Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc. Chapter 13 Physiology of the...

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.


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



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


Divisions of the Nervous System

Central nervous system Brain and spinal cord

Peripheral nervous system Somatic motor Autonomic (ANS)

• Parasympathetic• Sympathetic


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)


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


Parasympathetic Nervous System

Parasympathetic nervous system (PNS) drugs Digestion of food Excretion of waste Control of vision Conservation of energy


Sympathetic Nervous System Functions

Three main functions1. Regulation of cardiovascular system

• Maintaining blood flow to the brain• Redistributing blood• Compensating for loss of blood



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)



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


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.


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


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


Sympathomimetic Drugs

Primarily used for effects on the Heart and blood vessels

• Hypertension, heart failure, angina pectoris Lungs

• Primarily asthma


Basic Mechanisms by Which ANS Regulates Physiologic Processes

Patterns of innervation and control Feedback regulation Autonomic tone

ANS = autonomic nervous system.



Patterns of innervation and control Dual innervation opposed: heart rate Dual innervation complementary: erection and

ejaculation Only one division: blood vessels


Fig. 13-1. Opposing effects of parasympathetic and sympathetic nerves.



Feedback regulation Baroreceptor reflex and blood pressure Feedback loop Sensor, effector neurons


Fig. 13-2. Feedback loop of the autonomic nervous system.



Autonomic tone Only one division provides basal control to organ.

• Most organs: predominant tone is PNS• Vascular system: predominant tone is SNS


Anatomic Considerations

Parasympathetic nervous system Sympathetic nervous system Somatic motor system


Fig. 13-3. The basic anatomy of the parasympathetic and sympathetic nervous systems and the somatic motor system.


Sympathetic Nervous System

Neurons Preganglionic neurons Postganglionic neurons

Medulla of the adrenal gland Functional equivalent of the postganglionic SNS

neuron


Sympathetic Nervous System

Two general sites of action Synapses: preganglionic/postganglionic Junction: postganglionic neurons/effector organs


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


Fig. 13-4. Transmitters employed at specific junctions of the peripheral nervous system.


Receptors of the Peripheral Nervous System

Two basic categories

of receptors

Cholinergic receptors• Mediated by

acetylcholine

Adrenergic receptors• Mediated by

epinephrine and norepinephrine


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


Fig. 13-5. Drug structure and receptor selectivity.


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!


Locations of Receptor Subtypes

Knowledge of the sites at which specific receptor subtypes are located will help predict which organs a drug will affect.


Classification of Cholinergic and Adrenergic Receptors

Adrenergic Receptors

Mediated by epinephrine and norepinephrine


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


Fig. 13-6. Locations of cholinergic and adrenergic receptor subtypes.


Functions of AdrenergicReceptor Subtypes

Alpha1 Vasoconstriction Ejaculation Contraction of bladder neck and prostate

Alpha2 Located in presynaptic junction Minimal clinical significance



Beta1 Heart

Increases• Heart rate• Force of contraction• Velocity of conduction in atrioventricular (AV) node

Kidney Renin release



Beta2 Bronchial dilation Relaxation of uterine muscle Vasodilation Glycogenolysis

Dopamine Dilates renal blood vessels


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.


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


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.)


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.)

Copyright © 2013, 2010 by Saunders, an imprint of Elsevier Inc. Chapter 13 Physiology of the...

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