Ch 15 The Autonomic Nervous System PDF

Chapter 15 The Autonomic Nervous System and Visceral Reflexes PowerPoint slides adapted from Saladin by Stephen Runyan, Ph.D. Copyright © McGraw-Hill Education. Permission required for reproduction or display. 1 General Properties of the Autonomic Nervous System...

Chapter 15 The Autonomic Nervous System and Visceral Reflexes PowerPoint slides adapted from Saladin by Stephen Runyan, Ph.D. Copyright © McGraw-Hill Education. Permission required for reproduction or display. 1 General Properties of the Autonomic Nervous System The autonomic nervous system (ANS) is a motor nervous system that controls glands, cardiac muscle, and smooth muscle – Also called visceral motor system – Primary organs of the ANS: Viscera of thoracic and abdominal cavities Some structures of the body wall – Cutaneous blood vessels – Sweat glands – Piloerector muscles 2 General Properties of the Autonomic Nervous System Autonomic nervous system (ANS) – It carries out actions involuntarily - without our conscious intent or awareness Visceral effectors do not depend on the ANS to function; only to adjust their activity to the body’s changing needs Unlike the somatic nervous system, it exhibits denervation hypersensitivity—exaggerated responses of cardiac and smooth muscle if autonomic nerves are severed 3  Visceral Reflexes The ANS is responsible for visceral reflexes— unconscious, automatic, stereotyped responses to stimulation involving visceral receptors and effectors Slower than somatic reflexes Visceral reflex arcs include the following: 1. Receptors: nerve endings that detect stretch, tissue damage, blood chemicals, body temperature, and other internal stimuli 2. Afferent neurons: lead to CNS 3. Integrating center: interneurons in the CNS 4. Efferent neurons: carry motor signals away from the CNS 5. Effectors: carry out end response 4  Visceral Reflexes Baroreflex: (1) high blood pressure detected by arterial stretch receptors; (2) afferent neuron carries signal to CNS; (3) efferent signals of ANS travel to the heart via the vagus nerve; (4) heart then slows, reducing blood pressure This an example of a homeostatic negative feedback loop Figure 15.1 5  Divisions of the ANS The ANS has two divisions that differ in anatomy and function but often innervate the same target organs – May have cooperative or contrasting effects Sympathetic division - “Fight-or-flight” – Prepares the body for physical activity – stimulated by exercise, trauma, arousal, competition, anger, or fear Increases alertness, heart rate, BP, airflow, blood glucose levels, blood flow to skeletal and cardiac muscle etc. Reduces blood flow to the skin and digestive tract Parasympathetic division - “Resting and digesting” – Calms many body functions reducing energy expenditure and assists in bodily maintenance Digestion and waste elimination 6  Divisions of the ANS The ANS exhibits autonomic tone which is a normal background rate of activity that represents the balance of the two systems according to the body’s needs – Parasympathetic tone Maintains smooth muscle tone in intestines Holds resting heart rate down to about 70 to 80 beats per minute – if the vagus nerves are cut, the heart beats at its intrinsic rate of ~100 beats/min. – Sympathetic tone Keeps most blood vessels partially constricted and maintains blood pressure Neither system is purely excitatory or inhibitory – the sympathetic division excites the heart but inhibits digestive and urinary function, while parasympathetic has the opposite effect 7  Autonomic Output Pathways ANS contrasts to somatic motor pathway – In somatic pathways A single motor neuron from brainstem or spinal cord issues a myelinated axon that reaches all the way to skeletal muscle 8  Autonomic Output Pathways – In autonomic pathways The signal must travel across two neurons to get to the target organ – these two neurons meet in an autonomic ganglion The presynaptic neuron is the first neuron with a soma in the brainstem or spinal cord The second is the postganglionic neuron whose axon extends the rest of the way to the target cell 9 Autonomic Output Pathways Table 15.1 10 ANS versus Somatic Nervous System Both have motor fibers but differ in: – Effectors – Efferent pathways and ganglia – Target organ responses to neurotransmitters 11 Anatomy of the Sympathetic Division Also called the thoracolumbar division because it arises from the thoracic and lumbar (T1 to L2) spinal cord Relatively short preganglionic and long postganglionic fibers Preganglionic neurosomas are located in the lateral horns of the spinal cord gray matter – Lead to nearby sympathetic chain of ganglia (paravertebral ganglia) Series of longitudinal ganglia adjacent to both sides of the vertebral column 12  The Sympathetic Division Each paravertebral ganglion is connected to a spinal nerve by two branches: communicating rami – Preganglionic fibers are small myelinated fibers that travel from spinal nerve to the ganglion by way of the white communicating ramus (myelinated) – Postganglionic fibers leave the ganglion by way of the gray communicating ramus (unmyelinated) Forms a bridge back to the spinal nerve – Postganglionic fibers extend the rest of the way to the target organ 13  The Sympathetic Division After entering the sympathetic chain, the preganglionic fibers may follow any of three courses 1. Some end in ganglia which they enter and synapse immediately with a postganglionic neuron 2. Some travel up or down the chain and synapse in ganglia at other levels These fibers link the paravertebral ganglia into a chain 3. Some pass through the chain without synapsing and continue as splanchnic nerves 14  The Sympathetic Division Nerve fibers leave the sympathetic chain by three routes: 1. Spinal nerve route Some postganglionic fibers exit a ganglion by way of the gray ramus Return to the spinal nerve and travel the rest of the way to the target organ Most sweat glands, piloerector muscles, and blood vessels of the skin and skeletal muscles 15  The Sympathetic Division 2. Sympathetic nerve route Other nerves leave by way of sympathetic nerves that extend to the heart, lungs, esophagus, and thoracic blood vessels Issue fibers from there to the effectors in the head – Sweat, salivary, nasal glands; piloerector muscles; blood vessels; dilators of iris Some fibers form cardiac nerves to the heart 16  The Sympathetic Division 3. Splanchnic nerve route Some fibers pass through the sympathetic ganglia without synapsing – Continue on as the splanchnic nerves – Lead to second set of ganglia: collateral (prevertebral) ganglia and synapse there 17  The Sympathetic Division Collateral ganglia contribute to a network called the abdominal aortic plexus – Three major collateral ganglia in this plexus Celiac, superior mesenteric, and inferior mesenteric 18  The Sympathetic Division Postganglionic fibers accompany arteries of the same names and their branches to their target organs – Solar plexus: collective name for the celiac and superior mesenteric ganglia Nerves radiate from ganglia like rays of the sun 19  The Sympathetic Division The sympathetic division exhibits both neuronal convergence and neuronal divergence – Each preganglionic cell branches and synapses on 10 to 20 postganglionic cells – One preganglionic neuron can excite multiple postganglionic fibers leading to different target organs – Has relatively widespread effects 20  The Adrenal Glands Paired adrenal (suprarenal) glands located on superior poles of kidneys Each is two glands with different functions – Adrenal cortex (outer layer) Secretes steroid hormones – Adrenal medulla (inner core) Essentially a sympathetic ganglion consisting of modified postganglionic neurons (without fibers) Stimulated by preganglionic sympathetic neurons Sympathoadrenal system is the name for the adrenal medulla and sympathetic nervous system Secretes a mixture of hormones into bloodstream Catecholamines—85% epinephrine (adrenaline) and 15% norepinephrine (noradrenaline) 21  The Parasympathetic Division The parasympathetic division is also called the craniosacral division – Arises from the brain and sacral (S2 – S4) spinal cord Preganglionic fibers end in terminal ganglia in or near target organs – Long preganglionic, short postganglionic fibers 22  The Parasympathetic Division The parasympathetic division is relatively selective in stimulation of target organ – There is only a little neural divergence (less than divergence exhibited by sympathetic division) 23  Parasympathetic Cranial Nerves Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Preganglionic neurons Pterygopalatine Oculomotor nerve (III) ganglion Postganglionic neurons Oculomotor n. Ciliary ganglion Lacrimal gland – Narrows pupil and focuses lens (CN III) Eye Facial n. (CN VII) Submandibular ganglion Otic ganglion Submandibular salivary gland Parotid Facial nerve (VII) – Tear, nasal, and salivary glands Glossopharyngeal n. salivary gland (CN IX) Vagus n. (CN X) Cardiac plexus Heart Glossopharyngeal nerve (IX) – Parotid salivary gland Pulmonary plexus Regions of spinal cord Esophageal Cervical plexus Lung Vagus nerve (X) Thoracic Lumbar Celiac ganglion Stomach Sacral Abdominal aortic plexus Liver and gallbladder – Viscera as far as proximal half of colon Spleen Pelvic Pancreas – Cardiac, pulmonary, and splanchnic nerves Kidney and ureter esophageal plexuses Transverse colon Inferior Descending Hypogastric colon plexus Small intestine Rectum Pelvic nerves Ovary Penis Bladder Figure 15.7 24 Scrotum Uterus The Parasympathetic Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Pterygopalatine Preganglionic neurons Division ganglion Postganglionic neurons Oculomotor n. Ciliary ganglion Lacrimal gland (CN III) Eye Facial n. (CN VII) Submandibular ganglion Submandibular The parasympathetic fibers salivary gland Otic ganglion Parotid Glossopharyngeal n. salivary gland from S2 to S4 of the spinal (CN IX) Vagus n. cord form pelvic splanchnic (CN X) Heart Cardiac plexus nerves that lead to the Regions of Pulmonary plexus inferior hypogastric plexus spinal cord Cervical Esophageal plexus Lung Thoracic Lumbar Celiac ganglion Stomach Pelvic nerves run through Sacral Abdominal Liver and aortic gallbladder plexus the inferior hypogastric Pelvic Spleen Pancreas plexus to their terminal splanchnic nerves Kidney and ureter ganglion on the target organs Inferior Transverse colon Descending – Distal half of colon, rectum, Hypogastric colon plexus Small intestine urinary bladder, and Rectum reproductive organs Pelvic nerves Penis Bladder Ovary Scrotum Uterus 25 Figure 15.7 Comparison of Autonomic Divisions Table 15.3 26  The Enteric Nervous System The enteric nervous system is the nervous system of the digestive tract – It does not arise from the brainstem or spinal cord (no CNS components) – Innervates smooth muscle and glands Composed of neurons found in the walls of the digestive tract Exhibits local reflex arcs Regulates motility of the esophagus, stomach, and intestines and secretion of digestive enzymes and acid Normal digestive function also requires regulation by sympathetic and parasympathetic systems 27  Megacolon Hirschsprung disease is a hereditary defect causing absence of enteric nervous system – No innervation in the sigmoid colon and rectum – Constricts permanently prevents passage of feces – Feces becomes impacted above constriction – Megacolon results - a massive dilation of the bowel accompanied by abdominal distension and chronic constipation – May result in colonic gangrene, perforation of bowel, and peritonitis – Megacolon may also be caused by parasites from kissing bugs in South America that lead to Chagas disease. 28 Autonomic Effects on Target Organs How the autonomic nervous system have contrasting effects on organs? – e.g. norepinephrine from the sympathetic nervous system constricts blood vessels but dilates the bronchioles of the lungs Two fundamental reasons: – Sympathetic and parasympathetic fibers secrete different neurotransmitters (norepinephrine and acetylcholine) – The receptors on target cells vary Target cells respond to the same neurotransmitter differently depending on the type of receptor they have for it There are two different classes of receptors for acetylcholine and two classes of receptors for norepinephrine 29  Neurotransmitters and Their Receptors Acetylcholine (ACh) is secreted by all preganglionic neurons in both divisions and by postganglionic parasympathetic neurons – A few sympathetic postganglionic neurons also secrete ACh – sweat glands and some blood vessels – Axons that secrete ACh are called cholinergic fibers – Any receptor that binds ACh is called a cholinergic receptor 30 Neurotransmitters and Their Receptors Two types of cholinergic receptors – Muscarinic receptors (agonist = muscarine) All cardiac muscle, smooth muscle, and gland cells that receive cholinergic innervation have muscarinic receptors Can be excitatory or inhibitory due to subclasses of muscarinic receptors (excites intestinal smooth muscle and inhibits cardiac muscle) Functions through second messenger systems Antagonist = atropine – Nicotinic receptors (agonist = nicotine) On all ANS postganglionic neurons, in the adrenal medulla, and at neuromuscular junctions of skeletal muscle Excitatory when ACh binding occurs Receptors are ligand-gated ion channels Antagonist = curare 31 Neurotransmitters and Their Receptors Norepinephrine (NE) is secreted by nearly all sympathetic postganglionic neurons – Called adrenergic fibers – Receptors for NE are called adrenergic receptors 32 Neurotransmitters and Their Receptors Two types of adrenergic receptors – Alpha-adrenergic receptors Usually excitatory – constricts dermal blood vessels but inhibits intestinal motility Two subclasses use different second messengers (α1 and α2) – Beta-adrenergic receptors Usually inhibitory – dilates bronchioles but increases heart rate and contractility Two subclasses with different effects, but both act through cAMP as a second messenger (β1 and β2) 33  Neurotransmitters and Their Receptors Autonomic effects on glandular secretion are often an indirect result of their effect on blood vessels – Vasodilation: increased blood flow; increased secretion – Vasoconstriction: decreased blood flow; decreased secretion Sympathetic effects tend to last longer than parasympathetic effects – ACh is quickly broken down by acetylcholinesterase – effects last only seconds – NE by is reabsorbed by nerve terminals, diffuses to adjacent tissues, and much passes into bloodstream where it may exert effects for several minutes 34 Neurotransmitters and Their Receptors Many substances are released as neurotransmitters that modulate ACh and NE function – Sympathetic fibers may also secrete enkephalin, substance P, neuropeptide Y, somatostatin, neurotensin, or gonadotropin-releasing hormone – Some parasympathetic fibers stimulate endothelial cells to release the gas nitric oxide, which causes vasodilation by inhibiting smooth muscle tone Function is crucial to penile erection 35  Dual Innervation Dual innervation—most viscera receive nerve fibers from both parasympathetic and sympathetic divisions Both divisions do not normally innervate an organ equally – Parasypmathetic exerts more influence on digestive organs – Sympathetic has greater effect on ventricular muscle of heart 36  Dual Innervation Antagonistic effects oppose each other – Exerted through dual innervation of same effector cells Heart rate decreases (parasympathetic) Heart rate increases (sympathetic) – Exerted because each division innervates different cells Pupillary dilator muscle (sympathetic) dilates pupil Constrictor pupillae (parasympathetic) constricts pupil 37  Dual Innervation Cooperative effects result when the two divisions act on different effectors to produce a unified effect – Parasympathetics increase salivary serous cell secretion (enzyme-rich) – Sympathetics increase salivary mucous cell secretion – Both the enzymes and mucus are necessary components of saliva 38  Control Without Dual Innervation Some effectors receive only sympathetic fibers – Adrenal medulla, arrector pili muscles, sweat glands, and many blood vessels The most significant example of control without dual innervation is the regulation of blood pressure and routes of blood flow 39  Control Without Dual Innervation Sympathetic fibers to a blood vessel have a baseline firing rate, which keeps the vessels in a state of partial constriction called vasomotor tone – An increase in firing frequency results in vasoconstriction – A decrease in firing frequency results in vasodilation – Thus, the sympathetic division alone exerts opposite effects on the vessels 40  Control Without Dual Innervation Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The sympathetic division can shift blood flow from one organ Artery to another as needed 1 Sympathetic nerve fiber 1 Strong sympathetic tone 2 It dilates blood vessels to 3 2 Smooth muscle contraction skeletal muscles and heart in Vasomotor tone 3 Vasoconstriction times of emergency (a) Vasoconstriction Blood vessels to skin 1 vasoconstrict to minimize 1 Weaker sympathetic bleeding if injury occurs during 2 tone 2 Smooth muscle emergency 3 relaxation 3 Vasodilation (b) Vasodilation Figure 15.10 41 Central Control of Autonomic Function ANS regulated by several levels of CNS – Cerebral cortex has an influence: anger, fear, anxiety Powerful emotions influence the ANS because of the connections between our limbic system and the hypothalamus – The hypothalamus is the major visceral motor control center Nuclei for primitive functions—hunger, thirst, sex 42 Central Control of Autonomic Function – Midbrain, pons, and medulla oblongata contain: Nuclei for cardiac and vasomotor control, salivation, swallowing, sweating, bladder control, and pupillary changes – Spinal cord reflexes Defecation and micturition reflexes are integrated in spinal cord 43  Drugs and the Nervous System Neuropharmacology is the study of effects of drugs on the nervous system Sympathomimetics enhance sympathetic activity – Stimulate receptors or increase norepinephrine release Cold medicines that dilate the bronchioles or constrict nasal blood vessels Sympatholytics suppress sympathetic activity – Block receptors or inhibit norepinephrine release Beta blockers reduce high BP interfering with effects of epinephrine/norepinephrine on heart and blood vessels 44  Drugs and the Nervous System Parasympathomimetics enhance activity while parasympatholytics suppress parasympathetic activity Many drugs also act on neurotransmitters in CNS – Prozac blocks reuptake of serotonin to prolong its mood- elevating effect (SSRI) – Amphetamines chemically resemble norepinephrine and dopamine, associated with elevated mood – Cocaine blocks dopamine reuptake leading to a rush of good feelings 45  Drugs and the Nervous System Caffeine competes with adenosine (the presence of which causes sleepiness) by binding to its receptors This prevents adenosine from inhibiting ACh secretion. More ACh is secreted, and a person feels more alert. 46