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Chapter 14 Autonomic Nervous System © Annie Leibovitz/Contact Press Images MDufilho Autonomic Nervous System • Automatic nervous system (ANS) consists of motor neurons that: – Innervate smooth muscles, cardiac muscle, and glands – Make adjustments to ensure optimal support for body activities – Operate via subconscious control • Also called involuntary nervous system or general visceral motor system MDufilho 11/5/2015 2 Figure 14.1 Place of the ANS in the structural organization of the nervous system. Central nervous system (CNS) Peripheral nervous system (PNS) Sensory (afferent) division 11/5/2015 MDufilho Motor (efferent) division Somatic nervous system Autonomic nervous system (ANS) Sympathetic division Parasympathetic division 3 14.1 ANS versus Somatic Nervous System • Both have motor fibers but differ in: – Effectors – Efferent pathways and ganglia – Target organ responses to neurotransmitters MDufilho 11/5/2015 4 Figure 14.2 Comparison of motor neurons in the somatic and autonomic nervous systems. SOMATIC NERVOUS SYSTEM Cell bodies in central nervous system Neurotransmitter at effector Peripheral nervous system Effector organs Effect Single neuron from CNS to effector organs ACh Stimulatory Heavily myelinated axon Skeletal muscle Two-neuron chain from CNS to effector organs NE SYMPATHETIC Acetylcholine (ACh) 11/5/2015 MDufilho Lightly myelinated preganglionic axons Nonmyelinated postganglionic axon Ganglion ACh Epinephrine and norepinephrine Adrenal medulla PARASYMPATHETIC AUTONOMIC NERVOUS SYSTEM ACh Blood vessel ACh ACh Lightly myelinated preganglionic axon Ganglion Nonmyelinated postganglionic axon Smooth muscle (e.g., in gut), glands, cardiac muscle Stimulatory or inhibitory, depending on neurotransmitter and receptors on effector organs Norepinephrine (NE) 5 Role of the Parasympathetic Division – D activities • Keeps body energy use as low as possible, even while carrying out maintenance activities – Directs digestion, diuresis, defecation • Referred to as “rest-and-digest” system • Example: person relaxing and reading after a meal – Blood pressure, heart rate, and respiratory rates are low – Gastrointestinal tract activity is high – Pupils constricted, lenses accommodated for close vision MDufilho 11/5/2015 6 Role of the Sympathetic Division – E activities • Mobilizes body during activity • Referred to as “fight-or-flight” system • Exercise, excitement, emergency, embarrassment activates sympathetic system – Increased heart rate; dry mouth; cold, sweaty skin; dilated pupils • During vigorous physical activity: – Shunts blood to skeletal muscles and heart – Dilates bronchioles – Causes liver to release glucose MDufilho 11/5/2015 7 Figure 14.3 Key anatomical differences between ANS divisions. Sympathetic Parasympathetic Eye Salivary glands Eye Brain stem Skin* Cranial Sympathetic ganglia Heart Lungs Stomach 1 Fibers originate in the brain stem (cranial fibers) or sacral spinal cord. 1 Fibers originate in the thoracic and lumbar spinal cord. 2a Preganglionic 2a Preganglionic Liver and gallbladder Bladder Genitals 11/5/2015 MDufilho Lungs Heart fibers are long. fibers are short. Stomach 2b Postganglionic 2b Postganglionic fibers are long. Pancreas fibers are short. Pancreas T1 Salivary glands 3 Ganglia are within or near visceral effector organs. Sacral 3 Ganglia are close to spinal cord. L1 Liver and gallbladder Adrenal gland Bladder Genitals 8 Application • Work as a group on chart 11/5/2015 MDufilho 9 14.6 Neurotransmitters • Major neurotransmitters of ANS are acetylcholine (ACh) and norepinephrine (NE) – Ach (same as ACh used by somatic motor neuron) is released by cholinergic fibers at: • All ANS preganglionic axons and • All parasympathetic postganglionic axons – NE is released by adrenergic fibers at: • Almost all sympathetic postganglionic axons, except those at sweat glands (release ACh) • Effects of neurotransmitter depends on whether it binds to cholinergic receptor or adrenergic receptor MDufilho 11/5/2015 10 Figure 14.2 Comparison of motor neurons in the somatic and autonomic nervous systems. SOMATIC NERVOUS SYSTEM Cell bodies in central nervous system Neurotransmitter at effector Peripheral nervous system Effector organs Effect Single neuron from CNS to effector organs ACh Stimulatory Heavily myelinated axon Skeletal muscle Two-neuron chain from CNS to effector organs NE SYMPATHETIC Acetylcholine (ACh) 11/5/2015 MDufilho Lightly myelinated preganglionic axons Nonmyelinated postganglionic axon Ganglion ACh Epinephrine and norepinephrine Adrenal medulla PARASYMPATHETIC AUTONOMIC NERVOUS SYSTEM ACh Blood vessel ACh ACh Lightly myelinated preganglionic axon Ganglion Nonmyelinated postganglionic axon Smooth muscle (e.g., in gut), glands, cardiac muscle Stimulatory or inhibitory, depending on neurotransmitter and receptors on effector organs Norepinephrine (NE) 11 Cholinergic Receptors • Two types of cholinergic receptors bind ACh 1. Nicotinic receptors 2. Muscarinic receptors • Named after drugs that bind to them and mimic ACh effects: nicotine and muscarine (mushroom poison) MDufilho 11/5/2015 12 Adrenergic Receptors • Two major classes that respond to NE or epinephrine – Alpha () receptors • Divided into subclasses: 1, 2 – Beta () receptors • Divided into subclasses: 1, 2, 3 MDufilho 11/5/2015 13 Effects of Drugs • Epinephrine (adrenaline) – Hormone released or drug – Sympathomimetic effect – Hormone is released during “flight or fight” – Given as drug during heart attacks and anaphylactic shock - improve blood pressure and circulation – Binds to all adrenergic receptors (1, 2, 1, 2 , 3) – mimicks effect of NE – Active ingredient in OTC inhalers 11/5/2015 14 MDufilho Effects of Drugs • Albuterol – Proventil or Ventolin – Sympathomimetic effects – selective – Binds to 2 receptors – dilation of bronchioles – Given to asthma patients – relieves bronchospasms 11/5/2015 15 MDufilho Effects of Drugs • Propanolol (Inderal) – Sympatholytic effects – Rx as a “beta blocker to control hypertension – Vessels of skin and viscera receive constant impulses from sympathetic neurons – sympathetic tone – Action - decrease the number of sympathetic impulses from medulla to blood vessels – vasodilation and reduced blood pressure – Action – Binds and blocks NE receptors to heart – decreases heart rate to reduce cardiac output and BP 11/5/2015 16 MDufilho Effects of Drugs • Atropine – Anticholinergic; blocks muscarinic ACh receptors – Used to prevent salivation during surgery, and to dilate pupils for examination • Neostigmine – Inhibits acetylcholinesterase that breaks down ACh – Used to treat myasthenia gravis 11/5/2015 MDufilho 17 Effects of Drugs • Over-the-counter drugs for colds, allergies, and nasal congestion – Stimulate -adrenergic receptors • Beta-blockers – Drugs that attach to 2 receptors to dilate lung bronchioles in asthmatics; other uses • See Tables 14.3 and 14.4 p. 53 11/5/2015 MDufilho 18 Sympathetic Tone • Almost all blood vessel smooth muscle is entirely innervated by sympathetic fibers only, so this division controls blood pressure, even at rest MDufilho 11/5/2015 19 Sympathetic Tone (cont.) • Sympathetic tone (vasomotor tone): continual state of partial constriction of blood vessels – If blood pressure drops, sympathetic fibers fire faster than normal to increase constriction of blood vessels and cause blood pressure to rise – If blood pressure rises, sympathetic fibers fire less than normal, causing less constriction (dilation) of vessels, which leads to decrease in blood pressure – Allows sympathetic system to shunt blood where needed MDufilho 11/5/2015 20 Parasympathetic Tone • Parasympathetic division normally dominates heart and smooth muscle of digestive and urinary tract organs, and it activates most glands except for adrenal and sweat glands – Slows the heart and dictates normal activity levels of digestive and urinary tracts – These organs also exhibit parasympathetic tone where they are always slightly activated • The sympathetic division can override these effects during times of stress • Drugs that block parasympathetic responses increase heart rate and cause fecal and urinary retention MDufilho 11/5/2015 21 14.8 Control of ANS Function • ANS is under control of CNS centers in: – Brain stem and spinal cord, hypothalamus, and cerebral cortex – Hypothalamus is generally main integrative center of ANS activity • Cerebral input may modify ANS but does so subconsciously – Works through limbic system structures on hypothalamic centers MDufilho 11/5/2015 22 Figure 14.9 Levels of ANS control. Communication at subconscious level Cerebral cortex (frontal lobe) Limbic system (emotional input) Hypothalamus The “boss”: Overall integration of ANS Brain stem (reticular formation, etc.) Regulates pupil size, heart, blood pressure, airflow, salivation, etc. Spinal cord Reflexes for urination, defecation, erection, and ejaculation 11/5/2015 MDufilho 23 14.9 Disorders of the ANS • Many ANS disorders involve deficient control of smooth muscle activity – Hypertension (high blood pressure) • Overactive sympathetic vasoconstrictor response to stress • Heart must work harder, and artery walls are subject to increased wear and tear • Can be treated with adrenergic receptor-blocking drugs MDufilho 11/5/2015 24 14.9 Disorders of the ANS – Raynaud’s disease • Painful, exaggerated vasoconstriction in fingers and toes – Digits turn pale, then cyanotic – Treated with vasodilators – Autonomic dysreflexia • Life-threatening, uncontrolled activation of autonomic neurons in quadriplegics and people with spinal cord injuries above T6 • Blood pressure skyrockets, posing increased risk for stroke MDufilho 11/5/2015 25 Clinical – Homeostatic Imbalance 14.1 • Autonomic neuropathy: damage to autonomic nerves that is a common complication of diabetes mellitus • Early signs include sexual dysfunction • Other frequent symptoms include dizziness after standing suddenly (poor blood pressure control), urinary incontinence, sluggish eye pupil reactions, and impaired sweating • Best way to prevent diabetic neuropathy is to maintain good blood glucose levels MDufilho 11/5/2015 26 Developmental Aspects of the Autonomic Nervous System • ANS efficiency declines in old age, partially because of structural changes at preganglionic axon terminals • Effects of age on ANS – Constipation – Dry eyes and frequent eye infections – Orthostatic hypotension MDufilho 11/5/2015 • Low blood pressure after position change • Pressure receptors are less responsive to blood pressure changes • Cardiovascular centers fail to maintain healthy blood pressure 27