Autonomic Nervous System PDF 2024

Human Physiology 2024/10/09 The Autonomic Nervous System Dr. Wafaa Fawzi Lec.4 College of Pharmacy The Autonomic Nervous System and Homeostasis The autonomi...

Human Physiology 2024/10/09 The Autonomic Nervous System Dr. Wafaa Fawzi Lec.4 College of Pharmacy The Autonomic Nervous System and Homeostasis The autonomic nervous system contributes to homeostasis by responding to subconscious visceral sensations and exciting or inhibiting smooth muscle, cardiac muscle, and glands. The peripheral nervous system (PNS) includes cranial and spinal nerves and is divided into a somatic nervous system (SNS), autonomic nervous system (ANS), and enteric nervous system (ENS). Like the somatic nervous system, the autonomic nervous system (ANS) operates via reflex arcs. the output part of the ANS has two divisions: the sympathetic division and the parasympathetic division. Most organs have dual innervation; that is, they receive impulses from both sympathetic and parasympathetic neurons. A continual flow of nerve impulses from (1) autonomic sensory neurons in visceral organs and blood vessels propagate into (2) integrating centers in the CNS. Then, impulses in (3) autonomic motor neurons propagate to various effector tissues, thereby regulating the activity of smooth muscle, cardiac muscle, and many glands. (4) The enteric division is a specialized network of nerves and ganglia forming an independent nerve network within the wall of the gastrointestinal (GI) tract. However, centers in the hypothalamus and brain stem do regulate ANS reflexes. Autonomic Nervous System The main input to the ANS comes from autonomic (visceral) sensory neurons. Mostly, these neurons are associated with interoceptors, which are sensory receptors located in blood vessels, visceral organs, muscles, and the nervous system that monitor conditions in the internal environment. Examples of interoceptors are chemoreceptors that monitor blood CO2 level and mechanoreceptors that detect the degree of stretch in the walls of organs or blood vessels. In some organs, nerve impulses from one division of the ANS stimulate the organ to increase its activity (excitation), and impulses from the other division decrease the organ’s activity (inhibition). For example, an increased rate of nerve impulses from the sympathetic division increases heart rate, and an increased rate of nerve impulses from the parasympathetic division decreases heart rate. Changes in the diameter of the pupils, dilation and constriction of blood vessels and adjustment of the rate and force of the heartbeat are examples of autonomic motor responses. Unlike skeletal muscle, tissues innervated by the ANS often function to some extent even if their nerve supply is damaged. For example, the heart continues to beat when it is removed for transplantation into another person, smooth muscle in the lining of the gastrointestinal tract contracts rhythmically on its own, and glands produce some secretions in the absence of ANS control. Anatomy of autonomic motor pathways Anatomical Components Each division of the ANS has two motor neurons. The first of the two motor neurons in any autonomic motor pathway is called a preganglionic neuron (Fig.1b). Its cell body is in the 1 brain or spinal cord; its axon exits the CNS as part of a cranial or spinal nerve. The axon of a preganglionic neuron is myelinated fiber that usually extends to an autonomic ganglion (is a collection of neuronal cell bodies in the PNS), where it synapses with a postganglionic neuron, the second neuron in the autonomic motor pathway. Its cell body and dendrites are located in an autonomic ganglion, where it forms synapses with one or more preganglionic axons. The axon of a postganglionic neuron is unmyelinated fiber that terminates in a visceral effector. Thus, preganglionic neurons convey nerve impulses from the CNS to autonomic ganglia, and postganglionic neurons relay the impulses from autonomic ganglia to visceral effectors (smooth muscle, cardiac muscle, or a gland). Alternatively, in some autonomic pathways, the first motor neuron extends to specialized cells called chromaffin cells in the adrenal medullae (inner portions of the adrenal glands) rather than an autonomic ganglion. In addition, all somatic motor neurons release only acetylcholine (ACh) as their NT, but autonomic motor neurons release either ACh or norepinephrine (NE). Figure.1 Motor neuron pathways in the (a) somatic nervous system and (b) autonomic nervous system (ANS). Note that autonomic motor neurons release either acetylcholine (ACh) or norepinephrine (NE); somatic motor neurons release ACh. Preganglionic Neurons In the sympathetic division, the preganglionic neurons have their cell bodies in the lateral horns of the gray matter in the 12 thoracic segments and the first two (and sometimes three) lumbar segments of the spinal cord (Fig.2). For this reason, the sympathetic division is also called the thoracolumbar division, and the axons of the sympathetic preganglionic neurons are known as the thoracolumbar outflow. Cell bodies of preganglionic neurons of the parasympathetic division are located in the nuclei of 4 cranial nerves in the brain stem, specifically the oculomotor nerve, facial nerve, glossopharyngeal nerve, and vagus nerve (III, VII, IX, and X) and in the lateral gray matter of the second through three sacral segments of the spinal cord (S2–4), referred to as the pelvic splanchnic nerves, (Fig.3). Hence, the parasympathetic division is also known as the craniosacral division, and the axons of the parasympathetic preganglionic neurons are referred to as the craniosacral outflow. 2 Autonomic Ganglia: There are two major groups of autonomic ganglia: (1) sympathetic ganglia, which are components of the sympathetic division of the ANS, and (2) parasympathetic ganglia, which are components of the parasympathetic division of the ANS. Sympathetic ganglia The sympathetic ganglia are the sites of synapses between sympathetic preganglionic and postganglionic neurons. There are two major types of sympathetic ganglia: Sympathetic trunk ganglia and prevertebral ganglia. Sympathetic trunk ganglia (also called vertebral chain ganglia or paravertebral ganglia) lie in a vertical row on either side of the vertebral column. These ganglia extend from the base of the skull to the coccyx (Fig.2). Postganglionic axons from sympathetic trunk ganglia primarily innervate organs above the diaphragm, such as the head, neck, shoulders, and heart. Sympathetic trunk ganglia in the neck have specific names. They are the superior, middle, and inferior cervical ganglia. The remaining sympathetic trunk ganglia do not have individual names. Because the sympathetic trunk ganglia are near the spinal cord, most sympathetic preganglionic axons are short and most sympathetic postganglionic axons are long. The second group of sympathetic ganglia, the prevertebral (collateral) ganglia, lies anterior to the vertebral column and close to the large abdominal arteries. In general, postganglionic axons from prevertebral ganglia innervate organs below the diaphragm. There are five major prevertebral ganglia (Fig.2): (1) The celiac ganglion (2) The superior mesenteric ganglion (3) The inferior mesenteric ganglion (4) The aorticorenal ganglion and (5) the renal ganglion. A single sympathetic preganglionic fiber has many axon collaterals (branches) and may synapse with 20 or more postganglionic neurons. This pattern of projection is an example of divergence and helps explain why many sympathetic responses affect almost the entire body simultaneously. After exiting their ganglia, the postganglionic axons typically terminate in several visceral effectors. Parasympathetic ganglia Preganglionic axons of the parasympathetic division synapse with postganglionic neurons in terminal ganglia. Most of these ganglia are located close to or actually within the wall of a visceral organ, axons of parasympathetic preganglionic are long. Terminal ganglia in the head have specific names. They are the ciliary ganglion, pterygopalatine ganglion, submandibular ganglion, and otic ganglion (Fig.3). In the ganglion, the presynaptic neuron usually synapses with only four or five postsynaptic neurons, all of which supply a single visceral effector, allowing parasympathetic responses to be localized to a single effector. this pattern is called convergence. Autonomic Plexuses In the thorax, abdomen, and pelvis, axons of both sympathetic and parasympathetic neurons form tangled networks called autonomic plexuses, many of which lie along major arteries. The autonomic plexuses also may contain axons of autonomic sensory neurons. The major plexuses in the thorax are the cardiac plexus, which supplies the heart, and the pulmonary plexus, which supplies the bronchial tree (Fig.5). The abdomen and pelvis also contain major autonomic plexuses (Fig.4). The celiac plexus is the largest autonomic plexus and surrounds the celiac trunk (is the first major branch of the abdominal aortas) the abdominal aortas largest artery in the abdominal cavity, as part of the aorta, it is a direct continuation of the descending aorta (of the thorax). The celiac plexus contains two large celiac ganglia, 3 two aorticorenal ganglia, and a dense network of autonomic axons and is distributed to the stomach, spleen, pancreas, liver, gallbladder, kidneys, adrenal medullae, testes, and ovaries. The superior mesenteric plexus contains the superior mesenteric ganglion and supplies the small and large intestines. The inferior mesenteric plexus contains the inferior mesenteric ganglion, which innervates the large intestine. Axons mesenteric ganglion also extends through the hypogastric plexus to supply the pelvic viscera. The renal plexus contains the renal ganglion and supplies the renal arteries within the kidneys and ureters. ANS neurotransmitters and receptors Based on the NT they produce and release at synapse as well as at point of contact with visceral effectors, autonomic neurons are classified as either cholinergic or adrenergic. The receptors for the NTs are integral membrane proteins located in the plasma membrane of the postsynaptic neuron or effector cell. Cholinergic neurons and receptors Cholinergic neurons release the NT acetylcholine (ACh). In the ANS, the cholinergic neurons include (1) all sympathetic and parasympathetic preganglionic neurons, (2) sympathetic postganglionic neurons that innervate most sweat glands, and (3) all parasympathetic postganglionic neurons (Fig.5). ACh binds with specific cholinergic receptors, in the postsynaptic plasma membrane. The two types of cholinergic receptors, both of which bind ACh, are nicotinic receptors and muscarinic receptors. Nicotinic receptors are present in the plasma membrane of dendrites and cell bodies of both sympathetic and parasympathetic postganglionic neurons (Fig.6 a, b), the plasma membranes of chromaffin cells of the adrenal medullae, and in the motor end plate at the neuromuscular junction. Muscarinic receptors are present in the plasma membranes of all effectors (smooth muscle, cardiac muscle, and glands) innervated by parasympathetic postganglionic axons. In addition, most sweat glands receive their innervation from cholinergic sympathetic postganglionic neurons and possess muscarinic receptors (Fig.6 b). Activation of nicotinic receptors by ACh causes depolarization and thus excitation of the postsynaptic cell, which can be a postganglionic neuron, an autonomic effector. Activation of muscarinic receptors by ACh sometimes causes depolarization (excitation) and sometimes causes hyperpolarization (inhibition), depending on which particular cell bears the muscarinic receptors. For example, binding of ACh to muscarinic receptors inhibits (relaxes) smooth muscle sphincters in the gastrointestinal tract. By contrast, ACh excites muscarinic receptors in smooth muscle fibers in the circular muscles of the iris of the eye, causing them to contract. Adrenergic neurons and receptors In the ANS, adrenergic neurons release norepinephrine (NE), also known as noradrenalin (Fig.5 a). Most sympathetic postganglionic neurons are adrenergic. NE causing either excitation or inhibition of the effector cell. Adrenergic receptors bind both norepinephrine and epinephrine. The norepinephrine can be either released as a NT by sympathetic postganglionic neurons or released as a hormone into the blood by chromaffin cells of the adrenal medullae; epinephrine is released as a hormone. The two main types of adrenergic receptors are alpha (α) receptors and beta (β) receptors, which are found on visceral effectors innervated by most sympathetic postganglionic axons. 4 These receptors are further classified into subtypes- α1, α 2, β 1, β 2, and β 3. Cells of most effectors contain either alpha or beta receptors; some visceral effector cells contain both. Norepinephrine stimulates alpha receptors more strongly than beta receptors; epinephrine is a potent stimulator of both alpha and beta receptors. Compared to ACh, norepinephrine lingers in the synaptic cleft for a longer time. Thus, effects triggered by adrenergic neurons typically are longer lasting than those triggered by cholinergic neurons. Fig.2 Structure of the sympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated structures are shown for only one side of the body for diagrammatic purposes, the sympathetic division actually innervates tissues and organs on both sides. 5 Fig.3 Structure of the parasympathetic division of the autonomic nervous system. Solid lines represent preganglionic axons; dashed lines represent postganglionic axons. Although the innervated structures are shown only for one side of the body for diagrammatic purposes, the parasympathetic division actually innervates tissues and organs on both sides. Fig.4 Autonomic plexuses in the thorax, abdomen, and pelvis. 6 Fig.5. Cholinergic neurons and adrenergic neurons in the sympathetic and parasympathetic divisions. Physiology of the ANS Autonomic tone Most body organs receive innervation from both divisions of the ANS, which typically work in opposition to one another. The balance between sympathetic and parasympathetic activity, called autonomic tone, is regulated by the hypothalamus. Typically, the hypothalamus turns up sympathetic tone at the same time it turns down parasympathetic tone, and vice versa. The two divisions can affect body organs differently because their postganglionic neurons release different neurotransmitters and because the effector organs possess different adrenergic and cholinergic receptors. Sympathetic Responses During physical or emotional stress, the sympathetic division dominates the parasympathetic division. High sympathetic tone favors body functions that can support vigorous physical activity and rapid production of ATP. Besides physical exertion, various emotions-such as fear, embarrassment, or rage- stimulate the sympathetic division. Visualizing body changes that occur during “E situations” such as exercise, emergency, excitement, and embarrassment will help you remember most of the sympathetic responses. Activation of the sympathetic division and release of hormones by the adrenal medullae set in motion a series of physiological responses collectively called the fight-or-flight response, which includes the following effects: The pupils of the eyes dilate. Heart rate, force of heart contraction, and blood pressure increase. The airways dilate, allowing faster movement of air into and out of the lungs. The blood vessels that supply the kidneys and gastrointestinal tract constrict, which decreases blood flow through these tissues. The result is a slowing of urine formation and digestive activities, which are not essential during exercise. Blood vessels that supply organs involved in exercise or fighting off danger-skeletal muscles, cardiac muscle, liver, and adipose tissue-dilate, allowing greater blood flow through these tissues. Liver cells perform glycogenolysis (breakdown of glycogen to glucose), and adipose tissue cells perform lipolysis (breakdown of triglycerides to fatty acids and glycerol). Release of glucose by the liver increases blood glucose level. 7  Processes that are not essential for meeting the stressful situation are inhibited. For example, muscular movements of the gastrointestinal tract and digestive secretions slow down or even stop. The effects of sympathetic stimulation are longer lasting and more widespread than the effects of parasympathetic stimulation for three reasons: (1) Sympathetic postganglionic axons diverge more extensively; as a result, many tissues are activated simultaneously. (2) Acetylcholinesterase quickly inactivates acetylcholine, but norepinephrine lingers in the synaptic cleft for a longer period. (3) Epinephrine and norepinephrine secreted into the blood from the adrenal medullae intensify and prolong the responses caused by NE liberated from sympathetic postganglionic axons. These blood-borne hormones circulate throughout the body, affecting all tissues that have alpha and beta receptors. In time, blood-borne NE and epinephrine are destroyed by enzymes in the liver. Parasympathetic Responses The parasympathetic division enhances rest-and-digest activities. Parasympathetic responses support body functions that conserve and restore body energy during times of rest and recovery. In the quiet intervals between periods of exercise, parasympathetic impulses to the digestive glands and the smooth muscle of the gastrointestinal tract predominate over sympathetic impulses. This allows energy-supplying food to be digested and absorbed. At the same time, parasympathetic responses reduce body functions that support physical activity. The acronym SLUDD can be helpful in remembering five parasympathetic responses. It stands for salivation (S), lacrimation (L), urination (U), digestion (D), and defecation (D). All of these activities are stimulated mainly by the parasympathetic division. Besides the increasing SLUDD responses, other important parasympathetic responses are “three decreases”: decreased heart rate, decreased diameter of airways (bronchoconstriction), and decreased diameter of the pupils. 8