Autonomic Nervous System and Visceral Reflexes

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Autonomic Nervous System and Visceral Reflexes

• 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 muscle
• Receptors
• detect internal stimuli -- stretch, blood
  chemicals, etc.
• Afferent neurons
• connect to interneurons in the CNS
• Efferent neurons
• carry motor signals to effectors
• ANS is the efferent neurons of these reflex arcs
• 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
• Sympathetic division
• prepares body for physical activity
• increases heart rate, BP, airflow, blood glucose
  levels, etc
• Parasympathetic division
• calms many body functions and assists in bodily
  maintenance
• digestion and waste elimination
• 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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Sympathetic Nervous System

• 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
• enter ganglia and synapse on postganglionic cell
• travel to higher or lower ganglia and synapse
• pass through chain without synapsing to reach
  collateral ganglia via splanchnic nerves

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Sympathetic Nervous System

• Neuronal divergence predominates
• each preganglionic cell branches and synapses on
  multiple postganglionic cells
• produces widespread effects on multiple organs

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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
• nicotinic receptors
• on all ANS postganglionic neurons, in the adrenal
  medulla, and at neuromuscular junctions (skeletal
  muscle)
• excitatory when ACh binding occurs
• 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