The Autonomic Nervous System Chapter 15 The Autonomic Nervous System

INTRODUCTION The autonomic nervous system (ANS) operates via reflex arcs. Operation of the ANS to maintain homeostasis, however, depends on a continual flow of sensory afferent input, from receptors in organs, and efferent motor output to the same effector organs. Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons. Functionally, the ANS usually operates without conscious control. The ANS is regulated by the hypothalamus and brain stem. Principles of Human Anatomy and Physiology, 11e

Chapter 15 The Autonomic Nervous System Regulate activity of smooth muscle, cardiac muscle & certain glands Structures involved general visceral afferent neurons general visceral efferent neurons integration center within the brain Receives input from limbic system and other regions of the cerebrum Principles of Human Anatomy and Physiology, 11e

SOMATIC AND AUTONOMIC NERVOUS SYSTEMS The somatic nervous system contains both sensory and motor neurons. The somatic sensory neurons receive input from receptors of the special and somatic senses. These sensations are consciously perceived. Somatic motor neurons innervate skeletal muscle to produce conscious, voluntary movements. The effect of a motor neuron is always excitation. Principles of Human Anatomy and Physiology, 11e

SOMATIC AND AUTONOMIC NERVOUS SYSTEMS The autonomic nervous system contains both autonomic sensory and motor neurons. Autonomic sensory input is not consciously perceived. The autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities of cardiac muscle, smooth muscle, and glands. Most autonomic responses can not be consciously altered or suppressed. Principles of Human Anatomy and Physiology, 11e

SOMATIC vs AUTONOMIC NERVOUS SYSTEMS All somatic motor pathways consist of a single motor neuron Autonomic motor pathways consists of two motor neurons in series The first autonomic neuron motor has its cell body in the CNS and its myelinated axon extends to an autonomic ganglion. It may extend to the adrenal medullae rather than an autonomic ganglion The second autonomic motor neuron has its cell body in an autonomic ganglion; its nonmyelinated axon extends to an effector. Principles of Human Anatomy and Physiology, 11e

Somatic versus Autonomic NS Principles of Human Anatomy and Physiology, 11e

Basic Anatomy of ANS Preganglionic neuron cell body in brain or spinal cord axon is myelinated fiber that extends to autonomic ganglion Postganglionic neuron cell body lies outside the CNS in an autonomic ganglion axon is unmyelinated fiber that terminates in a visceral effector Principles of Human Anatomy and Physiology, 11e

Sympathetic vs. Parasympathetic NS Principles of Human Anatomy and Physiology, 11e

AUTONOMIC NERVOUS SYSTEM The output (efferent) part of the ANS is divided into two principal parts: the sympathetic division the parasympathetic division Organs that receive impulses from both sympathetic and parasympathetic fibers are said to have dual innervation. Table 15.1 summarizes the similarities and differences between the somatic and autonomic nervous systems. Principles of Human Anatomy and Physiology, 11e

Sympathetic ANS vs. Parasympathetic ANS Principles of Human Anatomy and Physiology, 11e

Divisions of the ANS 2 major divisions parasympathetic sympathetic Dual innervation one speeds up organ one slows down organ Sympathetic NS increases heart rate Parasympathetic NS decreases heart rate Principles of Human Anatomy and Physiology, 11e

Divisions of the ANS 2 major divisions parasympathetic sympathetic Dual innervation one speeds up organ one slows down organ Sympathetic NS increases heart rate Parasympathetic NS decreases heart rate Principles of Human Anatomy and Physiology, 11e

Sympathetic Ganglia These ganglia include the sympathetic trunk or vertebral chain or paravertebral ganglia that lie in a vertical row on either side of the vertebral column (Figures 15.2). Other sympathetic ganglia are the prevertebral or collateral ganglia that lie anterior to the spinal column and close to large abdominal arteries. celiac superior mesenteric inferior mesenteric ganglia (Figures 15.2 and 15.4). Principles of Human Anatomy and Physiology, 11e

Parasympathetic Ganglia Parasympathetic ganglia are the terminal or intramural ganglia that are located very close to or actually within the wall of a visceral organ. Examples of terminal ganglia include (Figure 15.3) ciliary, submandibular, otic ganglia Principles of Human Anatomy and Physiology, 11e

Sympathetic ANS vs. Parasympathetic ANS Principles of Human Anatomy and Physiology, 11e

Dual Innervation, Autonomic Ganglia Parasympathetic (craniosacral) division preganglionic cell bodies in nuclei of 4 cranial nerves and the sacral spinal cord Ganglia terminal ganglia in wall of organ Sympathetic (thoracolumbar) division preganglionic cell bodies in thoracic and first 2 lumbar segments of spinal cord Ganglia trunk (chain) ganglia near vertebral bodies prevertebral ganglia near large blood vessel in gut (celiac, superior mesenteric, inferior mesenteric) Principles of Human Anatomy and Physiology, 11e

Structures of Sympathetic NS Preganglionic cell bodies at T1 to L2 Postganglionic cell bodies sympathetic chain ganglia along the spinal column prevertebral ganglia at a distance from spinal cord celiac ganglion superior mesenteric ganglion inferior mesenteric ganglion Principles of Human Anatomy and Physiology, 11e

Postganglionic Neurons: Sympathetic vs. Parasympathetic Sympathetic preganglionic neurons pass to the sympathetic trunk. They may connect to postganglionic neurons in the following ways. (Figure 17.5). May synapse with postganglionic neurons in the ganglion it first reaches. May ascend or descend to a higher of lower ganglion before synapsing with postganglionic neurons. May continue, without synapsing, through the sympathetic trunk ganglion to a prevertebral ganglion where it synapses with the postganglionic neuron. Parasympathetic preganglionic neurons synapse with postganglionic neurons in terminal ganglia (Figure 17.3). Principles of Human Anatomy and Physiology, 11e

Pathways of Sympathetic Fibers Spinal nerve route out same level Sympathetic chain route up chain & out spinal nerve Collateral ganglion route out splanchnic nerve to collateral ganglion Principles of Human Anatomy and Physiology, 11e

Organs Innervated by Sympathetic NS Structures innervated by each spinal nerve sweat glands, arrector pili mm., blood vessels to skin & skeletal mm. Thoracic & cranial plexuses supply: heart, lungs, esophagus & thoracic blood vessels plexus around carotid artery to head structures Splanchnic nerves to prevertebral ganglia supply: GI tract from stomach to rectum, urinary & reproductive organs Principles of Human Anatomy and Physiology, 11e

Ganglia & Plexuses of Sympathetic NS Principles of Human Anatomy and Physiology, 11e

Structure of the Parasympathetic Division Preganglionic axons extend from the brain stem in four cranial nerves. (Figure 15.3). The cranial outflow consists of four pairs of ganglia and the plexuses associated with the vagus (X) nerve. The sacral parasympathetic outflow consists of preganglionic axons in the anterior roots of the second through fourth sacral nerves and they form the pelvic splanchnic nerve. (Figure15.3) Principles of Human Anatomy and Physiology, 11e

Anatomy of Parasympathetic NS Preganglionic cell bodies found in 4 cranial nerve nuclei in brainstem S2 to S4 spinal cord Postganglionic cell bodies very near or in the wall of the target organ in a terminal ganglia Principles of Human Anatomy and Physiology, 11e

Parasympathetic Cranial Nerves Oculomotor nerve ciliary ganglion in orbit ciliary muscle & pupillary constrictor muscle inside eyeball Facial nerve pterygopalatine and submandibular ganglions supply tears, salivary & nasal secretions Glossopharyngeal otic ganglion supplies parotid salivary gland Vagus nerve supply heart, pulmonary and GI tract as far as the midpoint of the colon Principles of Human Anatomy and Physiology, 11e

Parasympathetic Sacral Nerve Fibers Form pelvic splanchnic nerves Preganglionic fibers end on terminal ganglia in walls of target organs Innervate smooth muscle and glands in colon, ureters, bladder & reproductive organs Principles of Human Anatomy and Physiology, 11e

ANS Neurotransmitters Classified as either cholinergic or adrenergic neurons based upon the neurotransmitter released Adrenergic Cholinergic Principles of Human Anatomy and Physiology, 11e

Cholinergic Neurons and Receptors Cholinergic neurons release acetylcholine all preganglionic neurons all parasympathetic postganglionic neurons few sympathetic postganglionic neurons (to most sweat glands) Excitation or inhibition depending upon receptor subtype and organ involved. Principles of Human Anatomy and Physiology, 11e

Cholinergic Neurons and Receptors The two types of cholinergic receptors are nicotinic and muscarinic receptors (Figure 15.6 a , b). Activation of nicotinic receptors causes excitation of the postsynaptic cell. Nicotinic receptors are found on dendrites & cell bodies of autonomic NS cells (and at NMJ.) Activation of muscarinic receptors can cause either excitation or inhibition depending on the cell that bears the receptors. Muscarinic receptors are found on plasma membranes of all parasympathetic effectors Principles of Human Anatomy and Physiology, 11e

Adrenergic Neurons and Receptors Adrenergic neurons release norepinephrine (NE) from postganglionic sympathetic neurons only Excites or inhibits organs depending on receptors NE lingers at the synapse until enzymatically inactivated by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT) Principles of Human Anatomy and Physiology, 11e

Adrenergic Neurons and Receptors The main types of adrenergic receptors are alpha and beta receptors. These receptors are further classified into subtypes. Alpha1 and Beta1 receptors produce excitation Alpha2 and Beta2 receptors cause inhibition Beta3 receptors (brown fat) increase thermogenesis Effects triggered by adrenergic neurons typically are longer lasting than those triggered by cholinergic neurons. Table 15.2 describes the location of the subtypes of cholinergic and adrenergic receptors and summarizes the responses that occur when each type of receptor is activated. Principles of Human Anatomy and Physiology, 11e

Receptor Agonists and Antagonists An agonist is a substance that binds to and activates a receptor, mimicking the effect of a natural neurotransmitter or hormone. An antagonist is a substance that binds to and blocks a receptor, preventing a natural neurotransmitter or hormone from exerting its effect. Drugs can serve as agonists or antagonists to selectively activate or block ANS receptors. Principles of Human Anatomy and Physiology, 11e

Physiological Effects of the ANS Most body organs receive dual innervation innervation by both sympathetic & parasympathetic Hypothalamus regulates balance (tone) between sympathetic and parasympathetic activity levels Some organs have only sympathetic innervation sweat glands, adrenal medulla, arrector pili mm & many blood vessels Principles of Human Anatomy and Physiology, 11e

Sympathetic Responses Dominance by the sympathetic system is caused by physical or emotional stress -- “E situations” emergency, embarrassment, excitement, exercise Alarm reaction = flight or fight response dilation of pupils increase of heart rate, force of contraction & BP decrease in blood flow to nonessential organs increase in blood flow to skeletal & cardiac muscle airways dilate & respiratory rate increases blood glucose level increase Long lasting due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland Principles of Human Anatomy and Physiology, 11e

Parasympathetic Responses Enhance “rest-and-digest” activities Mechanisms that help conserve and restore body energy during times of rest Normally dominate over sympathetic impulses SLUDD type responses = salivation, lacrimation, urination, digestion & defecation and 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil Paradoxical fear when there is no escape route or no way to win causes massive activation of parasympathetic division loss of control over urination and defecation Principles of Human Anatomy and Physiology, 11e

Autonomic or Visceral Reflexes A visceral autonomic reflex adjusts the activity of a visceral effector, often unconsciously. changes in blood pressure, digestive functions etc filling & emptying of bladder or defecation Autonomic reflexes occur over autonomic reflex arcs. Components of that reflex arc: sensory receptor sensory neuron integrating center pre & postganglionic motor neurons visceral effectors Principles of Human Anatomy and Physiology, 11e

Control of Autonomic NS Not aware of autonomic responses because control center is in lower regions of the brain Hypothalamus is major control center input: emotions and visceral sensory information smell, taste, temperature, osmolarity of blood, etc output: to nuclei in brainstem and spinal cord posterior & lateral portions control sympathetic NS increase heart rate, inhibition GI tract, increase temperature anterior & medial portions control parasympathetic NS decrease in heart rate, lower blood pressure, increased GI tract secretion and mobility Principles of Human Anatomy and Physiology, 11e

Autonomic versus Somatic NS - Review Somatic nervous system consciously perceived sensations excitation of skeletal muscle one neuron connects CNS to organ Autonomic nervous system unconsciously perceived visceral sensations involuntary inhibition or excitation of smooth muscle, cardiac muscle or glandular secretion two neurons needed to connect CNS to organ preganglionic and postganglionic neurons Principles of Human Anatomy and Physiology, 11e