clusters of neurons linked by axonal bridges. There are typically 23 ganglia in the chain on either side of the spinal column. Three correspond to the cervical region, 12 are in the thoracic region, four are in the lumbar region, and four correspond to the sacral region. The cervical and sacral levels are not connected to the spinal cord directly through the spinal roots, but through ascending or descending connections through the bridges within the chain.\r\n\r\nA diagram that shows the connections of the sympathetic system is somewhat like a circuit diagram that shows the electrical connections between different receptacles and devices. In\u00a0Figure 1, the \u201ccircuits\u201d of the sympathetic system are diagrammed.\r\n\r\n[caption id=\"attachment_4863\" align=\"aligncenter\" width=\"601\"]\"This<\/a> Figure 1. Connections of Sympathetic Division of the Autonomic Nervous System.<\/strong> Neurons from the lateral horn of the spinal cord (preganglionic neurons) project to the chain ganglia on either side of the vertebral column or to collateral (prevertebral) ganglia that are anterior to the vertebral column in the abdominal cavity. Axons from these ganglionic neurons (postganglionic fibers) then project to target effectors throughout the body.[\/caption]\r\n\r\nTo continue with the analogy of the circuit diagram, there are three different types of \u201cjunctions\u201d that operate within the sympathetic system (Figure 2). The first type is most direct: the sympathetic nerve projects to the chain ganglion at the same level as the\u00a0target effector<\/strong>\u00a0(the organ, tissue, or gland to be innervated).\r\n\r\nAn example of this type is spinal nerve T1 that synapses with the T1 chain ganglion to innervate the trachea. The fibers of this branch are called\u00a0white<\/strong> rami communicantes<\/strong>\u00a0(singular = ramus communicans<\/em>); they are myelinated and therefore referred to as white (see\u00a0Figure\u00a02a). The axon from the central neuron (the preganglionic fiber shown as a solid line) synapses with the\u00a0ganglionic neuron<\/strong>\u00a0(with the postganglionic fiber shown as a dashed line). This neuron then projects to a target effector\u2014in this case, the trachea\u2014via\u00a0gray rami communicantes<\/strong>, which are unmyelinated axons.\r\n\r\nIn some cases, the target effectors are located superior or inferior to the spinal segment at which the preganglionic fiber emerges. With respect to the \u201cwiring\u201d involved, the synapse with the ganglionic neuron occurs at chain ganglia superior or inferior to the location of the central neuron. An example of this is spinal nerve T1 that innervates the eye. The spinal nerve tracks up through the chain until it reaches the\u00a0superior cervical ganglion<\/strong>, where it synapses with the postganglionic neuron (see\u00a0Figure 2b). The cervical ganglia are referred to as\u00a0paravertebral ganglia<\/strong>, given their location adjacent to prevertebral ganglia in the sympathetic chain.\r\n\r\nNot all axons from the central neurons terminate in the chain ganglia. Additional branches from the ventral nerve root continue through the chain and on to one of the collateral ganglia as the\u00a0greater splanchnic nerve<\/strong>\u00a0or\u00a0lesser splanchnic nerve<\/strong>. For example, the greater splanchnic nerve at the level of T5 synapses with a collateral ganglion outside the chain before making the connection to the postganglionic nerves that innervate the stomach (see\u00a0Figure 2c).\r\n\r\nCollateral ganglia<\/strong>, also called\u00a0prevertebral ganglia<\/strong>, are situated anterior to the vertebral column and receive inputs from splanchnic nerves as well as preganglionic sympathetic neurons. They are associated with controlling organs in the abdominal cavity, and are also considered part of the enteric nervous system. The three collateral ganglia are the\u00a0celiac ganglion<\/strong>, the\u00a0superior mesenteric ganglion<\/strong>, and the\u00a0inferior mesenteric ganglion<\/strong>\u00a0(see\u00a0Figure 1). The word celiac is derived from the Latin word \u201ccoelom,\u201d which refers to a body cavity (in this case, the abdominal cavity), and the word mesenteric refers to the digestive system.\r\n\r\n[caption id=\"attachment_4865\" align=\"alignright\" width=\"842\"]\"This<\/a> Figure 2. Sympathetic Connections and Chain Ganglia.<\/strong> The axon from a preganglionic sympathetic neuron in the spinal cord can project to the periphery in a number of different ways. (a) The fiber can project out to the ganglion at the same level and synapse on a postganglionic neuron. (b) A branch can project to more superior or inferior ganglion in the chain. (c) A branch can project through the white ramus communicans, but not terminate on a postganglionic neuron in the chain. Instead, it projects through one of the splanchnic nerves to a collateral ganglion or the adrenal medulla (not pictured).[\/caption]\r\n\r\nAn axon from the central neuron that projects to a sympathetic ganglion is referred to as a\u00a0preganglionic fiber<\/strong>\u00a0or neuron, and represents the output from the CNS to the ganglion. Because the sympathetic ganglia are adjacent to the vertebral column, preganglionic sympathetic fibers are relatively short, and they are myelinated. A\u00a0postganglionic fiber<\/strong>\u2014the axon from a ganglionic neuron that projects to the target effector\u2014represents the output of a ganglion that directly influences the organ.\r\n\r\nCompared with the preganglionic fibers, postganglionic sympathetic fibers are long because of the relatively greater distance from the ganglion to the target effector. These fibers are unmyelinated. (Note that the term \u201cpostganglionic neuron\u201d may be used to describe the projection from a ganglion to the target. The problem with that usage is that the cell body is in the ganglion, and only the fiber is postganglionic. Typically, the term neuron applies to the entire cell.)\r\n\r\nOne type of preganglionic sympathetic fiber does not terminate in a ganglion. These are the axons from central sympathetic neurons that project to the\u00a0adrenal medulla<\/strong>, the interior portion of the adrenal gland. These axons are still referred to as preganglionic fibers, but the target is not a ganglion. The adrenal medulla releases signaling molecules into the bloodstream, rather than using axons to communicate with target structures. The cells in the adrenal medulla that are contacted by the preganglionic fibers are called\u00a0chromaffin cells<\/strong>. These cells are neurosecretory cells that develop from the neural crest along with the sympathetic ganglia, reinforcing the idea that the gland is, functionally, a sympathetic ganglion.\r\n\r\nThe projections of the sympathetic division of the autonomic nervous system diverge widely, resulting in a broad influence of the system throughout the body. As a response to a threat, the sympathetic system would increase heart rate and breathing rate and cause blood flow to the skeletal muscle to increase and blood flow to the digestive system to decrease. Sweat gland secretion should also increase as part of an integrated response.\r\n\r\nAll of those physiological changes are going to be required to occur together to run away from the hunting lioness, or the modern equivalent. This divergence is seen in the branching patterns of preganglionic sympathetic neurons\u2014a single preganglionic sympathetic neuron may have 10\u201320 targets. An axon that leaves a central neuron of the lateral horn in the thoracolumbar spinal cord will pass through the white ramus communicans and enter the sympathetic chain, where it will branch toward a variety of targets. At the level of the spinal cord at which the preganglionic sympathetic fiber exits the spinal cord, a branch will synapse on a neuron in the adjacent chain ganglion.\r\n\r\nSome branches will extend up or down to a different level of the chain ganglia. Other branches will pass through the chain ganglia and project through one of the splanchnic nerves to a collateral ganglion. Finally, some branches may project through the splanchnic nerves to the adrenal medulla. All of these branches mean that one preganglionic neuron can influence different regions of the sympathetic system very broadly, by acting on widely distributed organs.\r\n

Parasympathetic Division of the Autonomic Nervous System<\/h2>\r\nThe parasympathetic division of the autonomic nervous system is named because its central neurons are located on either side of the thoracolumbar region of the spinal cord (para<\/em>- = \u201cbeside\u201d or \u201cnear\u201d). The parasympathetic system can also be referred to as the\u00a0craniosacral system<\/strong>\u00a0(or outflow) because the preganglionic neurons are located in nuclei of the brain stem and the lateral horn of the sacral spinal cord.\r\n\r\nThe connections, or \u201ccircuits,\u201d of the parasympathetic division are similar to the general layout of the sympathetic division with a few specific differences (Figure 3). The preganglionic fibers from the cranial region travel in cranial nerves, whereas preganglionic fibers from the sacral region travel in spinal nerves. The targets of these fibers are terminal ganglia<\/strong>, which are located near\u2014or even within\u2014the target effector. These ganglia are often referred to as\u00a0intramural ganglia<\/strong>\u00a0when they are found within the walls of the target organ. The postganglionic fiber projects from the terminal ganglia a short distance to the target effector, or to the specific target tissue within the organ. Comparing the relative lengths of axons in the parasympathetic system, the preganglionic fibers are long and the postganglionic fibers are short because the ganglia are close to\u2014and sometimes within\u2014the target effectors.\r\n\r\nThe cranial component of the parasympathetic system is based in particular nuclei of the brain stem. In the midbrain, the\u00a0Eddinger\u2013Westphal nucleus<\/strong>\u00a0is part of the oculomotor complex, and axons from those neurons travel with the fibers in the oculomotor nerve (cranial nerve III) that innervate the extraocular muscles. The preganglionic parasympathetic fibers within cranial nerve III terminate in the\u00a0ciliary ganglion<\/strong>, which is located in the posterior orbit. The postganglionic parasympathetic fibers then project to the smooth muscle of the iris to control pupillary size. In the upper medulla, the salivatory nuclei contain neurons with axons that project through the facial and glossopharyngeal nerves to ganglia that control salivary glands. Tear production is influenced by parasympathetic fibers in the facial nerve, which activate a ganglion, and ultimately the lacrimal (tear) gland.\r\n\r\nNeurons in the\u00a0dorsal nucleus of the vagus nerve<\/strong>\u00a0and the\u00a0nucleus ambiguus<\/strong>\u00a0project through the vagus nerve (cranial nerve X) to the terminal ganglia of the thoracic and abdominal cavities. Parasympathetic preganglionic fibers primarily influence the heart, bronchi, and esophagus in the thoracic cavity and the stomach, liver, pancreas, gallbladder, and small intestine of the abdominal cavity. The postganglionic fibers from the ganglia activated by the vagus nerve are often incorporated into the structure of the organ, such as the\u00a0mesenteric plexus<\/strong>\u00a0of the digestive tract organs and the intramural ganglia.\r\n\r\n[caption id=\"attachment_4864\" align=\"aligncenter\" width=\"667\"]\"This<\/a> Figure 3.<\/strong> Connections of Parasympathetic Division of the Autonomic Nervous System<\/strong> Neurons from brain-stem nuclei, or from the lateral horn of the sacral spinal cord, project to terminal ganglia near or within the various organs of the body. Axons from these ganglionic neurons then project the short distance to those target effectors.[\/caption]\r\n\r\n
\r\n

Everyday Connections:\u00a0Fight or Flight? What About Fright and Freeze?<\/b><\/h3>\r\n

The original usage of the epithet \u201cfight or flight\u201d comes from a scientist named Walter Cannon who worked at Harvard in 1915. The concept of homeostasis and the functioning of the sympathetic system had been introduced in France in the previous century. Cannon expanded the idea, and introduced the idea that an animal responds to a threat by preparing to stand and fight or run away. The nature of this response was thoroughly explained in a book on the physiology of pain, hunger, fear, and rage.<\/p>\r\nWhen students learn about the sympathetic system and the fight-or-flight response, they often stop and wonder about other responses. If you were faced with a lioness running toward you as pictured at the beginning of this chapter, would you run or would you stand your ground? Some people would say that they would freeze and not know what to do. So isn\u2019t there really more to what the autonomic system does than fight, flight, rest, or digest. What about fear and paralysis in the face of a threat?\r\n\r\nThe common epithet of \u201cfight or flight\u201d is being enlarged to be \u201cfight, flight, or fright\u201d or even \u201cfight, flight, fright, or freeze.\u201d Cannon\u2019s original contribution was a catchy phrase to express some of what the nervous system does in response to a threat, but it is incomplete. The sympathetic system is responsible for the physiological responses to emotional states. The name \u201csympathetic\u201d can be said to mean that (sym<\/em>- = \u201ctogether\u201d; -pathos<\/em> = \u201cpain,\u201d \u201csuffering,\u201d or \u201cemotion\u201d).\r\n\r\n<\/div>\r\n

\r\n\r\nWatch this\u00a0video\u00a0to learn more about the nervous system.\r\n\r\nhttps:\/\/youtu.be\/RyP8L3qTW9Q\r\n\r\nAs described in this video, the nervous system has a way to deal with threats and stress that is separate from the conscious control of the somatic nervous system. The system comes from a time when threats were about survival, but in the modern age, these responses become part of stress and anxiety. This video describes how the autonomic system is only part of the response to threats, or stressors. What other organ system gets involved, and what part of the brain coordinates the two systems for the entire response, including epinephrine (adrenaline) and cortisol?\r\n\r\n<\/div>","rendered":"
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 <\/p>\n<\/div>\n

The nervous system can be divided into two functional parts: the somatic nervous system and the autonomic nervous system. The major differences between the two systems are evident in the responses that each produces. The somatic nervous system causes contraction of skeletal muscles. The autonomic nervous system controls cardiac and smooth muscle, as well as glandular tissue. The somatic nervous system is associated with voluntary responses (though many can happen without conscious awareness, like breathing), and the autonomic nervous system is associated with involuntary responses, such as those related to homeostasis.<\/p>\n

The autonomic nervous system regulates many of the internal organs through a balance of two aspects, or divisions. In ition to the endocrine system, the autonomic nervous system is instrumental in homeostatic mechanisms in the body. The two divisions of the autonomic nervous system are the\u00a0sympathetic division<\/strong>\u00a0and the\u00a0parasympathetic division<\/strong>. The sympathetic system is associated with the\u00a0fight-or-flight response<\/strong>, and parasympathetic activity is referred to by the epithet of\u00a0rest and digest<\/strong>. Homeostasis is the balance between the two systems. At each target effector, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. One causes heart rate to increase, whereas the other causes heart rate to decrease.<\/p>\n

\n

Watch this\u00a0video\u00a0to learn more about adrenaline and the fight-or-flight response.<\/p>\n