copyright © 2013, 2010 by saunders, an imprint of elsevier inc. chapter 13 physiology of the...
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Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.
Chapter 13
Physiology of the Peripheral Nervous System
2Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.
Physiology of the Peripheral Nervous System
Divisions of the nervous system Overview of autonomic nervous system
functions Basic mechanisms by which the autonomic
nervous system regulates physiologic processes
Anatomic considerations Introduction to neurotransmitters of the
peripheral nervous system
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Physiology of the Peripheral Nervous System
Introduction to receptors of the peripheral nervous system
Exploring the concept of receptor subtypes Locations of receptor subtypes Functions of cholinergic and adrenergic
receptor subtypes Receptor specificity of adrenergic
neurotransmitters Neurotransmitter life cycles
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Divisions of the Nervous System
Central nervous system Brain and spinal cord
Peripheral nervous system Somatic motor Autonomic (ANS)
• Parasympathetic• Sympathetic
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Overview of Autonomic Nervous System Functions
Three principal functions Regulate the heart Regulate secretory glands (salivary, gastric,
sweat, and bronchial) Regulate smooth muscles (bronchi, blood vessels,
urogenital system, and GI tract)
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Parasympathetic Nervous System
Seven regulatory functions Slowing the heart rate Increasing gastric secretions Emptying the bladder Emptying the bowel Focusing the eye for near vision Constricting the pupil Contracting bronchial smooth muscle
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Parasympathetic Nervous System
Parasympathetic nervous system (PNS) drugs Digestion of food Excretion of waste Control of vision Conservation of energy
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Sympathetic Nervous System Functions
Three main functions1. Regulation of cardiovascular system
• Maintaining blood flow to the brain• Redistributing blood• Compensating for loss of blood
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Sympathetic Nervous System Functions
Three main functions (cont’d):2. Regulation of body temperature
• Regulates blood flow to the skin• Promotes secretion of sweat• Induces piloerection (erection of hair)
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Sympathetic Nervous System Functions
Three main functions (cont’d):3. Implementation of “fight-or-flight” reaction
• Increasing heart rate and blood pressure• Shunting blood away from the skin and viscera• Dilating the bronchi• Dilating the pupils• Mobilizing stored energy
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Homeostatic Objectives of SNS
Maintenance of blood flow to the brain Redistribution of blood flow during exercise Compensation for loss of blood, primarily by
causing vasoconstriction
SNS = sympathetic nervous system.
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SNS Body Temperature Regulation
Regulates blood flow to the skin Dilating surface vessels: accelerates heat loss Constricting cutaneous vessels: conserves heat
Promotes secretion of sweat glands: helps the body cool
Induces piloerection: promotes heat conservation
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Fight-or-Flight Response
Increasing heart rate and blood pressure Shunting blood away from the skin/viscera
and into skeletal muscle Dilating the bronchi to improve oxygenation Dilating the pupils Mobilizing stored energy, thereby providing
glucose for the brain and fatty acids for the muscles
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Sympathomimetic Drugs
Primarily used for effects on the Heart and blood vessels
• Hypertension, heart failure, angina pectoris Lungs
• Primarily asthma
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Basic Mechanisms by Which ANS Regulates Physiologic Processes
Patterns of innervation and control Feedback regulation Autonomic tone
ANS = autonomic nervous system.
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Basic Mechanisms by Which ANS Regulates Physiologic Processes
Patterns of innervation and control Dual innervation opposed: heart rate Dual innervation complementary: erection and
ejaculation Only one division: blood vessels
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Fig. 13-1. Opposing effects of parasympathetic and sympathetic nerves.
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Basic Mechanisms by Which ANS Regulates Physiologic Processes
Feedback regulation Baroreceptor reflex and blood pressure Feedback loop Sensor, effector neurons
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Fig. 13-2. Feedback loop of the autonomic nervous system.
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Basic Mechanisms by Which ANS Regulates Physiologic Processes
Autonomic tone Only one division provides basal control to organ.
• Most organs: predominant tone is PNS• Vascular system: predominant tone is SNS
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Anatomic Considerations
Parasympathetic nervous system Sympathetic nervous system Somatic motor system
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Fig. 13-3. The basic anatomy of the parasympathetic and sympathetic nervous systems and the somatic motor system.
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Sympathetic Nervous System
Neurons Preganglionic neurons Postganglionic neurons
Medulla of the adrenal gland Functional equivalent of the postganglionic SNS
neuron
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Sympathetic Nervous System
Two general sites of action Synapses: preganglionic/postganglionic Junction: postganglionic neurons/effector organs
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Neurotransmitters of the Peripheral Nervous System
•Employed at most junctions of the peripheral nervous system
Acetylcholine
•Released by most postganglionic neurons
Norepinephrine
•Released by the adrenal medulla
Epinephrine
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Fig. 13-4. Transmitters employed at specific junctions of the peripheral nervous system.
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Receptors of the Peripheral Nervous System
Two basic categories
of receptors
Cholinergic receptors• Mediated by
acetylcholine
Adrenergic receptors• Mediated by
epinephrine and norepinephrine
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Receptors of the Peripheral Nervous System
Subtypes of cholinergic and
adrenergic receptors•Subtypes of cholinergic receptors•NicotinicN
•NicotinicM
•Muscarinic•Subtypes of adrenergic receptors•Alpha1
•Alpha2
•Beta1
•Beta2
•Dopamine
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Fig. 13-5. Drug structure and receptor selectivity.
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Exploring the Concept of Receptor Subtypes
What do we mean by the term receptor subtype?
How do we know that receptor subtypes exist?
How can drugs be more selective than natural neurotransmitters at receptor subtypes?
Why do receptor subtypes exist? Do receptor subtypes matter to us? You bet!
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Locations of Receptor Subtypes
Knowledge of the sites at which specific receptor subtypes are located will help predict which organs a drug will affect.
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Classification of Cholinergic and Adrenergic Receptors
Adrenergic Receptors
Mediated by epinephrine and norepinephrine
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Functions of Cholinergic and Adrenergic Receptor Subtypes
Functions of cholinergic receptor subtypes Activation of nicotinicN (neuronal) receptors Activation of nicotinicM (muscle) receptors Activation of muscarinic receptors
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Fig. 13-6. Locations of cholinergic and adrenergic receptor subtypes.
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Functions of AdrenergicReceptor Subtypes
Alpha1 Vasoconstriction Ejaculation Contraction of bladder neck and prostate
Alpha2 Located in presynaptic junction Minimal clinical significance
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Functions of AdrenergicReceptor Subtypes
Beta1 Heart
Increases• Heart rate• Force of contraction• Velocity of conduction in atrioventricular (AV) node
Kidney Renin release
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Functions of AdrenergicReceptor Subtypes
Beta2 Bronchial dilation Relaxation of uterine muscle Vasodilation Glycogenolysis
Dopamine Dilates renal blood vessels
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Receptor Specificity of the Adrenergic Neurotransmitters
Epinephrine can activate all alpha and beta receptors, but not dopamine receptors.
Norepinephrine can activate alpha1, apha2, and beta1 receptors, but not beta2 or dopamine receptors.
Dopamine can activate alpha1, beta1, and dopamine receptors.
Note: Dopamine is the only neurotransmitter capable of activating dopamine receptors.
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Neurotransmitter Life Cycles
Many drugs produce their effects by interfering with specific life cycles. Life cycle of acetylcholine Life cycle of norepinephrine Life cycle of epinephrine
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Fig. 13-7. Life cycle of acetylcholine.Note that transmission is terminated by enzymatic degradation of ACh and not by uptake of intact ACh back into the nerve terminal. (Acetyl CoA = acetylcoenzyme A, ACh = acetylcholine, AChE = acetylcholinesterase.)
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Fig. 13-8. Life cycle of norepinephrine.Note that transmission is terminated by reuptake of NE into the nerve terminal and not by enzymatic degradation. Note also the structural similarity between epinephrine and norepinephrine. (DA = dopamine, MAO = monoamine oxidase, NE = norepinephrine.)