Transcript
Principles of Human Anatomy and Physiology, 11e 1
Chapter 15
The Autonomic Nervous System
Lecture Outline
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INTRODUCTION
• The autonomic nervous system (ANS) operates via reflex arcs.
• Regulate activity of smooth muscle, cardiac muscle & certain glands
• Receives input from limbic system and other regions of the cerebrum
• Operation of the ANS to maintain homeostasis, however, depends on a continual flow of sensory afferent input, from receptors in organs, and efferent motor output to the same effector organs.
• Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons.
• Functionally, the ANS usually operates without conscious control.
• The ANS is regulated by the hypothalamus and brain stem.
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SOMATIC AND AUTONOMIC NERVOUS SYSTEMS
• The somatic nervous system contains both sensory and motor neurons.
• The somatic sensory neurons receive input from receptors of the special and somatic senses.
• Somatic motor neurons innervate skeletal muscle to produce conscious, voluntary movements.
• The autonomic nervous system contains both autonomic sensory and motor neurons.
• The ANS also receives sensory input from somatic senses and special sensory neurons.
• The autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities of cardiac muscle, smooth muscle, and glands.
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Somatic versus Autonomic NS
All somatic motor pathways consist of a single motor neuronAutonomic motor pathways consists of two motor neurons in series
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Basic Anatomy of ANS
• Preganglionic neuron
– cell body in brain or spinal cord
• Postganglionic neuron
– cell body lies outside the CNS in an autonomic ganglion
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AUTONOMIC NERVOUS SYSTEM
• The output (efferent) part of the ANS is divided into two principal parts:
– the sympathetic division
– the parasympathetic division
– Organs that receive impulses from both sympathetic and parasympathetic fibers are said to have dual innervation.
• Dual innervation
– one speeds up organ
– one slows down organ
– Sympathetic NS increases heart rate
– Parasympathetic NS decreases heart rate
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Sympathetic ANS vs. Parasympathetic ANS
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Structures of Sympathetic NS
• Preganglionic cell bodies at T1 to L2
• 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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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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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 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
– all preganglionic neurons
– all parasympathetic postganglionic neurons
– few sympathetic postganglionic neurons (to most sweat glands)
• Excitation or inhibition depending upon receptor subtype and organ involved.
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Cholinergic Neurons and Receptors
• Cholinergic receptors are integral membrane proteins in the postsynaptic plasma membrane.
• The two types of cholinergic receptors are nicotinic and muscarinic receptors.
– Activation of nicotinic receptors causes excitation of the postsynaptic cell.
• Nicotinic receptors are found on dendrites & cell bodies of autonomic NS cells (and at NMJ.)
– Activation of muscarinic receptors can cause either excitation or inhibition depending on the cell that bears the receptors.
• 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
• 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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PHYSIOLOGICAL EFFECTS OF THE ANS - Summary
• The sympathetic responses prepare the body for emergency situations (the fight-or-flight responses).
• The parasympathetic division regulates activities that conserve and restore body energy (energy conservation-restorative system).
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Autonomic or Visceral Reflexes
• A visceral autonomic reflex adjusts the activity of a visceral effector, often unconsciously.
– changes in blood pressure, digestive functions etc
– filling & emptying of bladder or defecation
• Autonomic reflexes occur over autonomic reflex arcs. Components of that reflex arc:
– sensory receptor
– sensory neuron
– integrating center
– pre & postganglionic motor neurons
– visceral effectors
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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 versus Somatic NS - Review
• 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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