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CHAPTER 15 THE AUTONOMIC NERVOUS SYSTEM I. REVIEW OF SOMATIC & AUTONOMIC NERVOUS SYSTEMS SOMATIC --Sensory neurons relay info for special/somatic senses (consciously perceived) --Motor neurons innervate skeletal muscles voluntary movement **always excitatory **damage results in lack of stimulation lack of muscle tone --Axon of single motor neuron extends all the way from CNS to fibers of motor unit --All motor neurons release ACh only AUTONOMIC --Sensory receptors (interoceptors) in blood vessels, viscera, and muscles monitor internal changes **chemo/mechano receptors **not consciously perceived --Motor neurons regulate visceral activities by increasing or decreasing activities in effectors **can still function if damaged **cannot consciously change responses --Motor pathways consist of 2 motor neurons in series **1st has cell body in CNS & myelinated axon extends to autonomic ganglion **2nd has cell body in ganglion & axon extends to effector organ **in some pathways, 1st neuron terminates on adrenal medulla --Motor neurons release either ACh or NE --2 motor divisions **sympathetic **parasympathetic **most effectors receive dual innervation nerve impulses from one division increase activity of an organ and from other division, decrease activity II. ANATOMY OF AUTONOMIC MOTOR PATHWAYS A. Functional Divisions 1. Sympathetic division = “accelerator” 2. Parasympathetic division = “brakes” B. Neuron Characteristics 1. Sympathetic division a. preganglionic neuron has its cell body in gray matter of thoracolumbar region (T1-L2) of spinal cord i. synapse on autonomic ganglion outside spinal cord ii. axons usually short iii. sympathetic trunk ganglia = collections of ganglia lateral to vertebral column (form sympathetic chains— Fig 15.2) iv. collateral ganglia are located about ½ way to effector organs (outside chain) b. postganglionic neuron continues from ganglion to effector organ i. much longer than preganglionic fibers ii. terminate in varicosities c. single preganglionic fiber has many branches (20 or more postganglionic fibers extending from single ganglia) allows for widespread, simultaneous response 2. Parasympathetic division a. preganglionic neurons originate in brain/brain stem or sacral spinal cord i. axons = craniosacral outflow ii. longer axons than in sympathetic division iii. synapse a terminal ganglion on or near effector organ b. postganglionic axons are short & terminate in varicosities on effector organ c. preganglionic neuron synapses with only 4 or 5 postganglionic neurons supply single effector & allow localized responses C. Sympathetic structure 1. Preganglionic neurons exit spinal cord thru anterior roots along with somatic motor neurons 2. Preganglionic neurons can synapse w/ postganglionic neurons in sympathetic trunk either at the level of entry or at a higher/lower point in sympathetic chain 3. Sympathetic trunk ganglia can extend length of body, but preganglionic axons always originate in thoracolumbar region (thoracolumbar outflow) 4. Some preganglionic neurons extend to adrenal medullae a. stimulate adrenal medulla to release hormones (NE, Epi) b. no postganglionic neurons D. Parasympathetic structure 1. Preganglionic axons exit brain or sacral region as part of cranial or spinal nerve 2. 80% of craniosacral outflow leaves brains as part of Vagus (CN X) nerve axons innervate heart, airways, liver, stomach, gallbladder, pancreas, intestines 3. Preganglionic neurons end interminal ganglia near or in effector organs, so postganglionic axons are short III. AUTONOMIC NT & RECEPTORS A. NT 1. Sympathetic preganglionic neurons secrete ACh (cholinergic) 2. Sympathetic postganglionic neurons secrete NE or ACh (primarily NE) 3. All parasympathetic fibers are cholinergic B. Receptors 1. Cholinergic—bind ACh a. nicotinic receptors respond to ACh from preganglionic neurons i. found in ALL autonomic ganglia (symp & PS) & in motor end plate of NMJ ii. binds nicotine iii. activation causes depolarization & excitation of postsynaptic cell (postganglionic fiber, visceral effector, skel muscle) b. muscarinic receptors respond to ACh released from PS postganglionic fibers i. found in all PS effectors (smooth/cardiac muscle, & glands) ii. bind muscarine iii. activation can cause either depolarization or hyperpolarization depending on cell type **in GI tract, binding relaxes smooth muscle sphincters increases movement thru tract **in iris of eye, causes contraction of smooth muscle constriction of pupil iv. anti-muscarinic agents generally relax smooth muscle **atropine blocks muscarinic receptors & decreases digestion/secretions/urination during surgeries; dilation of pupils for ophtho procedures c. ACh binds both receptor types but nicotinic receptors do not bind muscarine & vice versa allows for pharmacological intervention by activating/inhibiting specific receptors d. AChE quickly inactivates ACh (short-lived effects) 2. Adrenergic receptors a. bind NE and epinephrine (Epi) b. found on visceral effectors innervated by most sympathetic postganglionic neurons c. α receptors i. bind both NE & Epi (NE binds more strongly) ii. found in sphincters of digestive tract, blood vessels, pancreas, sweat glands iii. stimulation usually results in contraction of smooth muscle d. β receptors i. β1 binds NE, Epi **found primarily in heart **activation increases heart rate & force of contractions increases blood flow ii. β2 primarily bind epinephrine **found in blood vessels of heart, skeletal muscles & smooth muscle of lungs, in urinary bladder, liver, adipocytes, digestive organs **activation relaxes smooth muscle iii. β3 found only in brown adipose tissues **in infants only **activation results in thermogenesis e. NE activation @ synapse is terminated by: i. reuptake by secreting neuron ii. inactivation by MAO or COMT f. NE effects are longer lasting than cholinergic effects C. Agonists/Antagonists 1. Agonists bind & activate receptor which mimics effect of naturally occurring NT or hormone a. parasympathomimetic or sympathomimetic agents b. phenylephrine = α agonist which causes constriction of blood vessels in nasal passages ingredient in decongestant medications 2. Antagonists bind & block a receptor which prevents the binding of NT & prevents exertion of NT/hormonal effects a. antisympathetic & antiparasympathetic agents b. atropine: anti PS (muscarinic) agent (see III-B-1 above) c. β blockers are used to treat high blood pressure (can be selective or non-selective) IV. PHYSIOLOGICAL RESPONSES OF ANS A. Autonomic Tone 1. Regulated by hypothalamus a. increase sympathetic tone & decrease PS tone simultaneously b. different effects on body organs because different NT released from postganglionic fibers i. ACh (PS) ii. NE (sympathetic) 2. In normal conditions, PS division dominates sympathetic 3. Sweat glands, adipose, skeletal muscles, blood vessels, kidneys, skin, adrenal medulla and the spleen receive sympathetic innervation only a. increasing sympathetic stimulation has one effect b. decreasing sympathetic stimulation has opposite effect B. Sympathetic Tone 1. Dominates during emotional/physical stress and during exercise 2. Increased sympathetic tone favors functions that support vigorous activities & synthesis of ATP (energy) (reduces functions that favor energy storage) 3. Effects on visceral organs a. heart: increased rate & force of contraction increased BP b. blood vessels: to digestive organs/skin/kidneys—constriction; to muscles/heart/adipose tissue/liver—dilation c. lungs: dilation of airways, decreased mucus synthesis d. bladder/digestive tract: relaxation/filling; decreased motility e. adipose: increased lipolysis to provide energy f. exocrine: increased sweating, decreased pancreatic secretions g. endocrine: decreased insulin, increased glucagon, release of NE/Epi from adrenal medulla h. brain: increased alertness i. eye: contraction of radial muscle (dilation of pupils) 4. Effects are longer lasting/more widespread than PS a. postganglionic neurons diverge b. NE lingers longer than ACh (several seconds vs. 20 msec) c. Epi & NE in blood intensify response blood/tissues do not contain MAO/COMT C. Parasympathetic Tone 1. Dominates symp. during normal, resting conditions 2. Increased PS tone favors functions that conserve/restore energy stores during rest/recovery 3. Effects on visceral organs a. heart: decreased rate/force of contraction b. blood vessels: only ones innervated by PS are associated w/ genitalia c. lungs: constriction of airways, increased mucus production d. digestive tract: increased motility, relaxation of sphincters, stimulation of digestive secretions e. gallbladder, urinary bladder—emptying f. eye: constriction of pupils, accommodation for near vision g. exocrine: pancreas releases digestive enzymes h. endocrine: pancreas increases insulin secretion V. INTEGRATION/CONTROL OF AUTONOMIC FUNCTION A. Autonomic Reflexes 1. Integration @ spinal cord, input from higher levels of consciousness 2. Components similar to somatic reflex arc 3. Regulate controlled body functions a. blood pressure: adjust heart rate/contraction strength/vessel diameter b. digestion: adjust muscle tone/motility within GI tract c. defecation/urination: regulates opening/closing of sphincters B. Control by Higher Centers 1. Hypothalamus integrates ANS/SNS a. input: sensory from viscera, special senses, changes in temp/osmolarity; emotional input from limbic system b. output i. influences brain stem: cardiovascular, salivation, swallowing, vomiting centers ii. influences spinal cord: urination/defecation c. posterior/lateral regions control sympathetic responses d. anterior/medial regions control PS responses 2. Medulla is directly responsible for autonomic motor output & is regulated by the HT 3. Higher brain functions w/in frontal cortex can also impact autonomic activity thru HT-medullary pathways