Copyright © John Wiley & Sons, Inc. All rights reserved. Chapter 15 The Autonomic Nervous System Lecture slides prepared by Curtis DeFriez, Weber State

Copyright © John Wiley & Sons, Inc. All rights reserved.

Chapter 15

The Autonomic

Nervous SystemLecture slides prepared by Curtis DeFriez, Weber State University


Introduction to the ANS In this chapter, we examine the structural and

functional features of the autonomic nervous

system (ANS) and compare the organization and

actions of its two major parts, the sympathetic and

parasympathetic divisions.

The autonomic nervous system contributes to

homeostasis by responding to subconscious

visceral sensations and exciting or inhibiting

smooth muscle, cardiac muscle, and many glands.


Introduction to the ANS Structurally, the ANS includes autonomic sensory

neurons, integrating centers in the CNS, and

autonomic motor neurons.

The enteric division is a specialized network of

nerves and ganglia forming an independent

nerve network within the wall of the

gastrointestinal (GI) tract. The enteric division

will not be further discussed in this chapter, but

we will return to it in Chapter 24.


Introduction to the ANS While both the ANS and the somatic nervous system

(SNS) include sensory and motor neurons, the ANS

has many distinctive features which set it apart.

Perhaps the biggest difference between these two

systems is the involvement of conscious control.

• In the SNS, feedback via tactile, thermal, pain, and

proprioceptive sensations are consciously

perceived, and skeletal muscle is the main tool

used to provide reflexive and voluntary movement.


Introduction to the ANS If a somatic motor neuron ceases to stimulate a

muscle, the result is a paralyzed, limp muscle that has no

tone.

Although we are generally not conscious of breathing,

the muscles that generate respiratory movements are

skeletal muscles controlled by somatic motor neurons.• If the respiratory motor neurons become

inactive, breathing stops.


Introduction to the ANS The ANS usually operates without conscious control,

though centers in the hypothalamus and brain stem

do provide regulation for ANS reflexes.

Sensory receptors called interoceptors located in

blood vessels, visceral organs, muscles, and the

nervous system monitor conditions in the internal

environment.

• Examples of interoceptors are chemoreceptors

that monitor blood CO2 level and

mechanoreceptors that detect the degree of

stretch in the walls of organs or blood vessels.


ANS Motor Pathways Autonomic motor neurons regulate visceral activities

by either increasing (exciting) or decreasing

(inhibiting) ongoing activities in their effector tissues.

Because autonomic responses cannot be consciously

altered to any great degree, some autonomic

responses are the basis for polygraph (“lie detector”)

tests.

However, practitioners of yoga and biofeedback

techniques may learn how to regulate at least some

of their autonomic activities through long practice.


Introduction to the ANS The anatomy of all autonomic pathways can

best be understood by picturing a double-

barrelled neuronal construct consisting of

a preganglionic neuron

leading to an intermediate

ganglion that contains

the cell bodies of post-

ganglionic neurons

(that innervate an effector).


Introduction to the ANS


Introduction to the ANSInteractions Animation

The ANS: An Introduction Animation

You must be connected to the internet to run this animation


Most body organs have dual ANS

innervation; that is, they receive impulses

from both sympathetic and

parasympathetic neurons.

Usually the nerve impulses from one

division stimulate an organ, while impulses

from the other division decrease activity.

Divisions of the ANS






Divisions of the ANS Furthermore, the responses of the various

organs to ANS stimulation neatly group into

two functional categories :

Like children on a teeter-totter, the

sympathetic divisions “fight or flight”

response is balanced against the “rest

and relax” (or rest and digest)

activities of the parasympathetic

division.


The Sympathetic Division The cell bodies of neurons which participate in motor

responses of the sympathetic nervous system are

located

in the lateral horns of the gray matter in the 12 thoracic

segments and the first two lumbar segments of the

cord.

Sympathetic preganglionic neurons exit the spinal cord

only between levels T1-L2 (hence the name

thoracolumbar division), though sympathetic ganglia

extend in the vicinity of the cord from the cervical to the

sacral region.


Some of the major groups of sympathetic

ganglia include:

The sympathetic trunk

(vertebral chain) ganglia

Prevertebral ganglia

The celiac, superior

mesenteric, inferior

mesenteric, aorticorenal

and renal ganglia

The Sympathetic Division


The Sympathetic Division Axons leave the sympathetic trunk in four possible ways:

They can enter and travel with spinal nerves.

They can form fine networks of periarterial

preganglionic traveling cephalad to synapse in the

cervical ganglia.

Postganglionic axons exiting the sympathetic trunk

can form sympathetic nerves to the heart and lungs.

Preganglionic axons can leave the sympathetic trunk

without synapsing and form splanchnic nerves.


The Sympathetic Division

Major groups of sympathetic

ganglia.


The Sympathetic Division A truism of the sympathetic division is that a

single sympathetic preganglionic fiber synapses

with many postganglionic branches (with 20 or

more) to create a diverging circuit.

The postganglionic axons typically terminate

in several different visceral effectors, making

the effects of sympathetic stimulation a

widespread massive response.

• This is why anger can be hard to control – it is

such a diffuse response.


The Sympathetic Division This schematic

illustrates the

outflow of the

sympathetic

division of the ANS

via thoracolumbar

pathways to the

many organs of the

body.


The Parasympathetic Division The cell bodies of preganglionic neurons which

participate in motor responses of the

parasympathetic nervous system are located in

nuclei of 4 cranial nerves in the brainstem (III, VII, IX

and X) and in the lateral gray matter of sacral areas

of the spinal cord (S2-S4).

The vagus nerve (CN X) carries nearly 80% of

the total parasympathetic flow to the organs of the

thorax and upper abdomen. Lower abdominal and

pelvic organs are innervated by the sacral output.


The Parasympathetic Division Parasympathetic ganglia are called terminal

ganglia because they are located far from their

origin at the “terminal” ends of the pathways (near

the target organs).

Four pairs of cranial parasympathetic ganglia

innervate structures in the head: The ciliary,

pterygopalatine, submandibular, and otic ganglia.

• The cranial-sacral division also has the ganglia

associated with the vagus (X) nerve and the

sacral nerves.


Most of the parasympathetic ganglia are located very close to the organs or intended action.


The sacral preganglionic axons branch off of sacral

spinal nerves to form pelvic splanchnic nerves

which synapse with parasympathetic postganglionic

neurons located in terminal ganglia in the walls

of the innervated viscera.From the terminal ganglia,

postganglionic axons innervate

smooth muscle and glands in

the walls of the colon, ureters,

urinary bladder, and

reproductive organs

The Parasympathetic Division


The Parasympathetic Division In contrast to the sympathetic system, the

parasympathetic response is more

controlled.

Presynaptic parasympathetic neurons usually

synapse with only 4–5 postsynaptic neurons, all

of which supply a single visceral effector.

Parasympathetic stimulation leads to a narrow,

focused action on specific organs.

• This is why it is possible to walk and chew

gum at the same time (not really!)


The division of the sympathetic and parasympathetic divisions of the ANS are compared in Table 15.3


ANS Neurotransmitters The total number of neurotransmitters used in the entire

nervous system is not known, but is well over 100.

Despite the variety of possible chemicals that could be

used to transmit chemical messages in the ANS, only

2, acetylcholine and norepinephrine, are used to

any great degree.

• Synapses at which ACh is used are termed

cholinergic.

• Synapses at which norepinephrine or epinephrine

are used are termed adrenergic.


ANS Neurotransmitters The neurotransmitter used in all of the synapses of

sympathetic and parasympathetic ganglia

(between the synapses of the preganglionic and

postganglionic fibers) is acetylcholine.

Receptors that respond to Ach released by these

cholinergic neurons are called cholinergic

receptors and there are 2 subtypes: nicotinic

receptors (found in the ganglia) and

muscarinic receptors (found in the synapses

with the effector organs).


ANS Neurotransmitters

Acetylcholine acts on a sub-type of cholinergic receptor (called nicotinic receptors) at ganglia of the ANS.


ANS Neurotransmitters The neurotransmitter used at

most sympathetic

postganglionic synapses is

norepinephrine.

The exception to this rule

is that ACh is used at

sympathetic

postganglionic synapses

for sweat glands.


ANS Neurotransmitters The neurotransmitter

used at all

parasympathetic

postganglionic

synapses

is Ach.

These are all a variety

of cholinergic

receptors

called muscarinic.


ANS Neurotransmitters

Neurons and Neurotransmitters of the Parasympathetic Nervous System

Preganglionic Postganglionic

Cell body in brain or spinal cord

Cell body in intramural ganglion

Acetylcholine (ACh) Acetylcholine (ACh)


ANS NeurotransmittersNeurons and Neurotransmitters ofthe Sympathetic Nervous System

Preganglionic Postganglionic

oCell body in lateral horn of ospinal cord

Cell body in sympathetico chain

gangliono

oAcetylcholine (ACh)

(norepinephrine, NE) ol

except sweat glands (Ach) o


ANS NeurotransmittersInteractions Animation

ANS Neurotransmitters and Neurons Anima

tion



Physiology of the ANS Sympathetic stimulation leads to secretion of

norepinephrine by the adrenal glands, an

increase in the rate and strength of the

heartbeat, constriction of blood vessels of non-

essential organs, dilation of vessels of essential

organs (skeletal muscle and the cerebral

cortex), an increase in the rate and depth of

breathing, hepatic conversion of glycogen to

glucose, and decrease in GI activity.


Physiology of the ANS






Physiology of the ANS SLUDD is as an acronym used to describe the

responses of the parasympathetic nervous system:

Salivation (increased)

Lacrimation (increased)

Urination (increased)

Digestion (increased)

Defecation (increased)

• … and 3 decreases (in the rate and force of the

heart beat, airway size and rate of breathing, and

pupil size)


Physiology of the ANSInteractions Animation

The balance of autonomic sympathetic-parasympathetic

tone is regulated by feedback loops between the spinal

cord and brainstem, with input from the limbic system

and oversight by the hypothalamus.

Physiological Effects of the ANS Animation



End of Chapter 15Copyright 2012 John Wiley & Sons, Inc. All rights

reserved. Reproduction or translation of this work beyond

that permitted in section 117 of the 1976 United States

Copyright Act without express permission of the copyright

owner is unlawful. Request for further information should

be addressed to the Permission Department, John Wiley &

Sons, Inc. The purchaser may make back-up copies for

his/her own use only and not for distribution or resale.

The Publisher assumes no responsibility for errors,

omissions, or damages caused by the use of these

programs or from the use of the information herein.

Documents

Copyright © John Wiley & Sons, Inc. All rights reserved. Chapter 15 The Autonomic Nervous System Lecture slides prepared by Curtis DeFriez, Weber State