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Chapter 15

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Autonomic Nervous System and Visceral Reflexes

• Autonomic nervous system (ANS)– general properties– anatomy

• Autonomic effects on target organs

• Central control of autonomic function

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ANS - General Properties

• Motor nervous system controls glands, cardiac and smooth muscle – also called visceral motor system

• Regulates unconscious processes that maintain homeostasis– BP, body temperature, respiratory airflow

• ANS actions are automatic– biofeedback techniques• train people to control hypertension, stress and

migraine headaches

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Visceral Reflexes• Unconscious, automatic responses to

stimulation of glands, cardiac or smooth muscle1. Receptors – detect internal stimuli -- stretch, blood chemicals, etc.

2. Afferent neurons – connect to interneurons in the CNS

3. Efferent neurons – carry motor signals to effectors– ANS is the efferent neurons of these reflex arcs

4. Effectors – glands, smooth or cardiac muscle

• ANS modifies effector activity

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Visceral Reflex to High BP

• High blood pressure detected by arterial stretch receptors (1), afferent neuron (2) carries signal to CNS, efferent (3) signals travel to the heart (4), heart slows reducing BP

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Divisions of ANS• Two divisions innervate same target organs– may have cooperative or contrasting effects

1. Sympathetic division – prepares body for physical activity

• increases heart rate, BP, airflow, blood glucose levels, etc

2. Parasympathetic division – calms many body functions and assists in bodily

maintenance– digestion and waste elimination

• Autonomic tone is the normal rate of activity that represents the balance of the two systems

• Effects of each depend upon neurotransmitters released

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Somatic versus Autonomic Pathways

ANS = 2 neurons from CNS to effectors• presynaptic neuron cell body in CNS • postsynaptic neuron cell body in peripheral ganglion

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• Origin of presynaptic neurons– lateral horns of spinal cord (T1-L2)

• Sympathetic chain ganglia (paravertebral)– 3 cervical, 11 thoracic, 4 lumbar, 4 sacral and 1

coccygeal ganglia– white and gray communicating rami suspend

ganglia from spinal nerve– pathways of preganglionic fibers

1. enter ganglia and synapse on postganglionic cell2. travel to higher or lower ganglia and synapse3. pass through chain without synapsing to reach

collateral ganglia via splanchnic nerves

Sympathetic Nervous System

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• Neuronal divergence predominates – each preganglionic cell branches and

synapses on multiple postganglionic cells– produces widespread effects on multiple

organs

Sympathetic Nervous System

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Efferent Pathways

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Preganglionic Pathways

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Ganglia and Abdominal Aortic Plexus

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Sympathetic Innervation

• Effectors in body wall are innervated by sympathetic fibers in spinal nerves

• Effectors in head and thoracic cavity are innervated by fibers in sympathetic nerves

• Effectors in abdominal cavity are innervated by sympathetic fibers in splanchnic nerves– celiac, superior and inferior mesenteric

ganglion

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Adrenal Glands

• Paired glands sit on superior pole of each kidney• Cortex (outer layer)– secretes steroid hormones

• Medulla (inner core)– a modified sympathetic ganglion

• stimulated by preganglionic sympathetic neurons

– secretes neurotransmitters (hormones) into blood• catecholamines (85% epinephrine and 15% norepinephrine)

• Sympathoadrenal system is the closely related functioning adrenal medulla and symphathetic nervous system

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Parasympathetic Nervous System

• Origin of preganglionic fibers– pons and medulla (for cranial nerve nuclei)– sacral spinal cord segments S2-S4

• Pathways of preganglionic fibers– cranial nerves III, VII, IX and X– arising from sacral spinal cord• pelvic splanchnic nerves and inferior hypogastric

plexus

• Terminal ganglia in/near target organs – long preganglionic, short postganglionic fibers

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Efferent Pathways

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Parasympathetic Cranial Nerves

• Oculomotor nerve (III) – narrows pupil and focuses

lens

• Facial nerve (VII)– tear, nasal and salivary

glands

• Glossopharyngeal (IX)– parotid salivary gland

• Vagus nerve (X)– viscera as far as proximal

half of colon– Cardiac, pulmonary, and

esophageal plexus

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Enteric Nervous System

• Nervous system of the digestive tract

• Composed of 100 million neurons found in the walls of the digestive tract (no components in CNS)

• Has its own reflex arcs

• Regulates motility of viscera and secretion of digestive enzymes and acid in concert with the ANS

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Neurotransmitters and Receptors

• Effects of ANS – determined by types of neurotransmitters

released and types of receptors on target cells

• Sympathetic has longer lasting effects– neurotransmitters persist in synapse and

some reach the bloodstream

• Many substances released as neurotransmitters– enkephalin, substance P, neuropeptide Y,

neurotensin, nitric oxide (NO)• NO inhibits muscle tone in BV walls (vasodilation)

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Neurotransmitters and Receptors

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Cholinergic Receptors for ACh

• Acetylcholine (Ach) binds to 2 classes of receptors

1. nicotinic receptors• on all ANS postganglionic neurons, in the adrenal

medulla, and at neuromuscular junctions (skeletal muscle)

• excitatory when ACh binding occurs

2. muscarinic receptors• on all gland, smooth muscle and cardiac muscle

cells that receives cholinergic innervation• excitatory or inhibitory due to subclasses of

muscarinic receptors

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Adrenergic Receptors for NE

• Norepinephrine binds to 2 classes of receptors – alpha adrenergic receptors (often excitatory)– beta adrenergic receptors (often inhibitory)

• Exceptions– existence of subclasses of each receptor type• alpha 1 and 2; beta 1 and 2

• Function by means of 2nd messengers– cyclic AMP and alpha 1 receptors

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Dual Innervation

• Most of viscera receive nerve fibers from both parasympathetic and sympathetic divisions

• Both divisions do not normally innervate an organ equally

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Dual Innervation• Antagonistic effects – oppose each other– exerted through dual innervation of same

effector• 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

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Dual Innervation

• Cooperative effects seen when 2 divisions act on different effectors to produce a unified effect– parasympathetics increase salivary serous cell

secretion– sympathetics increase salivary mucous cell

secretion

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Dual Innervation of the Iris

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Without Dual Innervation

• Some effectors receive only sympathetic– adrenal medulla, arrector pili muscles, sweat

glands and many blood vessels

• Sympathetic tone – a baseline firing frequency– vasomotor tone provides partial constriction• increase in firing frequency = vasoconstriction• decrease in firing frequency = vasodilation• can shift blood flow from one organ to another as

needed– sympathetic stimulation increases blood to skeletal and

cardiac muscles -- reduced blood to skin

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Sympathetic and Vasomotor Tone

Sympathetic division prioritizes blood vessels to skeletal muscles and heart in times of emergency.

Blood vessels to skin vasoconstrict to minimize bleeding if injury occurs during stress or exercise.

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Control of Autonomic Function

• ANS regulated by several levels of CNS– cerebral cortex has an influence– hypothalamus (major visceral motor control

center)• nuclei for primitive functions – hunger, thirst

–midbrain, pons, and medulla oblongata• nuclei for cardiac and vasomotor control, salivation,

swallowing, sweating, bladder control, and pupillary changes

– spinal cord reflexes• defecation and micturition reflexes integrated in

cord• brain can inhibit these responses consciously

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Drugs• Sympathomimetics enhance sympathetic activity– stimulate receptors or norepinephrine release

• Sympatholytics suppress sympathetic activity– block receptors or inhibit norepinephrine release

• Parasympathomimetics enhance activity while parasympatholytics suppress activity

• Management of clinical depression– Prozac blocks reuptake of serotonin to prolong its

mood-elevating effect– MAO inhibitors interfere with breakdown of monoamine

neurotransmitters

• Caffeine competes with adenosine (inhibitory; causes sleepiness) by binding to its receptors