Chapter 14

The Autonomic Nervous System

J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.

Autonomic Nervous System Overview

automatic, involuntary

primarily involved in maintaining homeostasis of the internal environment

visceral efferent neurons innervate visceral effectors: smooth muscle, cardiac muscle, exocrine glands and endocrine glands

The ANS Is Clinically Significant!

If you plan to succeed in a Health-Related Career, you will need to understand the Autonomic Nervous System very well!

Two Functional Divisions Parasympathetic and Sympathetic Divisions Structurally, each division consists of nerves, nerve

plexuses, and autonomic ganglia Each motor command is carried over a two-cell circuit Most effector organs and tissues receive impulses from

both ANS divisions, a dual or parallel innervation The two divisions often serve as antagonists to each

other in adjusting and maintaining internal homeostasis Parasympathetic system dominates in sleep and other

relaxed or resting states Sympathetic dominates during skeletal muscle

activities and various emergency situations (fright, panic, rage, aggression)

There is a constant interplay between the two divisions

Comparison of Somatic to Autonomic somatic: one motor neuron to skeletal muscle

effectors autonomic: two motor neurons to visceral effectors

Autonomic Visceral Reflex Arc

Two Types of Autonomic Neurons Preganglionic neurons

cell bodies in the CNS (brain or spinal cord)

transmit Action Potentials from the CNS

Postganglionic neuronscell bodies in

autonomic ganglia in the periphery

transmit APs to effectors

Two Cell Motor Pathways in the ANS

preganglionic neurons in the sympathetic division, the cell body is located

in the lateral gray horns (thoraco-lumbarthoraco-lumbar) of the spinal cord

in the parasympathetic division, the cell body is located in various nuclei of brain stem or in the lateral gray horns (cranio-sacralcranio-sacral)

postganglionic neurons the postganglionic fiber sends impulses to a target

organ the effects at the target organ are due to type of

neurotransmitter and specific cell surface receptors on the effector cells

Dual Innervation

The Sympathetic and Parasympathetic Divisions of the ANS innervate many of the same organs

Different effects are due to specific molecular differences in the neurotransmitters and in the receptor types on the effectors

ANS Dual Innervation The Parasympathetic

Division exerts short-lived, highly localized control.

The Sympathetic Division exerts long-lasting, diffuse effects.

Due to differences in target responsiveness to neurotransmitters

Parasympathetic Ganglia parasympathetic

terminal ganglia = intramural ganglia ganglia are located very

close to or in the wall of the visceral organs

each preganglionic neuron synapses with a only few postganglionic neurons

parasympathetic preganglionic fibers are long

parasympathetic postganglionic fibers are short

Sympathetic Ganglia sympathetic trunk =

vertebral chain ganglia (paravertebral ganglia) a vertical row on either side

of the vertebral column these ganglia are

interconnected thoracic and lumbar origin each preganglionic neuron

synapses with many postganglionic neurons

other sympathetic ganglia are located in the walls of major abdominal arteries

short preganglionic fibers long postganglionic fibers

ANS Neurotransmitters & Receptors Neurotransmitters

Preganglionic - Acetylcholine Postganglionic

Parasympathetic - acetylcholine Sympathetic – norepinephrine

& in a few locations acetylcholine

Receptors Parasympathetic

nicotinic - excitatory muscarinic - excitatory or

inhibitory

Sympathetic alpha - excitatory beta - excitatory or inhibitory

ANS Neurotransmitters

Acetylcholine (ACh) and norepinephrine (NE) are the two major neurotransmitters of the ANS

Cholinergic fibers = ACh-releasing fibers

ACh is released by all preganglionic axons and all parasympathetic postganglionic axons

Adrenergic fibers = NE-releasing fibers Most sympathetic postganglionic axons

Neurotransmitter effects can be excitatory or inhibitory depending upon the receptor type

Neurotransmitters and Receptors of the Autonomic Nervous System

Receptor Class Agonist AntagonistNicotinic Nicotone CurareMuscarinic Muscarine Atropine

Nicotinic Receptors

Nicotinic receptors are found on: Motor end plates (skeletal muscle) All postganglionic neurons of both

sympathetic and parasympathetic divisions The hormone-producing cells of the adrenal

medulla

The effect of ACh binding to nicotinic receptors is always excitatory

Muscarinic Receptors

Muscarinic receptors occur on all effector cells stimulated by parasympathetic cholinergic fibers and by those few effectors stimulated by sympathetic cholinergic fibers

The effect of ACh binding at muscarinic receptors: Can be either inhibitory or excitatory Depends on the receptor type of the target

organ

Adrenergic Receptors

The two fundamental types of adrenergic receptors are alpha and beta

Each type has subclasses: (1, 2, 1, 2 , 3)

Effects of NE binding to: receptors is generally excitatory to

effectors receptors is generally inhibitory to effectors

A clinically important exception – NE binding to receptors in the heart is excitatory

ANS Neurotransmitters & Receptors

See Table 14.3 “Cholinergic and Adrenergic Receptors” (p. 543) for all the details

Cholinergic receptors = nicotinic and muscarinic

Adrenergic receptors = alpha1,2 and beta1,2,3

These details about receptor subtypes will be very important in later courses, such as Pharmacology (in Nursing)

ANS Neurotransmitter Performance Cholinergic fibers/neurons tend to cause

relatively short-lived effects due to the rapid hydrolysis of acetylcholine by cholinesterase in the synapse

Adrenergic fibers/neurons tend to cause relatively longer-lived effects due to the slower degradation of norepinephrine by catechol-o-methyltransferase (COMT) and monoamine oxidase (MAO) in the synapse or in body fluids

Adrenergic receptors also respond to the closely-related hormone, epinephrine = adrenalin, secreted by the adrenal medulla

Drugs Related to ANS Neurotransmitters

Drugs which mimic the action of ACh and NE at their receptors are termed cholinergic and adrenergic agonistsagonists respectively

Drugs which block or inhibit the action of ACh and NE at their receptors are termed cholinergic and adrenergic antagonistsantagonists (or “blockers”) respectively

Drugs which enhance the action of ACh and NE at their synapses by delaying enzymatic degradation are termed anticholinesterasesanticholinesterases monoamine oxidase inhibitorsmonoamine oxidase inhibitors (MAO-inhibitors)

Drugs Related to ANS Neurotransmitters

Table 14-3, p. 537, list examples of a number of autonomic drugs classes and some specific examples.

Take a look. You’ll probably recognize some. Many are available in over-the-counter preparations. There are many more.

Some of the drug classes will be on the exam

But no specific drugs will be on the exam

Autonomic Nervous System Review

Autonomic Nervous System Online Review

Neuroscience For Kids

http://faculty.washington.edu/chudler/neurok.html

This web site has many good reviews and illustrations.

Autonomic Nervous System Controls

Different regions of the CNS have responsibility for different functions.

The cerebral cortex, limbic system, hypothalamus, and the brain stem cooperate to initiate autonomic motor commands.

Autonomic Nervous System Controls

Most control is unconscious and originates from the hypothalamus

But strong conscious emotional states can trigger autonomic, usually sympathetic, responses

Autonomic Nervous System Actions See Table 14.4 (pg. 538) for complete details

for the responses of each effector organ Parasympathetic

S(alivation) L(acrimation) U(rination) D(efecation) metabolic “business as usual” “rest and digest” – “feed and breed” – basic survival

functions

Sympathetic fight or flight = emergency “survival” situations any increase in skeletal muscular activity

for these activities - increase heart rate, blood flow, breathing

decrease non-survival activities - food digestion, etc.

Parasympathetic Tone Parasympathetic tone:

Slows the heart Directs normal activities of the digestive

and urinary systems

The sympathetic division can override these effects during times of stress or muscular exertion

Drugs that block parasympathetic stimuli increase heart rate and interfere with fecal and urinary retention

Cooperative Effects

ANS cooperation is involved in the complex control of the cardiovascular system

ANS cooperation is also seen in control of the external genitalia during sexual activities

Parasympathetic fibers cause vasodilation and are responsible for erection of the penis and clitoris

Sympathetic fibers cause ejaculation of semen in males and reflex peristalsis in the female reproductive tract

Sympathetic Stimulation

Sympathetic stimulation is long-lasting because norepinephrine (NE):

NE is inactivated more slowly by MAO and COMT

NE is an indirectly acting neurotransmitter, triggering a second-messenger system

NE and epinephrine are released into the blood by the adrenal medulla in certain situations and remain there until inactivated by liver enzymes

Solitary Sympathetic Stimulation

Regulates some effectors not innervated by the parasympathetic division

Therefore, acting more as an on-off switch

These include the adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels

Solitary Sympathetic Stimulation

The sympathetic division controls: Thermoregulatory responses to heat

Cutaneous vasodilation and sweating Release of renin from the kidneys

Increased blood pressure from a complex regulatory response

Metabolic effects (in a complex coordination with the endocrine system) increases the metabolic rate of body cells elevates blood glucose levels for use by nervous tissue shifts cellular metabolism to fats for other tissues stimulates the reticular activating system (RAS) of the brain,

increasing mental alertness These actions serve to support the body during

strenuous physical activities and emergencies but may contribute to undesirable side effects in cases of long term stress such as illnesses

Sympathetic Tone The sympathetic division controls blood pressure,

keeping the blood vessels in a continual state of partial constriction (vasomotor tone)

Blood pressure rises or falls with sympathetic activity

Blood is also diverted to or away from different organ systems depending on the level of muscular activity or the presence of emergency or stressful states

Alpha-blocker drugs inhibit vasomotor tone and are used to treat hypertension

Autonomic Nervous System ActionsStructure Sympathetic

StimulationParasympathetic Stimulation

iris of the eye pupil dilation pupil constriction

salivary glands reduce salivation increase salivation

oral/nasal mucosa reduce mucus production

increase mucus production

heart increase rate and force of contraction

decrease rate and force of contraction

lung relax bronchial smooth muscle

constrict bronchial smooth muscle

Autonomic Nervous System ActionsStructure Sympathetic

StimulationParasympathetic Stimulation

stomach reduce peristalsis; decrease gastric secretions

increase peristalsis; increase gastric secretions

small intestine reduce peristalsis; decrease intestinal secretions

increase peristalsis; increase intestinal secretions

large intestine reduce peristalsis; decrease intestinal secretions

increase peristalsis; increase intestinal secretions

liver increase conversion of glycogen to glucose; release glucose into bloodstream

n/a

Autonomic Nervous System ActionsStructure Sympathetic

StimulationParasympathetic Stimulation

kidney decrease urinary output increase urinary output

urinary bladder wall relaxed; sphincter closed

wall contracted; sphincter relaxed

adrenal medulla secrete epinephrine and norepinephrine

n/a

sweat glands increase sweat secretion n/a

blood vessels increase supply to skeletal muscles; decrease supply to most viscera

decrease supply to skeletal muscles; increase supply to most viscera

Developmental Aspects of the ANS

During youth, ANS impairments are usually due to injury

In old age, ANS efficiency decreases, resulting in constipation, dry eyes, and orthostatic hypotension

Orthostatic hypotension is a form of low blood pressure that occurs when sympathetic vasoconstriction centers respond slowly to positional changes

Raynaud’s Disease causes sudden severe

vasoconstriction in the fingers, toes and, occasionally, the ears and nose

during a Raynaud’s attack, or episode, several skin color changes (pallor, cyanosis, rubor) may occur, which are often accompanied by paresthsias, a throbbing or burning sensation, cold, or numbness

ischemia can be so extreme as to cause gangrene

The exact cause of Raynaud’s Disease remains unknown.

Referred Pain Visceral afferents

run in the same nerves with somatic afferents

Pain in the viscera is transferred or interpreted as if it came from somatic areas

Heart attack afferents in T1 - T5

pain in the chest, arm, neck or face

Risk Factors For Hypertension age heredity

race gender weight

diet lifestyle/activity level stress: overstimulates

sympathetic division? alcohol tobacco

End Chapter 14

Some additional information is presented in slides beyond this end point for the lecture. While you will not be tested on this additional material in 2010, you will see it again in some professional courses!

Structure of the Autonomic Nervous System

Preganglionic neuron somata in the thoracic and lumbar spinal cord segments.

Preganglionic neurons synapse onto postganglionic neurons in the sympathetic chain ganglia.

The postganglionic neurons extend a long distance (usually) to the target organs.

Preganglionic neuron somata in the brain and sacral spinal cord.

Preganglionic neurons synapse onto postganglionic neuron ganglia near or in the target organs.

The postganglionic neurons do not extend a long distance to the target organs.

Parasympathetic and Sympathetic Divisions

Structurally, each division consists of nerves, nerve plexuses, and autonomic ganglia

Each motor command is carried in a two-cell circuit

Most effector organs and tissues receive impulses from both divisions, a dual or parallel innervation

The two divisions often serve as antagonists to each other in adjusting and maintaining internal homeostasis

Parasympathetic system dominates in sleep and relaxation

Sympathetic dominates during vigorous activity/strong emotion

sympathetic parasympathetic

preganglionic fiber

short, myelinated, cholinergic

long, myelinated, cholinergic

postganglionic fiber

long,

unmyelinated,

usually adrenergic

short, unmyelinated,

cholinergic

ganglia sympathetic chain

(= paravertebral), collateral

(= prevertebral),

adrenal medullae

terminal (on or in effector)

(= intramural)

dominant? fight or flight,

“E” situations

rest and relaxation,

“SLUD”

Parasympathetic Pathways Cranial Outflow vial

cranial nerves III, VII, IX, and X Cranial nerves III, VII, & IX

innervate structures in head Cranial Nerve X (Vagus)

innervates ventral body cavity; carries 90% of parasympathetic fibers

Sacral Outflow Innervates distal half of

large intestine, urinary bladder, ureters, & genitalia

Sympathetic Ganglia sympathetic trunk ( =

sympathetic chain or paravertebral) ganglia lateral to vertebrae

collateral (=prevertebral) ganglia are located near the major abdominal arteries

adrenal medullae function as sympathetic ganglia, but neurotransmitters released into blood

Sympathetic Paths

Sympathetic Paths All preganglionic fibers exit spinal cord via

ventral root of spinal nerve, travel into ventral ramus, and into sympathetic chain ganglion via white ramus communicans

Then, 1 of 4 (sometimes overlapping) possibilities:

1. synapse with postganglionic neurons at that level of sympathetic chain

2. travel up and/or down sympathetic chain to synapse with postganglionic neurons at other level(s)

3. Pass through sympathetic chain to prevertebral ganglion (via splanchnic nerves) and synapse with postganglionic neurons there

4. (pass through prevertebral ganglion to adrenal medullae)

Unique Roles of the Sympathetic Division

Regulates many functions not subject to parasympathetic influence

These include the activity of the adrenal medulla, sweat glands, arrector pili muscles, kidneys, and most blood vessels

The sympathetic division controls: Thermoregulatory responses to heat Release of renin from the kidneys Metabolic effects: increased metabolic rate,

increased blood glucose, fat breakdown (lipolysis)

Interactions of ANS Divisions Antagonistic: most common, organs

stimulated by one division inhibited by the other

Sympathetic and Parasympathetic Tone Sympathetic tone controls most blood vessels

(even at rest) Parasympathetic tone controls the normal activity

of the heart, digestive, and urinary systems

Cooperative Effects Important in normal sexual function

Thermoregulatory Responses to Heat Applying heat to the skin causes reflex

dilation of blood vessels

Systemic body temperature elevation results in widespread dilation of blood vessels

This dilation brings warm blood to the surface and activates sweat glands to cool the body

When temperature falls, blood vessels constrict and blood is retained in deeper vital organs

Release of Renin from the Kidneys

Sympathetic impulses activate the kidneys to release renin

Renin is an enzyme that promotes increased blood pressure

Metabolic Effects

The sympathetic division promotes metabolic effects that are not reversed by the parasympathetic division

Increases the metabolic rate of body cells Raises blood glucose levels Mobilizes fat as a food source Stimulates the reticular activating system

(RAS) of the brain, increasing mental alertness

Localized Versus Diffuse Effects

The parasympathetic division exerts short-lived, highly localized control

The sympathetic division exerts long-lasting, diffuse effects because NE:

1. Is inactivated more slowly than Ach

2. Acts indirectly, using a second-messenger system

3. And epinephrine are released into the blood and remain there until destroyed by the liver

Localized Versus Diffuse Effects The parasympathetic division exerts

localized control because preganglionic fiber travels directly to specific

targets and synapses with fewer ganglionic neurons

acetylcholine is rapidly degraded The sympathetic division exerts diffuse

effects because preganglionic fibers branch and synapse with

many ganglionic neurons stimulation of adrenal medullae causes

secretion of epinephrine into blood slower inactivation of norepinephrine and

epinephrine

Effects of Drugs (see Table 14.4, p. 545)

Atropine – blocks parasympathetic effects

Neostigmine – inhibits acetylcholinesterase and is used to treat myasthenia gravis

Tricyclic antidepressants – prolong the activity of NE on postsynaptic membranes

Over-the-counter drugs for colds, allergies, and nasal congestion – stimulate -adrenergic receptors

Beta-blockers – attach mainly to 1 receptors and reduce heart rate and prevent arrhythmias

Drugs Which Influence the ANS

Table 14.4.1

Drugs that Influence the ANS

Drugs that Influence the ANS

Drugs Which Influence the ANS

Table 14.4.2

End Additional Materialon Chapter 14