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Chapter 15
Lecture
Outline
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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
15-3
Visceral Reflexes
•
Unconscious, automatic responses to
stimulation of glands, cardiac or smooth muscle
1. 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
15-5
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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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
1. enter ganglia and synapse on postganglionic cell
2. travel to higher or lower ganglia and synapse
3. 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
15-20
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
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cardiac muscles -- reduced blood to skin
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
15-29
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
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