Chapter 13
Physiology of the Peripheral
Nervous System
Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.
Physiology of the Peripheral
Nervous System
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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
Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc.
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Physiology of the Peripheral
Nervous System
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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
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Central nervous system
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Brain and spinal cord
Peripheral nervous system
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Somatic motor
Autonomic (ANS)
• Parasympathetic
• Sympathetic
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Overview of Autonomic
Nervous System Functions
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Three principal functions
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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
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Seven regulatory functions
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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
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Parasympathetic nervous system (PNS)
drugs
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Digestion of food
Excretion of waste
Control of vision
Conservation of energy
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Sympathetic Nervous
System Functions
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Three main functions
1. 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
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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
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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
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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
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Regulates blood flow to the skin
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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
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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
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Primarily used for effects on the
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Heart and blood vessels
• Hypertension, heart failure, angina pectoris
Lungs
• Primarily asthma
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Basic Mechanisms by Which ANS
Regulates Physiologic Processes
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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
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Patterns of innervation and control
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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
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Feedback regulation
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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
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Autonomic tone
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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
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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
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Neurons
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Preganglionic neurons
Postganglionic neurons
Medulla of the adrenal gland
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Functional equivalent of the postganglionic SNS
neuron
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Sympathetic Nervous System
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Two general sites of action
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Synapses: preganglionic/postganglionic
Junction: postganglionic neurons/effector organs
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Neurotransmitters of the
Peripheral Nervous System
Acetylcholine
• Employed at most junctions of
the peripheral nervous system
Norepinephrine
• Released by most
postganglionic neurons
Epinephrine
• Released by the adrenal
medulla
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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
Cholinergic
receptors
Adrenergic
receptors
• Mediated by
acetylcholine
• Mediated by
epinephrine and
norepinephrine
Two basic
categories
of
receptors
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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
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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
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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
Cholinergic
Receptors
• Mediated by
acetylcholine
Adrenergic
Receptors
• Mediated by
epinephrine
and
norepinephrine
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Functions of Cholinergic and
Adrenergic Receptor Subtypes
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Functions of cholinergic receptor subtypes
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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 Adrenergic
Receptor Subtypes
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Alpha1
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Vasoconstriction
Ejaculation
Contraction of bladder neck and prostate
Alpha2
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Located in presynaptic junction
Minimal clinical significance
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Functions of Adrenergic
Receptor Subtypes
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Beta1
Heart
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Increases
• Heart rate
• Force of contraction
• Velocity of conduction in atrioventricular (AV) node
Kidney
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Renin release
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Functions of Adrenergic
Receptor Subtypes
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Beta2
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Bronchial dilation
Relaxation of uterine muscle
Vasodilation
Glycogenolysis
Dopamine
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Dilates renal blood vessels
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Receptor Specificity of the
Adrenergic Neurotransmitters
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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
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Many drugs produce their effects by
interfering with specific life cycles.
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Life cycle of acetylcholine
Life cycle of norepinephrine
Life cycle of epinephrine
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3
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.)
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