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Chapter 17 The Autonomic Nervous System
• Regulate activity of smooth muscle, cardiac muscle & certain glands
• Structures involved– general visceral afferent neurons– general visceral efferent neurons– integration center within the brain
• Receives input from limbic system and other regions of the cerebrum
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Autonomic versus Somatic NS
• Somatic nervous system– consciously perceived sensations– excitation of skeletal muscle– one neuron connects CNS to organ
• Autonomic nervous system– unconsciously perceived visceral sensations – involuntary inhibition or excitation of smooth
muscle, cardiac muscle or glandular secretion– two neurons needed to connect CNS to organ
• preganglionic and postganglionic neurons
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Autonomic versus Somatic NS
• Notice that the ANS pathway is a 2 neuron pathway while the Somatic NS only contains one neuron.
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Basic Anatomy of ANS
• Preganglionic neuron– cell body in brain or spinal cord
– axon is myelinated type B fiber that extends to autonomic ganglion
• Postganglionic neuron– cell body lies outside the CNS in an autonomic ganglion
– axon is unmyelinated type C fiber that terminates in a visceral effector
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Divisions of the ANS
• 2 major divisions– parasympathetic– sympathetic
• Dual innervation– one speeds up organ– one slows down organ– Sympathetic NS
increases heart rate– Parasympathetic NS
decreases heart rate
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Sources of Dual Innervation
• Sympathetic (thoracolumbar) division– preganglionic cell bodies in
thoracic and first 2 lumbar segments of spinal cord
• Parasympathetic (craniosacral) division– preganglionic cell bodies in
nuclei of 4 cranial nerves and the sacral spinal cord
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Locations of Autonomic Ganglia
• Sympathetic Ganglia– trunk (chain) ganglia near
vertebral bodies– prevertebral ganglia near large
blood vessel in gut• celiac • superior mesenteric• inferior mesenteric
• Parasympathetic Ganglia– terminal ganglia in wall of
organ
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Autonomic Plexuses
• Cardiac plexus• Pulmonary plexus• Celiac (solar) plexus• Superior mesenteric• Inferior mesenteric• Hypogastric
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Structures of Sympathetic NS
• Preganglionic cell bodies at T1 to L2
• Rami communicantes– white ramus = myelinated = preganglionic fibers– gray ramus = unmyelinated = postganglionic fibers
• Postganglionic cell bodies– sympathetic chain ganglia along the spinal column– prevertebral ganglia at a distance from spinal cord
• celiac ganglion
• superior mesenteric ganglion
• inferior mesenteric ganglion
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Ganglia & Plexuses of Sympathetic NS
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Pathways of Sympathetic Fibers
• Spinal nerve route– out same level
• Sympathetic chain route– up chain & out spinal n
• Collateral ganglion route– out splanchnic n to
collateral ganglion
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Organs Innervated by Sympathetic NS
• Structures innervated by each spinal nerve– sweat glands, arrector pili mm., blood vessels to skin &
skeletal mm.
• Thoracic & cranial plexuses supply:– heart, lungs,esophagus & thoracic blood vessels– plexus around carotid artery to head structures
• Splanchnic nerves to prevertebral ganglia supply:– GI tract from stomach to rectum, urinary & reproductive
organs
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Circuitry of Sympathetic NS
• Divergence = each preganglionic cell synapses on many postganglionic cells
• Mass activation due to divergence– multiple target organs– fight or flight response explained
• Adrenal gland– modified cluster of postganglionic cell bodies that
release epinephrine & norepinephrine into blood
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Anatomy of Parasympathetic NS
• Preganglionic cell bodies found in– 4 cranial nerve nuclei
in brainstem – S2 to S4 spinal cord
• Postganglionic cell bodies very near or in the wall of the target organ in a terminal ganglia
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Parasympathetic Cranial Nerves• Oculomotor nerve
– ciliary ganglion in orbit
– ciliary muscle & pupillary constrictor muscle inside eyeball
• Facial nerve– pterygopalatine and submandibular ganglions
– supply tears, salivary & nasal secretions
• Glossopharyngeal– otic ganglion supplies parotid salivary gland
• Vagus nerve– many brs supply heart, pulmonary and GI tract as far as
the midpoint of the colon
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Parasympathetic Sacral Nerve Fibers
• Form pelvic splanchnic nerves
• Preganglionic fibers end on terminal ganglia in walls of target organs
• Innervate smooth muscle and glands in colon, ureters, bladder & reproductive organs
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ANS Neurotransmitters
• Classified as either cholinergic or adrenergic neurons based upon the neurotransmitter released
• Adrenergic
• Cholinergic
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Cholinergic Neurons and Receptors
• Cholinergic neurons release acetylcholine from preganglionic neurons & from parasympathetic postganglionic neurons
• Excites or inhibits depending upon receptor type and organ involved
• Nicotinic receptors are found on dendrites & cell bodies of autonomic NS cells and at NMJ
• Muscarinic receptors are found on plasma membranes of all parasympathetic effectors
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Adrenergic Neurons and Receptors
• Adrenergic neurons release norepinephrine (NE) )– from postganglionic
sympathetic neurons only
– Excites or inhibits organs depending on receptors– Alpha1 and Beta1 receptors produce excitation– Alpha2 and Beta2 receptors cause inhibition– Beta3 receptors(brown fat) increase thermogenesis
• NE lingers at the synapse until enzymatically inactivated by monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT)
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Physiological Effects of the ANS
• Most body organs receive dual innervation– innervation by both sympathetic & parasympathetic
• Hypothalamus regulates balance (tone) between sympathetic and parasympathetic activity levels
• Some organs have only sympathetic innervation– sweat glands, adrenal medulla, arrector pili mm &
many blood vessels
– controlled by regulation of the “tone” of the sympathetic system
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Sympathetic Responses• Dominance by the sympathetic system is caused by physical
or emotional stress -- “E situations”– emergency, embarrassment, excitement, exercise
• Alarm reaction = flight or fight response– dilation of pupils– increase of heart rate, force of contraction & BP– decrease in blood flow to nonessential organs– increase in blood flow to skeletal & cardiac muscle– airways dilate & respiratory rate increases– blood glucose level increase
• Long lasting due to lingering of NE in synaptic gap and release of norepinephrine by the adrenal gland
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Parasympathetic Responses• Enhance “rest-and-digest” activities
• Mechanisms that help conserve and restore body energy during times of rest
• Normally dominate over sympathetic impulses• SLUDD type responses = salivation, lacrimation,
urination, digestion & defecation and 3 “decreases”--- decreased HR, diameter of airways and diameter of pupil
• Paradoxical fear when there is no escape route or no way to win– causes massive activation of parasympathetic division
– loss of control over urination and defecation
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Autonomic or Visceral Reflexes
• Autonomic reflexes occur over autonomic reflex arcs. Components of that reflex arc:– sensory receptor– sensory neuron– integrating center– pre & postganglionic motor neurons– visceral effectors
• Unconscious sensations and responses– changes in blood pressure, digestive functions etc– filling & emptying of bladder or defecation
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Control of Autonomic NS
• Not aware of autonomic responses because control center is in lower regions of the brain
• Hypothalamus is major control center– input: emotions and visceral sensory information
• smell, taste, temperature, osmolarity of blood, etc
– output: to nuclei in brainstem and spinal cord– posterior & lateral portions control sympathetic NS
• increase heart rate, inhibition GI tract, increase temperature
– anterior & medial portions control parasympathetic NS• decrease in heart rate, lower blood pressure, increased GI
tract secretion and mobility
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Autonomic Dysreflexia• Exaggerated response of sympathetic NS in cases
of spinal cord injury above T6
• Certain sensory impulses trigger mass stimulation of sympathetic nerves below the injury
• Result– vasoconstriction which elevates blood pressure– parasympathetic NS tries to compensate by slowing
heart rate & dilating blood vessels above the injury– pounding headaches, sweating warm skin above the
injury and cool dry skin below– can cause seizures, strokes & heart attacks
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