{"id":162,"date":"2019-10-17T17:02:07","date_gmt":"2019-10-17T17:02:07","guid":{"rendered":"https:\/\/pressbooks.publishdot.com\/nursingpharmacology\/chapter\/4-2-ans-basics\/"},"modified":"2021-12-07T11:12:06","modified_gmt":"2021-12-07T11:12:06","slug":"4-2-ans-basics","status":"publish","type":"chapter","link":"https:\/\/pressbooks.publishdot.com\/nursingpharmacology\/chapter\/4-2-ans-basics\/","title":{"raw":"4.2 Autonomic Nervous System Basics","rendered":"4.2 Autonomic Nervous System Basics"},"content":{"raw":"This section will review key anatomy concepts in the autonomic nervous system (ANS) related to the mechanism of action of medications. For more detailed information regarding the concepts reviewed, use the links provided to review detailed autonomic nervous system content in the Open Stax Anatomy and Physiology book:[footnote]Content can be found at https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/12-1-basic-structure-and-function-of-the-nervous-system<\/a>[\/footnote]<\/sup>\n\nReview the basic structure and function of the nervous system<\/a>\n\nReview the anatomy of sensory perception<\/a>.\n\nReview the anatomy of motor responses<\/a>.\n\nReview the divisions of the autonomic nervous system<\/a>.\n\nReview autonomic reflexes and homeostasis<\/a>.\n\nReview information on a few drugs that affect the autonomic nervous system<\/a>.\n

<\/a>Components and Functions of the Nervous System<\/h2>\n[caption id=\"\" align=\"aligncenter\" width=\"841\"]\"Outline Figure 4.1 Central and Peripheral Nervous System[\/caption]\n\nThe nervous system has two major components: the central nervous system (CNS) and the peripheral nervous system. See Figure 4.1.[footnote]\"1201 Overview of Nervous System.jpg<\/a>\" by CNX OpenStax<\/a>. is licensed under CC BY 4.0<\/a> Access for free at https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/12-1-basic-structure-and-function-of-the-nervous-system<\/a>[\/footnote]<\/sup> The [pb_glossary id=\"1854\"]central nervous system (CNS)[\/pb_glossary]<\/strong> is composed of the brain and the spinal cord. The[pb_glossary id=\"1855\"] peripheral nervous system[\/pb_glossary]<\/strong> includes nerves outside the brain and spinal cord and consists of sensory neurons and motor neurons. [pb_glossary id=\"843\"]Sensory neurons[\/pb_glossary]<\/strong> sense the environment and conduct signals to the brain that become a conscious perception of that stimulus. This conscious perception may lead to a motor response that is conducted from the brain to the peripheral nervous system via motor neurons to cause a movement. [pb_glossary id=\"844\"]Motor neurons[\/pb_glossary]<\/strong> consist of the [pb_glossary id=\"845\"]somatic nervous system [\/pb_glossary] <\/strong>that stimulates voluntary movement of muscles and the [pb_glossary id=\"846\"]autonomic nervous system[\/pb_glossary]<\/strong>[footnote]\"Component of the Nervous System\" by Blaire Babbit at Chippewa Valley Technical College<\/a> is licensed under CC BY 4.0<\/a>[\/footnote] that controls involuntary responses. This chapter will focus on the autonomic nervous system.\n\nThe two divisions\u00a0of the autonomic nervous system are the [pb_glossary id=\"853\"]sympathetic division (SNS)[\/pb_glossary]<\/strong> and the [pb_glossary id=\"854\"]parasympathetic division (PNS)[\/pb_glossary]<\/strong>. The SNS contains alpha and beta receptors, and the PNS contains nicotinic and muscarinic receptors. Each type of receptor has a specific action when stimulated. See Figure 4.2 for an image of the divisions of the nervous system and the receptors in the ANS.[footnote]\"Component of the Nervous System\" by Blaire Babbitt at Chippewa Valley Technical College<\/a> is licensed under CC BY 4.0<\/a>[\/footnote]\n\n \n\n[caption id=\"attachment_161\" align=\"aligncenter\" width=\"1024\"]\"Concept Figure 4.2 Components of the Nervous System and ANS receptors[\/caption]\n

<\/a>SNS and PNS Functions and Homeostasis<\/h3>\nThe sympathetic system is associated with the [pb_glossary id=\"2905\"]\"fight-or-flight\"[\/pb_glossary]<\/strong> response, and parasympathetic activity is often referred to as \"rest and digest.\" See Figure 4.3[footnote]Untitled image by Meredith Pomietlo for Chippewa Valley Technical College<\/a> is licensed under CC BY 4.0<\/a>[\/footnote]<\/sup> to compare the effects on PNS and SNS stimulation on target organs.\u00a0 The autonomic nervous system regulates many of the internal organs through a balance of these two divisions and is instrumental in homeostatic mechanisms in the body.[footnote]This work is a derivative of Anatomy and Physiology<\/a> by OpenStax<\/a> licensed under CC BY 4.0<\/a>. Access for free at Access for free at https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-introduction<\/a>[\/footnote]<\/sup>\n\n \n\n[caption id=\"attachment_161\" align=\"aligncenter\" width=\"598\"]\"Diagram Figure 4.3. Effects of PNS and SNS Stimulation on Target Organs[\/caption]\n\nStimulation of SNS primarily produces increased heart rate, increased blood pressure via the constriction of blood vessels, and bronchial dilation. In comparison, stimulation of the PNS causes slowing of the heart, lowering of blood pressure due to vasodilation, bronchial constriction, and focuses on stimulating intestinal motility, salivation, and relaxation of the bladder.\n\n[pb_glossary id=\"856\"]Homeostasis[\/pb_glossary]<\/strong> is the balance between the two systems. At each target organ, dual innervation determines activity. For example, the heart receives connections from both the sympathetic and parasympathetic divisions. SNS stimulation causes the heart rate to increase, whereas PNS stimulation causes the heart rate to decrease.\n\nTo respond to a threat - to \"fight or flight\" - the sympathetic system stimulates many different target organs to achieve this purpose. For example, if a person sees a grizzly bear in the wilderness, the individual has the choice to stand and fight the bear or to run away. For either choice, several things must occur for additional oxygen and glucose to be delivered to skeletal muscle to fight or run. The respiratory, cardiovascular, and musculoskeletal systems are all activated to breathe rapidly, cause bronchodilation in the lungs to inhale more oxygen, stimulate the heart to pump more blood, and increase blood pressure to deliver it to the muscles.[footnote]This work is a derivative of Anatomy and Physiology<\/a> by OpenStax<\/a> licensed under CC BY 4.0<\/a>. Access for free at https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-introduction<\/a>[\/footnote]<\/sup> The liver creates more glucose for energy for the muscles to use. The pupils dilate to see the threat (or the escape route) more clearly. Sweating prevents the body from overheating from excess muscle contraction.\u00a0 Since the digestive system is not needed during this time of threat, the body shunts oxygen-rich blood to the skeletal muscles. To coordinate all these targeted responses, catecholamines such as epinephrine and norepinephrine are released in the sympathetic system and disperse to the many neuroreceptors on the target organs simultaneously.[footnote]This work is a derivative of Anatomy and Physiology<\/a> by OpenStax<\/a> licensed under CC BY 4.0<\/a>. Access for free at Access for free at https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-introduction<\/a>[\/footnote]<\/sup>\n

<\/a>Chemical Signaling in the Autonomic Nervous System<\/h2>\n[caption id=\"\" align=\"aligncenter\" width=\"586\"]\"Image Figure 4.4 Autonomic System neurons conduct signals via the preganglionic neurons to postganglionic neurons to the target organs[\/caption]\n\n[pb_glossary id=\"859\"]Neurons [\/pb_glossary]<\/strong> conduct impulses to the synapse of a target organ. The [pb_glossary id=\"860\"]synapse[\/pb_glossary]<\/strong> is a connection between the neuron and its target cell. See Figures 4.4[footnote]\"Autonomic Nervous System<\/a>\" by CNX OpenStax<\/a> is licensed under CC BY 4.0<\/a>[\/footnote]<\/sup> and 4.5[footnote]\"The Synapse\" by CNX OpenStax<\/a> is licensed under CC BY 4.0<\/a> Access for free at https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/12-5-communication-between-neurons<\/a>[\/footnote]<\/sup> for images of synapse connections.\n\n[caption id=\"\" align=\"aligncenter\" width=\"592\"]\"Illustration Figure 4.5 The synapse is the connection between a neuron and its target cell where neurotransmitters are released[\/caption]\n

<\/a>Preganglionic Neurons<\/h3>\nThe synapse is composed of a preganglionic (presynaptic) neuron and a postganglionic (postsynaptic) neuron.<\/span> [pb_glossary id=\"861\"]Preganglionic neurons[\/pb_glossary] <\/strong>release <\/span>[pb_glossary id=\"862\"]acetylcholine (ACh)[\/pb_glossary]<\/strong> onto nicotinic receptors on the postganglionic neuron.\u00a0 Nicotine, found in tobacco products, also binds to and activates nicotinic receptors, mimicking the effects of ACh. This is worth noting, because if medications were developed to impact the nicotinic receptors, then it would impact both the SNS and PNS systems at the preganglionic level. Instead, most medications target the <\/span>[pb_glossary id=\"863\"]postganglionic neurons[\/pb_glossary]<\/strong>, because each type of postganglionic neuron has different neurotransmitters and different target receptors.<\/span>\n

<\/a>Postganglionic Neurons<\/h3>\nThere are different types of postganglionic neurons in the SNS and PNS branches of the autonomic nervous system. Postganglionic neurons of the PNS branch are classified as [pb_glossary id=\"866\"]cholinergic[\/pb_glossary]<\/strong>, meaning that acetylcholine (ACh) is released, whereas postganglionic neurons of the SNS are classifed as [pb_glossary id=\"868\"]adrenergic[\/pb_glossary]<\/strong>, meaning that norepinephrine (NE) is released. The terms cholinergic and adrenergic refer not only to the signal that is released, but also to the class of neuroreceptors that each binds. (See Figure 4.6 for an image of the release of ACh and NE and their attachment to the corresponding adrenergic or nicotinic receptors.)\n\nThe cholinergic system of the PNS includes two classes of postganglionic neuroreceptors: the nicotinic receptor and the muscarinic receptor. Both receptor types bind to ACh and cause changes in the target cell. The situation is similar to locks and keys. Imagine two locks\u2014one for a classroom and the other for an office\u2014opened by two separate keys. The classroom key will not open the office door, and the office key will not open the classroom door. This is similar to the specificity of nicotine and muscarine for their receptors. However, a master key can open multiple locks, such as a master key for the biology department that opens both the classroom and the office doors. This is similar to ACh that binds to both types of receptors.\n\nThe adrenergic system of the SNS has two major types of neuroreceptors: the alpha (\u03b1)-adrenergic receptor and beta (\u03b2)-adrenergic receptor. There are two types of \u03b1-adrenergic receptors, termed \u03b11 and \u03b12, and there are two types of \u03b2-adrenergic receptors, termed \u03b21 and \u03b22. An additional aspect of the adrenergic system is that there is a second neurotransmitter in addition to norepinephrine. The second neurotransmitter is called epinephrine. The chemical difference between norepinephrine and epinephrine is the addition of a methyl group (CH3) in epinephrine. The prefix \"nor-\" actually refers to this chemical difference in which a methyl group is missing. [footnote]\"Sympathetic and Parasympathetic Pre-and Postganglionic fibers and neuroreceptors\" by Dominic Slausen at Chippewa Valley Technical College<\/a> is licensed under CC BY 4.0<\/a>[\/footnote]\n\nThe term adrenergic should remind you of the word adrenaline, which is associated with the fight-or-flight response described earlier. Adrenaline and epinephrine are two names for the same molecule. The adrenal gland (in Latin, ad- = \"on top of\"; renal = \"kidney\") secretes adrenaline. The ending \"-ine\" refers to the chemical being derived, or extracted, from the adrenal gland.[footnote]This work is a derivative of Anatomy and Physiology<\/a> by OpenStax<\/a> licensed under CC BY 4.0<\/a>. Access for free at Access for free at https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/1-introduction<\/a>[\/footnote]<\/sup>\n\n[caption id=\"attachment_161\" align=\"alignright\" width=\"1324\"]\"Adrenergic Figure 4.6 Sympathetic and Parasympathetic Pre-and Postganglionic Fibers and Neuroreceptors[\/caption]\n\n \n
\n

\u00a0Interactive Activity<\/h3>\n[h5p id=\"11\"]\n\n<\/div>\n

<\/a>ANS Neuroreceptors and Effects<\/h3>\nThe effects of stimulating each type of neuroreceptor are outlined in this section and sample uses of medications are provided.\n

<\/a>Sympathetic Nervous System<\/h4>\nSNS receptors include Alpha-1, Alpha-2, Beta-1, and Beta-2 receptors. Epinephrine and norepinephrine stimulate these receptors, causing the overall fight-or-flight response in various target organs. Medications causing similar effects are called [pb_glossary id=\"875\"]adrenergic agonists[\/pb_glossary]<\/strong>, or [pb_glossary id=\"876\"]sympathomimetics[\/pb_glossary]<\/strong>, because they mimic the effects of the body's natural SNS stimulation. On the other hand, [pb_glossary id=\"877\"]adrenergic antagonists[\/pb_glossary]<\/strong> block the effects of the SNS receptors. Dopamine also stimulates these receptors, but it is dosage-based. Dopamine causes vasodilation of arteries in the kidney, heart, and brain, depending on the dosage. See Table 4.1 for a comparison of stimulation and inhibition of these SNS receptors.\n\nTable 4.1 Comparison of Medication Effects of Adrenergic Receptor Stimulation and Inhibition\n\n\n\n\n\n\n\n
\n
Receptor<\/strong><\/h5>\n<\/th>\n
\n
Effects of Stimulation<\/strong><\/h5>\n<\/th>\n
\n
Effects of Inhibition<\/strong><\/h5>\n<\/th>\n<\/tr>\n
\n
Alpha-1<\/h5>\n<\/th>\n
Contract smooth muscle\n\nCNS stimulation\n\nBlood vessels: vasoconstriction to nonessential organs\n\nGI: relax smooth muscle and decrease motility\n\nLiver: glyconeogenesis\n\nBladder: contraction\n\nUterus: contraction\n\nPupils: dilation\n\nMedication example: Pseudoephedrine to treat nasal congestion by vasoconstriction<\/td>\nRelax smooth muscle\n\nVasodilation\n\nBladder: Increase urine flow\n\n \n\nMedication example:\n\nTamsulosin to improve urine flow<\/td>\n<\/tr>\n
\n
Alpha-2<\/h5>\n<\/th>\n
Vasodilation\n\nMedication Example: Clonidine to treat hypertension<\/td>\nNot used clinically<\/td>\n<\/tr>\n
\n
Beta-1<\/h5>\n<\/th>\n
Primarily stimulates heart with increased heart rate and contractility\n\nAlso causes kidneys to release renin\n\nMedication example: Dobutamine to treat acute heart failure to increase cardiac output<\/td>\n\"Selective Beta blocker\" used to decrease heart rate and blood pressure\n\n \n\nMedication example: Metoprolol to decrease heart rate and blood pressure<\/td>\n<\/tr>\n
\n
Beta-2<\/h5>\n<\/th>\n
Primarily relax smooth muscle\n\nBlood vessels: vasodilation\n\nLungs: bronchodilation\n\nGI: decreased motility\n\nLiver: glyconeogenesis\n\nUterus: relaxation\n\nMedication example: Albuterol for bronchodilation<\/td>\n\"Nonselective Beta Blockers\" block Beta-1 and Beta-2 receptors so also cause bronchoconstriction\n\n \n\nMedication example: Propranolol blocks Beta-1 and Beta-2 receptor so lowers blood pressure but inadvertently causes bronchoconstriction<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\n

\u00a0Interactive Activity<\/h3>\n[h5p id=\"12\"]\n\n<\/div>\n

<\/a>Adrenergic Agonists<\/strong><\/h5>\nAdrenergic agonists stimulate Alpha-1, Alpha-2, Beta-1, or Beta-2 receptors. Stimulation of each type of receptor has different effects and are further explained below.\n\nAlpha-1 receptor agonists:<\/strong> Stimulation of Alpha-1 receptors causes vasoconstriction in the periphery, which increases blood pressure. Vasoconstriction also occurs in mucus membranes, which decreases swelling and secretions for patients experiencing upper respiratory infections. Examples of Alpha-1 agonist medications are pseudoephedrine or phenylephrine, used to treat nasal congestion.\n\nAlpha-2 receptor agonists:<\/strong> Stimulation of Alpha-2 receptors reduces CNS stimulation and is primarily used as an antihypertensive or a sedative. An example of an Alpha-2 agonist medication is clonidine, which is used to treat hypertension and is also used to treat attention deficit hyperactivity disorder.\n\nBeta-1 receptor agonists<\/strong>: Stimulation of Beta-1 receptors primarily affects the heart by increasing heart rate and contractility. It also causes the kidneys to release renin. Effects on the heart are described as having a positive [pb_glossary id=\"888\"]chronotropic[\/pb_glossary]<\/strong> (increases heart rate), positive [pb_glossary id=\"886\"] inotropic[\/pb_glossary]<\/strong> (increases force of contraction), and positive [pb_glossary id=\"887\"]dromotropic[\/pb_glossary]<\/strong> (increases speed of conduction between SA and AV node) properties. Medications that stimulate Beta-1 receptors are primarily used during cardiac arrest, acute heart failure, or shock. An example of a Beta-1 receptor agonist medication is dobutamine, which is used to increase cardiac output in someone experiencing acute heart failure or shock. See Figure 4.7[footnote]\"2018 Conduction System of Heart.jpg<\/a>\" by OpenStax College<\/a> is licensed under CC BY 3.0<\/a> [\/footnote]\u00a0 illustrating dromotropic properties of stimulating Beta-1 receptors.\n\n[caption id=\"\" align=\"aligncenter\" width=\"673\"]\"Illustration Figure 4.7 Dromotropic Properties Affect the Speed of Conduction Between SA and AV Nodes[\/caption]\n\nBeta-2 receptor agonists<\/strong>: Stimulation of Beta-2 receptors causes relaxation in smooth muscle in the lungs, GI, uterus, and liver. Medications that stimulate Beta-2 receptors are primarily used to promote bronchodilation, which opens the airway, and are often used to treat patients with asthma or chronic obstructive pulmonary disease (COPD). An example of a Beta-2 receptor agonist medication used in asthma is albuterol. See Figure 4.8[footnote]\"Bronchodilators<\/a>\" by BruceBlaus<\/a> is licensed under CC BY 4.0<\/a>[\/footnote]<\/sup>\u00a0 for an illustration of the effects of stimulating Beta-2 receptors in the lungs.\n\nSide effects of Beta-2 receptor agonists are related to stimulation of Beta-2 receptors in other locations in the body. For example, albuterol can cause tachycardia by stimulating Beta-2 receptors in the heart. Stimulation of Beta-2 receptors can also inadvertently cause [pb_glossary id=\"2190\"]hyperglycemia[\/pb_glossary] <\/strong>in patients with diabetes because of activation of Beta-2 receptors in the liver, causing [pb_glossary id=\"2195\"]glyconeogenesis[\/pb_glossary]<\/strong>.\n\n[caption id=\"\" align=\"aligncenter\" width=\"598\"]\"Images Figure 4.8 Effects of Medications Stimulating Beta 2 Receptors in the Lungs[\/caption]\n
<\/a>Adrenergic Antagonists<\/strong><\/h5>\nAdrenergic antagonist medications inhibit the Alpha-1, Alpha-2, Beta-1, and Beta-2 receptors. The effects of inhibition of each receptor are explained further below.\n\nAlpha-1 antagonists:<\/strong> Alpha-1 antagonists are primarily used to relax smooth muscle in the bladder and cause vasodilation.\n\nExamples include:\n