The Autonomic Nervous System and the Adrenal Medulla pdf - The Nervous System

way: Each sympathetic pathway from the cord to the stimulated tissue is composed

The sympathetic nerves are different from skeletal motor nerves in the following

then to the tissues and organs that are stimulated by the sympathetic nerves.

sympathetic chain

between cord segments T-1 and L-2 and pass first into the

The sympathetic nerve fibers originate in the spinal cord along with spinal nerves

nal organs.

), and (3) nerves extending from the ganglia to the different inter-

nerves on the side of the vertebral column, (2) two

paravertebral sympathetic chains of ganglia

pathetic nervous system. Shown specifically in the figure are (1) one of the two

Figure 60–1 shows the general organization of the peripheral portions of the sym-

Physiologic Anatomy of the Sympathetic Nervous System

follow.

, the characteristics and functions of which

The efferent autonomic signals are transmitted to the various organs of the

activities.

the autonomic ganglia, the brain stem, or the hypothalamus and then return

. That is, subconscious sensory signals from a visceral organ can enter

reflexes

The autonomic nervous system also often operates by means of

especially of the limbic cortex, can transmit signals to the lower centers and in

. Also, portions of the cerebral cortex,

, and

spinal cord

The autonomic nervous system is activated mainly by centers located in the

Nervous System

General Organization of the Autonomic

bladder may empty involuntarily, also within seconds.

seconds to cause fainting. Sweating can begin within seconds, and the urinary

extreme, the arterial pressure can be decreased low enough within 10 to 15

and within 10 to 15 seconds the arterial pressure can be doubled; or, at the other

instance, within 3 to 5 seconds it can increase the heart rate to twice normal,

the rapidity and intensity with which it can change visceral functions. For

activities, some of which are controlled almost

ing, sweating, body temperature, and many other

gastrointestinal secretion, urinary bladder empty-

control arterial pressure, gastrointestinal motility,

This system helps to

The portion of the nervous system that controls

and the Adrenal Medulla

The Autonomic Nervous System

748

most visceral functions of the body is called the
autonomic nervous system.

entirely and some only partially by the autonomic

nervous system.

One of the most striking characteristics of the autonomic nervous system is

, brain stem

hypothalamus

this way influence autonomic control.

visceral

subconscious reflex responses directly back to the visceral organ to control its

body through two major subdivisions called the sympathetic nervous system and
the parasympathetic nervous system

that are interconnected with the spinal

prevertebral ganglia (the celiac

and hypogastric

and

Preganglionic and Postganglionic Sympathetic Neurons

lateral horn cells of the spinal cord, through the

, all the way from the intermedio-

Preganglionic sympathetic nerve fibers pass,

Special Nature of the Sympathetic Nerve Endings in the Adrenal

into the thorax. Likewise, the abdominal organs receive

which the organ originated. For instance, the heart

The distribution of sympathetic nerves to each organ

greatly.

This distribution is only approximate and overlaps

the abdomen; and from T-12, L-1, and L-2 into the legs.

into the thorax; from T-7, T-8, T-9, T-10, and T-11 into

to terminate in the neck; from T-3, T-4, T-5, and T-6

the sympathetic chain to terminate in the head; from T-2

pathetic fibers from cord segment T-1 generally pass up

nerve fibers from the same segments. Instead, the

The

Segmental Distribution of the Sympathetic Nerve Fibers.

fibers, a fact that indicates their great importance.

piloerector muscles of the hairs. About 8 per cent of the

nerves. They control the blood vessels, sweat glands, and

pathetic fibers are all very small type C fibers, and they

levels of the cord, as shown in Figure 60–2. These sym-

gray rami

Sympathetic Nerve Fibers in the Skeletal Nerves.

organs.

From either of these two sources, the postganglionic

The postganglionic sympathetic neuron thus origi-

chain, finally synapsing in a

other ganglia of the chain; or (3) it can pass for variable

in the ganglion that it enters; (2) it can pass upward

of the fibers can be one of the following three: (1) It

Then the course

sympathetic chain.

canal, the preganglionic sympathetic fibers leave the

spinal nerve.

spinal cord; its fiber passes, as shown in Figure 60–2,

skeletal motor pathway. The cell body of each pregan-

, in contrast to only a single neuron in the

of two neurons, a

The Autonomic Nervous System and the Adrenal Medulla

Chapter 60

749

preganglionic neuron and a postgan-

glionic neuron

glionic neuron lies in the intermediolateral horn of the

through an anterior root of the cord into the corre-
sponding

Immediately after the spinal nerve leaves the spinal

spinal nerve and pass through a white ramus into one of
the ganglia of the

can synapse with postganglionic sympathetic neurons

or downward in the chain and synapse in one of the

distances through the chain and then through one
of the sympathetic nerves radiating outward from the

peripheral sympathetic

ganglion.

nates either in one of the sympathetic chain ganglia
or in one of the peripheral sympathetic ganglia.

fibers then travel to their destinations in the various

Some of the

postganglionic fibers pass back from the sympathetic
chain into the spinal nerves through

at all

extend to all parts of the body by way of the skeletal

fibers in the average skeletal nerve are sympathetic

sympathetic pathways that originate in the different
segments of the spinal cord are not necessarily distrib-
uted to the same part of the body as the somatic spinal

sym-

is determined partly by the locus in the embryo from

receives many sympathetic nerve fibers from the neck
portion of the sympathetic chain because the heart orig-
inated in the neck of the embryo before translocating

most of their sympathetic innervation from the lower
thoracic spinal cord segments because most of the prim-
itive gut originated in this area.

Medullae.

without synapsing

Bronchi

Heart

Eye

Celiac
ganglion

Blood

vessel

Sweat

gland

Piloerector

muscle

T-1

L-1

Hypogastric plexus

Pylorus

Adrenal
medulla

Kidney

Ureter

Intestine

Ileocecal valve

Anal sphincter

Detrusor

Trigone

sweat glands, and piloerector muscles.

postganglionic fibers in the gray rami leading from the sympa-

Sympathetic nervous system. The black dashed lines represent

Figure 60–1

thetic chains into spinal nerves for distribution to blood vessels,

Peripheral ganglion

Postganglionic nerve
fibers

Preganglionic nerve
fiber

Effector endings

Sensory endings

Anterior root

Sympathetic chain

Gray ramus

White ramus

Spinal nerve

Posterior root

Gut

Intermedio-
lateral horn

Nerve connections between the spinal cord, spinal nerves, sym-

Figure 60–2

pathetic chain, and peripheral sympathetic nerves.

a very few blood vessels are cholinergic.

glands, to the piloerector muscles of the hairs, and to

However,

the

. Conversely,

neurons of the parasympathetic system are also cholin-

neurons. Either

stances, when applied to the ganglia, will excite both

systems. Acetylcholine or acetylcholine-like sub-

cholinergic

is an alternate name for epinephrine.

, a term derived from

. Those that secrete norepinephrine are said

cholinergic

Those fibers that secrete acetylcholine are said to be

acetylcholine

mitter substances,

The sympathetic and parasympathetic nerve fibers

Secretion of Acetylcholine or

Cholinergic and Adrenergic Fibers—

abdomen, rather than in the excited organ itself.

arrangement of the sympathetic ganglia, because the

neurons to innervate the tissues of the organ. This loca-

a millimeter to several centimeters in length, leave the

and very, very short postganglionic fibers, a fraction of

The preganglionic fibers synapse with these,

neurons.

pathetic nerves, the

However, except in the case of a few cranial parasym-

both preganglionic and postganglionic neurons.

The parasympathetic system, like the sympathetic, has

ureters. Also, this sacral group of parasympathetics sup-

rectum, urinary bladder, and lower portions of the

These fibers then distribute to the descending colon,

plexus on each side of the cord at the S-2 and S-3 levels.

, which pass through the spinal nerve sacral

The sacral parasympathetic fibers are in the

lacrimal, nasal, and submandibular glands. And fibers

Fibers from the

the pupillary sphincter and ciliary muscle of the eye.

third cranial nerve

upper portions of the ureters.

of the colon, liver, gallbladder, pancreas, kidneys, and

esophagus, stomach, entire small intestine, proximal half

supply parasympathetic nerves to the heart, lungs,

thinks mainly of the two vagus nerves. The vagus nerves

abdominal regions of the body. Therefore, a physiologist

(cranial nerve X), passing to the entire thoracic and

the first and fourth sacral nerves. About 75 per cent of

VII, IX, and X; additional parasympathetic fibers leave

60–3, demonstrating that parasympathetic fibers leave

is shown in Figure

The

Physiologic Anatomy of the

norepinephrine.

nerve fibers, and it is the endings of these fibers that

glionic neurons; indeed, they even have rudimentary

These secretory cells embryologically are derived from

and finally into the two adrenal medullae. There they

sympathetic chains, then through the splanchnic nerves,

The Nervous System: C. Motor and Integrative Neurophysiology

750

Unit XI

end directly on modified neuronal cells that secrete epi-
nephrine and norepinephrine into the blood stream.

nervous tissue and are actually themselves postgan-

secrete the adrenal hormones epinephrine

and

Parasympathetic Nervous System

parasympathetic nervous system

the central nervous system through cranial nerves III,

the lowermost part of the spinal cord through the
second and third sacral spinal nerves and occasionally

all parasympathetic nerve fibers are in the vagus nerves

speaking of the parasympathetic nervous system often

Parasympathetic fibers in the

go to

seventh cranial nerve pass to the

from the ninth cranial nerve go to the parotid gland.

pelvic

nerves

plies nerve signals to the external genitalia to cause
erection.

Preganglionic and Postganglionic Parasympathetic Neurons.

preganglionic fibers pass uninter-

rupted all the way to the organ that is to be controlled.
In the wall of the organ are located the postganglionic

tion of the parasympathetic postganglionic neurons in
the visceral organ itself is quite different from the

cell bodies of the sympathetic postganglionic neurons
are almost always located in the ganglia of the sympa-
thetic chain or in various other discrete ganglia in the

Basic Characteristics
of Sympathetic and
Parasympathetic Function

Norepinephrine

secrete mainly one or the other of two synaptic trans-

or norepinephrine.

to be adrenergic

adrenalin, which

All preganglionic neurons are

in both

the sympathetic and the parasympathetic nervous

sympathetic and parasympathetic postganglionic

all or almost all of the postganglionic

ergic

most of the postganglionic sym-

pathetic neurons are adrenergic
postganglionic sympathetic nerve fibers to the sweat

Heart

Parotid gland

Otic ganglion

Submandibular gland

Submandibular ganglion

Nasal glands

Lacrimal glands

Sphenopalatine ganglion

Pupillary sphincter

Ciliary muscles of eye

Ciliary ganglion

Pylorus

Colon

Small intestine

Ileocecal valve

Anal sphincter

Bladder
Detrusor

Trigone

Sacral

Stomach

VII

III

Figure 60–3

Parasympathetic nervous system.

Therefore, when secreted into the blood, both norepi-

methyl transferase; this occurs mainly in the liver.

tissue, where they can be destroyed by catechol-

tissue are rapid. However, the norepinephrine and

a tissue remains active for only a few seconds, demon-

Ordinarily, the norepinephrine secreted directly into

, which is present diffusely in all tissues).

catechol-O-methyl trans-

nerve endings, and another is

, which is found in the

monoamine oxidase

of the remaining norepinephrine; and (3) destruction

secreted norepinephrine; (2) diffusion away from the

three ways: (1) reuptake into the adrenergic nerve

nerve endings, it is removed from the secretory site in

epinephrine into epinephrine, as follows:

In the adrenal medulla, this reaction goes still one

3. Transport of dopamine into the vesicles

cles. The basic steps are the following:

for synthesis of new acetylcholine.

minal nerve ending, where it is used again and again

muscular junctions of skeletal nerve fibers. The choline

the local connective tissue. This is the same mechanism

acetylcholinesterase

, catalyzed by the enzyme

choline

mitter function. Then it is split into an

cholinergic nerve ending, it persists in the tissue for a

centrated form until it is released. The basic chemical

The molecular structures of acetylcholine and nor-

Therefore, acetylcholine is called a

respective parasympathetic or sympathetic effects.

, but a few secrete acetylcholine. These

acetylcholine

Thus, the terminal nerve endings of the parasympa-

The Autonomic Nervous System and the Adrenal Medulla

Chapter 60

751

thetic system all or virtually all secrete

Almost all of the sympathetic nerve endings secrete
norepinephrine
hormones in turn act on the different organs to cause

parasympathetic

transmitter and norepinephrine is called a sympathetic
transmitter.

epinephrine are the following:

Synthesis of Acetylcholine, Its Destruction After Secretion, and
Its Duration of Action.

Acetylcholine is synthesized in the

terminal endings and varicosities of the cholinergic
nerve fibers where it is stored in vesicles in highly con-

reaction of this synthesis is the following:

Once acetylcholine is secreted into a tissue by a

few seconds while it performs its nerve signal trans-

acetate ion and

that is bound with collagen and glycosaminoglycans in

for acetylcholine signal transmission and subsequent
acetylcholine destruction that occurs at the neuro-

that is formed is then transported back into the ter-

Synthesis of Norepinephrine, Its Removal, and Its Duration of
Action.

Synthesis of norepinephrine begins in the axo-

plasm of the terminal nerve endings of adrenergic
nerve fibers but is completed inside the secretory vesi-

step further to transform about 80 per cent of the nor-

After secretion of norepinephrine by the terminal

endings themselves by an active transport process—
accounting for removal of 50 to 80 per cent of the

nerve endings into the surrounding body fluids and
then into the blood—accounting for removal of most

of small amounts by tissue enzymes (one of these
enzymes is

ferase

strating that its reuptake and diffusion away from the

epinephrine secreted into the blood by the adrenal
medullae remain active until they diffuse into some

O-

nephrine and epinephrine remain very active for 10 to

Norepinephrine

Epinephrine

methylation

æææ

Dopamine

Norepinephrine

hydroxylation

ææææ

Dopa

Dopamine

decarboxylation

ææææ

Tyrosine

Dopa

hydroxylation

ææææ

choline acetyl-

cetyl-CoA

Choline

Acetylcholine

transferase

ææææ

Acetylcholine

Norepinephrine

rior. Thus, the transmitter substance is secreted.

varicosities. The calcium ions in turn cause the termi-

ber membrane to calcium ions, allowing

bers, the depolarization process increases the perme-

When an action potential spreads over the terminal

nephrine synthesis.

mitochondria that supply adenosine triphosphate,

stored. Also in the varicosities are large numbers of

or pass over or near the cells to be stimulated, they

that are to be stimulated. Where these

vate as they pass by; or in some instances, they termi-

tion. However, many of the parasympathetic nerve

parasympathetic nerves, are similar to but much

autonomic nerve endings, especially those of the

glionic Nerve Endings.

Subsequent Removal of the Transmitter

Mechanisms of Transmitter Secretion and

at the Postganglionic Endings

Secretion of Acetylcholine and Norepinephrine by Postgan-

A few of the postganglionic

smaller than those of the skeletal neuromuscular junc-

fibers and almost all the sympathetic fibers merely
touch the effector cells of the organs that they inner-

nate in connective tissue located adjacent to the cells

filaments touch

usually have bulbous enlargements called varicosities;
it is in these varicosities that the transmitter vesicles of
acetylcholine or norepinephrine are synthesized and

which is required to energize acetylcholine or norepi-

fi
ability of the fi
these ions to diffuse into the nerve terminals or nerve

nals or varicosities to empty their contents to the exte-

but essentially no action on alpha receptors.

rine,

isopropyl norepineph-

nephrine and norepinephrine,

inhibitory. Therefore, alpha and beta receptors are not

are excitatory, whereas others are inhibitory. Likewise,

by the sympathetics. Note that certain alpha functions

Table 60

beta receptors, epinephrine will be the more effective

types of receptors in the organs. If they are all

types of receptors approximately equally. Therefore,

extent as well. Conversely, epinephrine excites both

beta receptors. Norepinephrine excites mainly alpha

Norepinephrine and epinephrine, both of which are

receptors.)

receptors. Also, there is a division of alpha receptors

. (The beta

tors,

There are also two major types of adrenergic recep-

Adrenergic Receptors—Alpha and

the other of the two types of receptors.

instance, at the neuromuscular junctions in skeletal

parasympathetic systems. (Nicotinic receptors are also

tine activates only nicotinic receptors; acetylcholine

and will not activate nicotinic receptors, whereas nico-

from toadstools, activates only muscarinic receptors

reason for these names is that muscarine, a poison

receptors. The

They are called

Two Principal Types of Acetylcholine

organs.

formational state. In each organ, the resulting effects

or excitation in others. This is usually determined by

of many different intracellular actions, the exact effect

. The cAMP in turn can initiate any one

on the inside of the cell, and

protrudes into the interior of the cell. For instance,

lar chemical) inside the cell. The enzyme often is

ment will cause an internal cell action, such as a direct

In some cells, the changed intracellular ion environ-

sium ions to diffuse out of the cell, and this usually

times, potassium channels are opened, allowing potas-

the cell. At other

the respective ions into the cell, usually depolarizing

For instance, sodium and/or calcium ion channels

permeability of the cell membrane to various ions.

interstices of the protein molecule, thus altering the

opens or closes an ion channel

is an integral part of the cell membrane, a conforma-

Membrane Permeability.

or inhibits the cell, most often by (1) causing a change

molecule. In turn, the altered protein molecule excites

stance binds with the receptor, this causes a confor-

through the cell membrane. When the transmitter sub-

outside of the cell membrane, bound as a prosthetic

on the effector cells. The receptor is on the

ulate an effector organ, it must

Before an acetylcholine, norepinephrine, or epineph-

over 1 to several minutes.

30 seconds; but their activity declines to extinction

The Nervous System: C. Motor and Integrative Neurophysiology

752

Unit XI

Receptors on the Effector Organs

rine secreted at an autonomic nerve ending can stim-

first bind with specific

receptors

group to a protein molecule that penetrates all the way

mational change in the structure of the protein

in cell membrane permeability to one or more ions or
(2) activating or inactivating an enzyme attached to
the other end of the receptor protein where it pro-
trudes into the interior of the cell.

Excitation or Inhibition of the Effector Cell by Changing Its

Because the receptor protein

tional change in structure of the receptor protein
often

through the

frequently become opened and allow rapid influx of

the cell membrane and exciting

inhibits the cell because loss of electropositive
potassium ions creates hypernegativity inside the cell.

effect of calcium ions to promote smooth muscle
contraction.

Receptor Action by Altering Intracellular “Second Messenger”
Enzymes.

Another way a receptor often functions is to

activate or inactivate an enzyme (or other intracellu-

attached to the receptor protein where the receptor

binding of norepinephrine with its receptor on the
outside of many cells increases the activity of the
enzyme adenylyl cyclase
this causes formation of cyclic adenosine monophos-
phate (cAMP)

depending on the chemical machinery of the effector
cell.

It is easy to understand how an autonomic trans-

mitter substance can cause inhibition in some organs

the nature of the receptor protein in the cell mem-
brane and the effect of receptor binding on its con-

are likely to be entirely different from those in other

Receptors—Muscarinic and Nicotinic
Receptors

Acetylcholine activates mainly two types of receptors.

muscarinic and nicotinic

activates both of them.

Muscarinic receptors are found on all effector cells

that are stimulated by the postganglionic cholinergic
neurons of either the parasympathetic nervous system
or the sympathetic system.

Nicotinic receptors are found in the autonomic

ganglia at the synapses between the preganglionic and
postganglionic neurons of both the sympathetic and

present at many nonautonomic nerve endings—for

muscle [discussed in Chapter 7].)

An understanding of the two types of receptors is

especially important because specific drugs are fre-
quently used as medicine to stimulate or block one or

Beta Receptors

alpha receptors and beta receptors

receptors in turn are divided into beta

and beta

recep-

tors because certain chemicals affect only certain beta

into alpha

and alpha

secreted into the blood by the adrenal medulla,
have slightly different effects in exciting the alpha and

receptors but excites the beta receptors to a lesser

the relative effects of norepinephrine and epinephrine
on different effector organs are determined by the

excitant.

–1 gives the distribution of alpha and beta

receptors in some of the organs and systems controlled

certain beta functions are excitatory and others are

necessarily associated with excitation or inhibition but
simply with the affinity of the hormone for the recep-
tors in the given effector organ.

A synthetic hormone chemically similar to epi-

has an extremely strong action on beta receptors

sion of contents along the tract. This propulsive effect

and relaxing the sphincters, thus allowing rapid propul-

stimulation, in general, increases overall degree of activ-

the gastrointestinal intramural plexus. Parasympathetic

, located in the walls of the gut.Also, both

The

Intramural Nerve Plexus of the Gastrointestinal System.

parasympathetic centers.

close embryological relation to sweat glands, are acti-

the shoulder joint. The apocrine glands, despite their

ulation. This secretion actually functions as a lubricant

lation, but they do not respond to parasympathetic stim-

The

could be called a parasympathetic function, even though

to be parasympathetic centers. Therefore, sweating

more, the sweat glands are stimulated primarily by

bers, which are adrenergic. Further-

bers to the palms and soles), in contrast to almost all

cholinergic

glands are

nerves. However, the sympathetic

when the sympathetic nerves are stimulated, but no

sweat glands

The

enzymes and mucus. But it also causes vasoconstriction

less by the autonomic nerves.

those of the mouth and stomach. On the other hand, the

parasympathetics are those of the upper tract, especially

copious quantities of watery secretion. The glands of

parasympathetic nervous system, usually resulting in

, and many

The

Glands of the Body.

tion of the eyes.

objects near at hand. The detailed focusing mechanism

become more convex, causing the eye to focus on

of the lens radial ligaments. This contraction releases

, which is a ringlike body of

of its radial ligaments. Parasympathetic excitation con-

the parasympathetic nervous system. The lens is nor-

Focusing of the lens is controlled almost entirely by

increase pupillary opening at these times.

that strikes the retina. Conversely, the sympathetics

which is explained in Chapter 51; this re

exly stimulated when excess light enters the eyes,

The parasympathetics that control the pupil are

the pupil, whereas parasym-

contracts the meridional

opening and (2) the focus of the lens.

autonomic nervous system. They are (1) the pupillary

Two functions of the eyes are controlled by the

Effects of Sympathetic and

ed in still greater detail, as follows.

2. Some of these functions need to

listed in Table 60

tions of these two nervous systems on each organ, as

thetic function, one must learn all the separate func-

Therefore, to understand sympathetic and parasympa-

There is no generalization one can use to explain

of the two systems.

occasionally act reciprocally to each other. But most

times inhibits it, demonstrating that the two systems

particular organ, parasympathetic stimulation some-

Also, when sympathetic stimulation excites a

others.

inhibitory effects in others. Likewise, parasympathetic

From this table, it can be seen again that

parasympathetic nerves or the sympathetic nerves.

Table 60

Actions of Sympathetic and

Excitatory and Inhibitory

The Autonomic Nervous System and the Adrenal Medulla

Chapter 60

753

Parasympathetic Stimulation

–2 lists the effects on different visceral func-

tions of the body caused by stimulating either the

sympathetic

stimulation causes excitatory effects in some organs but

stimulation causes excitation in some but inhibition in

organs are dominantly controlled by one or the other

whether sympathetic or parasympathetic stimulation
will cause excitation or inhibition of a particular organ.

–

be clarifi

Parasympathetic Stimulation
on Specific Organs

Eyes.

Sympathetic stimulation

fibers of the iris that dilate
pathetic stimulation contracts the circular muscle of the
iris to constrict the pupil.

refl

flex reduces the

pupillary opening and decreases the amount of light

become stimulated during periods of excitement and

mally held in a flattened state by intrinsic elastic tension

tracts the ciliary muscle
smooth muscle fibers that encircles the outside ends

the tension on the ligaments and allows the lens to

is discussed in Chapters 49 and 51 in relation to func-

nasal, lacrimal, salivary

gastrointestinal glands are strongly stimulated by the

the alimentary tract most strongly stimulated by the

glands of the small and large intestines are controlled
principally by local factors in the intestinal tract itself
and by the intestinal enteric nervous system and much

Sympathetic stimulation has a direct effect on most

alimentary gland cells to cause formation of a concen-
trated secretion that contains high percentages of

of the blood vessels that supply the glands and in this
way sometimes reduces their rates of secretion.

secrete large quantities of sweat

effect is caused by stimulating the parasympathetic

fibers to most sweat

(except for a few adrenergic

fi
other sympathetic fi

centers in the hypothalamus that are usually considered

it is controlled by nerve fibers that anatomically are dis-
tributed through the sympathetic nervous system.

apocrine glands in the axillae secrete a thick,

odoriferous secretion as a result of sympathetic stimu-

to allow easy sliding motion of the inside surfaces under

vated by adrenergic fibers rather than by cholinergic
fibers and are also controlled by the sympathetic centers
of the central nervous system rather than by the

gastrointestinal system has its own intrinsic set of nerves
known as the intramural plexus or the intestinal enteric
nervous system
parasympathetic and sympathetic stimulation originat-
ing in the brain can affect gastrointestinal activity
mainly by increasing or decreasing specific actions in

ity of the gastrointestinal tract by promoting peristalsis

is associated with simultaneous increases in rates of

Table 60–1

Bladder sphincter contraction

Calorigenesis (

Pilomotor contraction

Bronchodilation (

Intestinal sphincter contraction

Intestinal relaxation (

Intestinal relaxation

Increased myocardial

Iris dilation

Cardioacceleration (

Vasoconstriction

Vasodilation (

Alpha Receptor

Beta Receptor

Adrenergic Receptors and Function

)

strength (

)

Uterus relaxation (

)

Glycogenolysis (

)

Lipolysis (

)

Bladder wall relaxation (

)

sympathetic vascular constriction, but this occurs rarely

Under some conditions, the beta function of the sym-

restricted areas, such as in the blush area of the face.

limbs, are constricted by sympathetic stimulation.

Most systemic blood vessels, espe-

Systemic Blood Vessels.

strenuous activity.

pumping, allowing the heart to rest between bouts of

heart as a pump, as required during heavy exercise,

tion. To express these effects in another way, sympa-

overall activity of the heart. This is accomplished by

In general, sympathetic stimulation increases the

increases the tone of the sphincters. The net result is

very dependent on sympathetic stimulation. However,

described earlier.

secretion by many of the gastrointestinal glands,

The Nervous System: C. Motor and Integrative Neurophysiology

754

Unit XI

Normal function of the gastrointestinal tract is not

strong sympathetic stimulation inhibits peristalsis and

greatly slowed propulsion of food through the tract and
sometimes decreased secretion as well—even to the
extent of sometimes causing constipation.

Heart.

increasing both the rate and force of heart contraction.

Parasympathetic stimulation causes mainly opposite

effects—decreased heart rate and strength of contrac-

thetic stimulation increases the effectiveness of the

whereas parasympathetic stimulation decreases heart

cially those of the abdominal viscera and skin of the

Parasympathetic stimulation has almost no effects on
most blood vessels except to dilate vessels in certain

pathetics causes vascular dilation instead of the usual

except after drugs have paralyzed the sympathetic alpha

Table 60–2

Skeletal muscle

Increased glycogenolysis

None

Piloerector muscles

Contracted

None

Mental activity

Increased

None

Adrenal medullary secretion

Increased

None

Basal metabolism

Increased up to 100%

None

Lipids

Increased

None

Glucose

Increased

None

Coagulation

Increased

None

Skin

Constricted

None

)

None

Muscle

Constricted (adrenergic

Abdominal viscera

Constricted

None

Penis

Ejaculation

Erection

Trigone

Contracted

Relaxed

Detrusor

Relaxed (slight)

Contracted

Kidney

Decreased output and renin secretion

None

Gallbladder and bile ducts

Relaxed

Contracted

Liver

Glucose released

Slight glycogen synthesis

Sphincter

Increased tone (most times)

Relaxed (most times)

Lumen

Decreased peristalsis and tone

Increased peristalsis and tone

Blood vessels

Mildly constricted

? Dilated

Bronchi

Dilated

Constricted

)

Dilated

); constricted (

Coronaries

Dilated (

Increased force of contraction

Decreased force of contraction (especially of atria)

Muscle

Increased rate

Slowed rate

Blood vessels

Most often constricted

Most often little or no effect

Apocrine glands

Thick, odoriferous secretion

None

Sweat glands

Copious sweating (cholinergic)

Sweating on palms of hands

Nasal

enzyme-secreting glands)

Glands

Vasoconstriction and slight secretion

Stimulation of copious secretion (containing many enzymes for

Ciliary muscle

Slight relaxation (far vision)

Constricted (near vision)

Pupil

Dilated

Constricted

Organ

Effect of Sympathetic Stimulation

Effect of Parasympathetic Stimulation

Autonomic Effects on Various Organs of the Body

Eye

Lacrimal
Parotid
Submandibular
Gastric
Pancreatic

Heart

Lungs

Gut

Bladder

Systemic arterioles

Dilated (adrenergic

)

Dilated (cholinergic)

Blood

Increased strength

Fat cells

Lipolysis

None

bers.

by these hormones, especially by epinephrine,

bers. For instance,

missing.

factor, one mechanism substituting for the other if it is

almost all the necessary duties. Thus, the dual mecha-

blood and indirectly cause stimulation. Likewise,

most instances, substitute for the other. For instance,

of stimulation support each other, and either can, in

the adrenal medullary hormones. The two means

fore, the organs are actually stimulated in two ways:

directly by generalized sympathetic activation. There-

thetic Nervous System.

Value of the Adrenal Medullae to the Function of the Sympa-

over.

longed, lasting 2 to 4 minutes after the stimulation is

stimulation, except that the effects are greatly pro-

epinephrine, which together have almost the same

In summary, stimulation of the adrenal medullae

such as glycogenolysis in the liver and muscle, and

also increases the rates of other metabolic activities,

increasing the activity and excitability of the body. It

as much as 100 per cent above normal, in this way

great a metabolic effect as norepinephrine. Indeed, the

tissue metabolism. Epinephrine has 5 to 10 times as

cardiac output more.

elevates arterial pressure, whereas epinephrine raises

a major segment of the vessels of the body, this differ-

norepinephrine. Because the muscle vessels represent

constriction of the blood vessels in the muscles, in com-

epinephrine. Second, epinephrine causes only weak

greater effect in stimulating the beta receptors, has a

following respects: First, epinephrine, because of its

caused by norepinephrine, but the effects differ in the

gastrointestinal tract, dilation of the pupils of the eyes,

causes increased activity of the heart, inhibition of the

of essentially all the blood vessels of the body; it also

The circulating norepinephrine causes constriction

slowly over a period of 2 to 4 minutes.

The circulating epinephrine and norepinephrine

physiologic conditions.

20 per cent is norepinephrine, although the relative

the blood to all tissues of the body. On the average,

blood, and these two hormones in turn are carried in

Function of the Adrenal Medullae

acts, as explained in Chapters 80 and 81.

Finally, the sympathetics and parasympathetics are

basal metabolic rate, and increase in mental activity.

muscle, increase in skeletal muscle strength, increase in

centration, increase in glycogenolysis in both liver and

glucose from the liver, increase in blood glucose con-

parasympathetic stimulation. Sympathetic stimulation

gallbladder, ureter, urinary bladder, and bronchi, are

entodermal structures, such as the ducts of the liver,

to multiple body functions. In general, most of the

the sympathetic and parasympathetic control systems,

Functions of the Body.

rary loss of all or most arterial pressure.

rial pressure. But

Therefore, the usual effect is a slight decrease in arte-

has virtually no effect on vascular peripheral resistance.

Conversely, moderate parasympathetic stimulation

the same time.

ow, which usually causes a

ow of blood through the peripheral blood vessels.

factors: propulsion of blood by the heart and resistance

The arterial pressure is determined by two

usually far dominant over the beta effects.

vasoconstrictor effects, which, in blood vessels, are

The Autonomic Nervous System and the Adrenal Medulla

Chapter 60

755

Effect of Sympathetic and Parasympathetic Stimulation on Arter-
ial Pressure.

to fl
Sympathetic stimulation increases both propulsion by
the heart and resistance to fl
marked acute increase in arterial pressure but often very
little change in long-term pressure unless the sympa-
thetics stimulate the kidneys to retain salt and water at

via the vagal nerves decreases pumping by the heart but

very strong vagal parasympathetic

stimulation can almost stop or occasionally actually stop
the heart entirely for a few seconds and cause tempo-

Effects of Sympathetic and Parasympathetic Stimulation on Other

Because of the great importance of

they are discussed many times in this text in relation

inhibited by sympathetic stimulation but excited by

also has multiple metabolic effects such as release of

involved in execution of the male and female sexual

Stimulation of the sympathetic nerves to the adrenal
medullae causes large quantities of epinephrine and
norepinephrine to be released into the circulating

about 80 per cent of the secretion is epinephrine and

proportions can change considerably under different

have almost the same effects on the different organs
as the effects caused by direct sympathetic stimulation,
except that the effects last 5 to 10 times as long because
both of these hormones are removed from the blood

and so forth.

Epinephrine causes almost the same effects as those

greater effect on cardiac stimulation than does nor-

parison with much stronger constriction caused by

ence is of special importance because norepinephrine
greatly increases the total peripheral resistance and

the arterial pressure to a lesser extent but increases the

A third difference between the actions of epineph-

rine and norepinephrine relates to their effects on

epinephrine secreted by the adrenal medullae can
increase the metabolic rate of the whole body often to

glucose release into the blood.

causes release of the hormones epinephrine and nor-

effects throughout the body as direct sympathetic

Epinephrine and norepinephrine

are almost always released by the adrenal medullae at
the same time that the different organs are stimulated

directly by the sympathetic nerves and indirectly by

destruction of the direct sympathetic pathways to
the different body organs does not abrogate sympa-
thetic excitation of the organs because norepinephrine
and epinephrine are still released into the circulating

loss of the two adrenal medullae usually has little
effect on the operation of the sympathetic nervous
system because the direct pathways can still perform

nism of sympathetic stimulation provides a safety

Another important value of the adrenal medullae

is the capability of epinephrine and norepinephrine
to stimulate structures of the body that are not
innervated by direct sympathetic fi
the metabolic rate of every cell of the body is increased

even though only a small proportion of all the cells
in the body are innervated directly by sympathetic
fi

because of the lost vascular tone, but over a period of

rst, the blood

tone is lost. At

stellate ganglion is removed, and normal sympathetic

ow lasting a minute or so. Then the

200 ml/min; a test dose of norepinephrine causes only a

4, showing blood

strated in Figure 60

rine or acetylcholine, respectively. This effect is demon-

parasympathetic nerve is destroyed, the innervated

Organs After Denervation

will still be partially elevated 6 months later.

heart rate to 160 beats per minute in a dog, and this

many months. For instance, loss of parasympathetic

thetic system, the compensation sometimes requires

its normal basal level. However, in the parasympa-

thetic tone is lost. That is, intrinsic compensation soon

This intrinsic tone eventually restores almost normal

bers themselves.

that is, increased tone caused by

days, or weeks,

maximal vasodilation. However, over minutes, hours,

case of the blood vessels, for instance, cutting the sym-

loses its sympathetic or parasympathetic tone. In the

parasympathetic nerve is cut, the innervated organ

Denervation.

Effect of Loss of Sympathetic or Parasympathetic Tone After

the cardiovascular system are removed. Therefore, it is

indeed, enough to maintain the blood pressure almost

of norepinephrine. These quantities are considerable

The normal resting

Tone Caused by Basal Secretion of Epinephrine and Norepi-

nal activity.

increased, thereby promoting increased gastrointesti-

thereby inhibiting gastrointestinal motility, or it can be

required. This tone can be decreased by the brain,

serious constipation,

thus demonstrating that

gastrointestinal tract. Surgical removal of the parasym-

the continual background sympathetic tone, the sym-

normal, the arterioles can be dilated. If it were not for

more; conversely, by decreasing the stimulation below

above normal, these vessels can be constricted even

stricted to about one half their maximum diameter.

. For instance, sympathetic tone nor-

it allows a single nervous

The value of tone is that

activity are known, respectively, as

systems are continually active, and the basal rates of

Normally, the sympathetic and parasympathetic

Parasympathetic “Tone”

20 times per second. This compares with full activation

normal sympathetic or parasympathetic effect, and full

vation of autonomic effectors. In general, only one

low frequency of stimulation is required for full acti-

to Degree of Sympathetic and

Relation of Stimulus Rate

The Nervous System: C. Motor and Integrative Neurophysiology

756

Unit XI

Parasympathetic Effect

A special difference between the autonomic nervous
system and the skeletal nervous system is that only a

nerve impulse every few seconds suffices to maintain

activation occurs when the nerve fibers discharge 10 to

in the skeletal nervous system at 50 to 500 or more
impulses per second.

Sympathetic and

sympathetic tone

and parasympathetic tone.

system both to increase and to decrease the activity of
a stimulated organ
mally keeps almost all the systemic arterioles con-

By increasing the degree of sympathetic stimulation

pathetic system could cause only vasoconstriction,
never vasodilation.

Another interesting example of tone is the back-

ground “tone” of the parasympathetics in the

pathetic supply to most of the gut by cutting the vagus
nerves can cause serious and prolonged gastric and
intestinal “atony” with resulting blockage of much of
the normal gastrointestinal propulsion and consequent

parasympathetic tone to the gut is normally very much

nephrine by the Adrenal Medullae.

rate of secretion by the adrenal medullae is about
0.2

mg/kg/min of epinephrine and about 0.05 mg/kg/min

—

up to normal even if all direct sympathetic pathways to

obvious that much of the overall tone of the sympa-
thetic nervous system results from basal secretion of
epinephrine and norepinephrine in addition to the tone
resulting from direct sympathetic stimulation.

Immediately after a sympathetic or

pathetic nerves results within 5 to 30 seconds in almost

intrinsic tone in the smooth muscle of

the vessels increases—
increased smooth muscle contractile force that is not
the result of sympathetic stimulation but of chemical
adaptations in the smooth muscle fi

vasoconstriction.

Essentially the same effects occur in most other

effector organs whenever sympathetic or parasympa-

develops to return the function of the organ almost to

tone to the heart after cardiac vagotomy increases the

Denervation Supersensitivity of
Sympathetic and Parasympathetic

During the first week or so after a sympathetic or

organ becomes more sensitive to injected norepineph-

–

flow in the

forearm before removal of the sympathetics to be about

slight depression in fl

flow rises markedly

days to weeks the blood flow returns much of the way
back toward normal because of progressive increase in

Effect of same

Effect of test dose

400

Normal

of norepinephrine

Stellate
ganglionectomy

test dose of
norepinephrine

Blood flow in arm (ml/min)

200

Weeks

of the vasculature to norepinephrine.

test dose of norepinephrine before and after sympathectomy,

Effect of sympathectomy on blood flow in the arm, and effect of a

Figure 60–4

showing supersensitization

mainly by the stomach glands. Finally, the rectal emp-

the mouth glands, or in other instances secretion is

increase or decrease its rate of beating. Likewise, other

c. For instance, parasympathetic

response of the sympathetic system, control functions

motor or secretory activity.

gut mainly to the paravertebral ganglia, and then back

not even enter the spinal cord, merely passing from the

whereas cooling causes opposite effects. (3) Many of

causes local vasodilation and enhanced local sweating,

ex responses. For instance, heating a local skin area

organs innervated by the sympathetics. (2) Many

of heat regulation, the sympathetics control sweating

tant of these are the following: (1) During the process

of the sympathetic nervous system. The most impor-

At other times, activation occurs in isolated portions

, which we shall discuss shortly.

by fright or fear or severe pain. The result is a wide-

. This

mass discharge

plete unit, a phenomenon called

many instances, almost all portions of the sympathetic

Organs in Some Instances and

Stimulation of Discrete

ceral functions, all of which are discussed in detail at

ing, blood glucose concentration, and many other vis-

gallbladder emptying, kidney excretion of urine, sweat-

, and then

mainly a parasympathetic func-

sources converge on the sacral cord and, in the male,

stimuli from the sexual organs. Impulses from these

exes, which are ini-

bladder and relaxation of the urinary sphincters,

cord, and this in turn causes re

of the colon; these result in strong peristaltic contrac-

the spinal cord, and a re

of the alimentary canal, sensory impulses initiated by

When fecal matter

the mouth and stomach, causing secretion of digestive

of the brain stem. These in turn transmit signals through

to the vagal, glossopharyngeal, and salivatory nuclei

exes. For instance, the

The uppermost part of

excite the parasympathetics; this allows the arterial

transmitted to the brain stem, where they inhibit the

these become stretched by high pressure, signals are

internal carotid arteries and the arch of the aorta. When

walls of several major arteries, including especially the

exes. Brie

, which is described in Chapter

arterial blood pressure and the heart rate. One of these

Several re

organ systems; to illustrate their importance, a few are

Throughout this text, the functions

autonomic reflexes.

injected into the circulating blood, the effector reaction

tors. Therefore, when a dose of the hormone is now

synapses, a process called

The cause of

Mechanism of Denervation Supersensitivity.

extent in some organs than in others, occasionally

denervation supersensitivity.

epinephrine as previously. This phenomenon is called

before, demonstrating that the blood vessels have

tered, and the blood

Then, another test dose of norepinephrine is adminis-

partially compensating for the loss of sympathetic tone.

intrinsic tone of the vascular musculature itself, thus

The Autonomic Nervous System and the Adrenal Medulla

Chapter 60

757

flow decreases much more than

become about two to four times as responsive to nor-

It occurs in both sympa-

thetic and parasympathetic organs but to far greater

increasing the response more than 10-fold.

denervation supersensitivity is only partially known.
Part of the answer is that the number of receptors in
the postsynaptic membranes of the effector cells
increases—sometimes manyfold—when norepineph-
rine or acetylcholine is no longer released at the

“up-regulation” of the recep-

is vastly enhanced.

Autonomic Reflexes

Many visceral functions of the body are regulated by

of these reflexes are discussed in relation to individual

presented here briefly.

Cardiovascular Autonomic Reflexes.

flexes in the

cardiovascular system help to control especially the

is the baroreceptor reflex
18 along with other cardiovascular refl

fly,

stretch receptors called baroreceptors are located in the

sympathetic impulses to the heart and blood vessels and

pressure to fall back toward normal.

Gastrointestinal Autonomic Reflexes.

the gastrointestinal tract and the rectum are controlled
principally by autonomic refl
smell of appetizing food or the presence of food in
the mouth initiates signals from the nose and mouth

the parasympathetic nerves to the secretory glands of

juices sometimes even before food enters the mouth.

fills the rectum at the other end

stretching the rectum are sent to the sacral portion of

flex signal is transmitted back

through the sacral parasympathetics to the distal parts

tions that cause defecation.

Other Autonomic Reflexes.

Emptying of the urinary bladder

is controlled in the same way as emptying the rectum;
stretching of the bladder sends impulses to the sacral

flex contraction of the

thereby promoting micturition.

Also important are the sexual refl

tiated both by psychic stimuli from the brain and by

result first in erection,
tion

ejaculation, partially a sympathetic

function.

Other autonomic control functions include reflex

contributions to the regulation of pancreatic secretion,

other points in this text.

Mass Stimulation in Other
Instances by the Sympathetic
and Parasympathetic Systems

Sympathetic System Often Responds by Mass Discharge.

nervous system discharge simultaneously as a com-

frequently occurs when the hypothalamus is activated

spread reaction throughout the body called the alarm
or stress response

and blood flow in the skin without affecting other

“local reflexes” involving sensory afferent fibers travel
centrally in the peripheral nerves to the sympathetic
ganglia and spinal cord and cause highly localized
refl

the sympathetic reflexes that control gastrointestinal
functions operate by way of nerve pathways that do

to the gut through sympathetic nerves to control

Parasympathetic System Usually Causes Specific Localized
Responses.

In contrast to the common mass discharge

by the parasympathetic system are much more likely
to be highly specifi
cardiovascular reflexes usually act only on the heart to

parasympathetic reflexes cause secretion mainly by

tying reflex does not affect other parts of the bowel to
a major extent.

extent, therefore, the autonomic centers in the brain

tinal activity, and cause bladder emptying. To some

other hypothalamic centers control body temperature,

arterial pressure to more than twice normal. Likewise,

brain stem autonomic control centers. For instance,

functions of the body.

ered to be an autonomic function, it is one of the

discussed in Chapter 41. Although this is not consid-

pontine centers for regulation of respiration, which are

hypothalamus. Conversely, transection immediately

. Indeed, transection of the brain

trolled in the brain stem are arterial pressure, heart rate,

cussed at appropriate points in this text. Suf

of the urinary bladder. Control of each of these is dis-

gastrointestinal peristalsis, and degree of contraction

rate, glandular secretion in the gastrointestinal tract,

autonomic functions such as arterial pressure, heart

5), control different

many special nuclei (Figure 60

the medulla, pons, and mesencephalon, as well as in

Mesencephalic Control of the

Medullary, Pontine, and

s subsequent activities vigorous.

event, the sympathetic alarm reaction makes the

ght or to run. In either

. It is also called the

events ensue immediately. This is called the sympa-

pathetic discharge; most aforementioned sympathetic

ulating the hypothalamus, signals are transmitted

, which is elicited to a great extent by stim-

vated in many emotional states. For instance, in the

The sympathetic system is especially strongly acti-

states of stress: this is called the sympathetic

can excite the sympathetic system, it is

would otherwise be possible. Because either

The sum of these effects permits a person to

8. Increased rate of blood coagulation

7. Increased mental activity

6. Increased muscle strength

5. Increased glycolysis in the liver and in muscle

4. Increased blood glucose concentration

3. Increased rates of cellular metabolism throughout

2. Increased blood

1. Increased arterial pressure

body to perform vigorous muscle activity. Let us sum-

charge

that is, a

When large portions of the sympathetic nervous

Response of the

tiate rectal emptying.

Conversely, the bladder emptying re

ex, resulting in simulta-

time. Also, the rectal emptying re

secretion, these two also often occur together, and

parasympathetic functions. For instance, although sali-

Yet there is often association between closely allied

The Nervous System: C. Motor and Integrative Neurophysiology

758

Unit XI

vary secretion can occur independently of gastric

pancreatic secretion frequently occurs at the same

flex often initiates a

urinary bladder emptying refl
neous emptying of both the bladder and the rectum.

flex can help ini-

“Alarm”

“Stress”

Sympathetic Nervous System

system discharge at the same time—

mass dis-

—this increases in many ways the ability of the

marize these ways:

flow to active muscles concurrent

with decreased blood flow to organs such as the
gastrointestinal tract and the kidneys that are not
needed for rapid motor activity

the body

perform far more strenuous physical activity than

mental or

physical stress
frequently said that the purpose of the sympathetic
system is to provide extra activation of the body in

stress

response.

state of rage

downward through the reticular formation of the brain
stem and into the spinal cord to cause massive sym-

thetic alarm reaction

fight or flight

reaction because an animal in this state decides almost
instantly whether to stand and fi

animal’

Autonomic Nervous System

Many neuronal areas in the brain stem reticular sub-
stance and along the course of the tractus solitarius of

–

fice it to

point out here that the most important factors con-

and respiratory rate
stem above the midpontine level allows basal control
of arterial pressure to continue as before but prevents
its modulation by higher nervous centers such as the

below the medulla causes the arterial pressure to fall
to less than one-half normal.

Closely associated with the cardiovascular regula-

tory centers in the brain stem are the medullary and

invol-

untary

Control of Brain Stem Autonomic Centers by Higher Areas.

Signals from the hypothalamus and even from the
cerebrum can affect the activities of almost all the

stimulation in appropriate areas mainly of the poste-
rior hypothalamus can activate the medullary cardio-
vascular control centers strongly enough to increase

increase or decrease salivation and gastrointes-

stem act as relay stations for control activities

Heat control

Parasympathetic

Water
balance

Feeding
control

Hypothalamus

Adenohypophysis

Mamillary body

Respiratory center

Cardiac slowing

Cardiac acceleration

and vasoconstriction

Pneumotaxic center

Urinary bladder control

Pons

Medulla

Sympathetic

Autonomic control areas in the brain stem and hypothalamus.

Figure 60–5

carinic actions.

muscarinic actions, whereas

, have both nicotinic and

methacholine

drugs, such as

nicotinic drugs.

Therefore, drugs

nicotinic type of acetylcholine receptor.

throughout the body.

neurons of both systems, thereby causing at the same

the postganglionic neurons. Furthermore, injected

at their endings, and this acetylcholine in turn stimulates

The

skeletal muscle.

. These drugs

acetylcholine on the muscarinic type of cholinergic effec-

block the action of

and similar drugs, such as

stimuli, and the degree of action also increases.

parasympathetic nerve endings. As a consequence, the

ing rapid destruction of the acetylcholine

These drugs inhibit acetylcholinesterase, thus

, and

pyridostigmine

of acetylcholine at the neuromuscular junction. They

at the parasympathetic endings.They are the same drugs

muscarinic type of cholinergic receptors.

They act directly on the

methacholine.

Two commonly used parasympathomimetic drugs are

drugs.

thetic effects, and they are called

Yet a number of other drugs that are not so rapidly

uids before it can reach all the effector organs.

Drugs That Act on Cholinergic

section, but an important drug for blockade of both

autonomic ganglia. They are discussed in a later

5. Sympathetic activity can be blocked by drugs that

4. The sympathetic

phentolamine.

phenoxybenzamine

Two drugs that cause this effect are

3. The sympathetic

guanethidine.

endings can be blocked. This is caused by

2. Release of norepinephrine from the sympathetic

reserpine.

sympathetic nerve endings can be prevented. The

1. The synthesis and storage of norepinephrine in the

process, as follows:

Drugs That Block Adrenergic Activity.

sympathetic effects.

endings. The released norepinephrine in turn causes the

Their effect is to cause release of norepinephrine

mine.

ephedrine, tyramine,

These drugs include

instead of directly exciting adrenergic effector organs.

Drugs That Cause Release of Norepinephrine from Nerve Endings.

(beta receptors), and

minutes to 2 hours.

as 1 to 2 minutes, whereas the actions of some other

there are many others. They differ from one another in

are also sympathomimetic drugs, and

methoxamine

adrenergic drug. Epinephrine

stimulation. Therefore, norepinephrine is called a

From the foregoing discussion, it is obvious that intra-

Drugs That Act on Adrenergic

Autonomic Nervous System

Pharmacology of the

stomach or duodenum, constipation, heart palpitation,

system. Indeed, some higher areas of the brain can

of the brain stem, and (3) the autonomic nervous

through (1) the hypothalamus, (2) the reticular areas

In Chapters 58 and 59, it is pointed out also that

hypothalamus.

initiated at higher levels of the brain, especially in the

The Autonomic Nervous System and the Adrenal Medulla

Chapter 60

759

many of our behavioral responses are mediated

alter function of the whole autonomic nervous system
or of portions of it strongly enough to cause severe
autonomic-induced disease such as peptic ulcer of the

or even heart attack.

Effector Organs—
Sympathomimetic Drugs

venous injection of norepinephrine causes essentially
the same effects throughout the body as sympathetic

sympathomimetic or

and

the degree to which they stimulate different sympa-
thetic effector organs and in their duration of action.
Norepinephrine and epinephrine have actions as short

commonly used sympathomimetic drugs last for 30

Important drugs that stimulate specific adrenergic

receptors but not others are phenylephrine (alpha
receptors), isoproterenol

albuterol

(only beta

receptors).

Certain drugs have an indirect sympathomimetic action

and ampheta-

from its storage vesicles in the sympathetic nerve

Adrenergic activity

can be blocked at several points in the stimulatory

best known drug that causes this effect is

alpha receptors can be blocked.

and

beta receptors can be blocked.

A drug that blocks beta

and beta

receptors is

propranolol. One that blocks mainly beta

receptors is metoprolol.

block transmission of nerve impulses through the

sympathetic and parasympathetic transmission
through the ganglia is hexamethonium.

Effector Organs

Parasympathomimetic Drugs (Cholinergic Drugs).

Acetyl-

choline injected intravenously usually does not cause
exactly the same effects throughout the body as
parasympathetic stimulation because most of the acetyl-
choline is destroyed by cholinesterase in the blood and
body fl

destroyed can produce typical widespread parasympa-

parasympathomimetic

pilocarpine and

Drugs That Have a Parasympathetic Potentiating Effect—Anti-
cholinesterase Drugs.

Some drugs do not have a direct

effect on parasympathetic effector organs but do poten-
tiate the effects of the naturally secreted acetylcholine

as those discussed in Chapter 7 that potentiate the effect

include neostigmine,

ambenonium.

prevent-

liberated at

quantity of acetylcholine increases with successive

Drugs That Block Cholinergic Activity at Effector Organs—
Antimuscarinic Drugs.

Atropine

homatropine and scopolamine,

tor organs

do not affect the nicotinic action

of acetylcholine on the postganglionic neurons or on

Drugs That Stimulate or Block
Sympathetic and Parasympathetic
Postganglionic Neurons

Drugs That Stimulate Autonomic Postganglionic Neurons.

preganglionic neurons of both the parasympathetic and
the sympathetic nervous systems secrete acetylcholine

acetylcholine can also stimulate the postganglionic

time both sympathetic and parasympathetic effects

Nicotine is another drug that can stimulate postgan-

glionic neurons in the same manner as acetylcholine
because the membranes of these neurons all contain the

that cause autonomic effects by stimulating postgan-
glionic neurons are called

Some other

pilocarpine has only mus-

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Dampney RA, Horiuchi J, Tagawa T, et al: Medullary and

Pharmacol Sci 25:317, 2004.

receptors and the regulation of neuronal signalling. Trends

Dajas-Bailador F, Wonnacott S: Nicotinic acetylcholine

284:G357, 2003.

to the gut. Am J Physiol Gastrointest Liver Physiol

ex? IV. Current concepts of vagal efferent projections

derer: what

Chang HY, Mashimo H, Goyal RK: Musings on the wan-

tractus solitarius. Ann N Y Acad Sci 940:132, 2001

Andresen MC, Doyle MW, Jin YH, Bailey TW: Cellular

sure in many patients with hypertension, but these drugs

the effects of parasympathetic blockade. The ganglionic

the parasympathetic systems simultaneously. They are

These drugs block acetylcholine stimulation of

, and

neurons to the postganglionic neurons, including

trointestinal activity and, sometimes, slowing of the

pathetic postganglionic neurons at the same time,

The Nervous System: C. Motor and Integrative Neurophysiology

760

Unit XI

Nicotine excites both the sympathetic and parasym-

resulting in strong sympathetic vasoconstriction in the
abdominal organs and limbs but at the same time result-
ing in parasympathetic effects such as increased gas-

heart.

Ganglionic Blocking Drugs.

Many important drugs block

impulse transmission from the autonomic preganglionic

tetraethyl ammonium ion, hexamethonium ion

pen-

tolinium.
the postganglionic neurons in both the sympathetic and

often used for blocking sympathetic activity but seldom
for blocking parasympathetic activity because their
effects of sympathetic blockade usually far overshadow

blocking drugs especially can reduce the arterial pres-

are not very useful clinically because their effects are
difficult to control.

References

mechanisms of baroreceptor integration at the nucleus

’s new in our understanding of vago-vagal

refl

supramedullary mechanisms regulating sympathetic vaso-

quences of altering myocardial adrenergic receptor sig-

2001.

visceral perception and autonomic pattern generation.

diovascular and respiratory systems and their interactions