-- 17 --

Section A:Dysautonomias

-- 18 --

-- 19 --

The “Automatic”Nervous System

-- 20 --

-- 21 --

What is theAutonomic

Nervous System?We all have a nervous system. What exactly makes upthis system? What does it do? And what is the“autonomic” part of the nervous system?

This chapter is about your nervous system and how itfunctions when there is nothing wrong with it. You willneed to understand the basics before you can understandthe problems that can develop.

Your body has to be able to coordinate many differentactivities, just to keep you going. Some of these activitiesare automatic, like breathing and digesting. Some arevoluntary, like moving your legs to walk across the room.Your brain uses different parts of the nervous system toregulate these activities.

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Brain

Spinal Cordto

Organs

Perip

hera

l Ner

ves

CentralNervousSystem

The central nervous system is like a lollipopon a stick. The brain is the candy. The spinalcord is the stick.

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The central nervous system is made up ofthe brain and the spinal cord. The brain islike a command and control center. Thespinal cord is a rope of nerves that runs fromthe base of your brain down through yourback in your spinal column. The controlsignals travel from your brain to your limbsand organs by way of the peripheral nervoussystem. The peripheral nerves are all thenerves that lie outside the brain and spinalcord.

The peripheral nervous system has two maindivisions. The first is the somatic nervoussystem, which helps you deal with the “outerworld.” The second is designed to help youregulate your “inner world,” makingadjustments to the systems inside your body.This is the autonomic nervous system.

We are going to devote the rest of thischapter to explaining the components of theautonomic nervous system and how itworks.

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AutonomicNervousSystem

("Automatic")

SomaticNervousSystem

("Voluntary")

SkeletalMuscle

Smooth Muscle& Glands

Central Nervous System

("The Inner World") ("The Outer World")

Peripheral Nervous System

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Think of theautonomic nervoussystem as the“automatic nervoussystem.”

The autonomic nervous systemregulates the “inner world” of thebody.

The autonomic nervous system isresponsible for many of theautomatic, usually unconsciousprocesses that keep the bodygoing, such as…

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• keeping the rightblood flow tothe brain

• keeping the right bodytemperature

• keeping the right amount ofenergy production and fueldelivery

• getting rid of waste products

• warning signs in dangeroussituations, such as fast pulserate, increased blood pressure,sweating, pallor, andtrembling.

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By way of the autonomic, or “automatic”nervous system, the brain controls the“inner world.”

The autonomic nervous system is the main way the brainregulates the “inner world.” The somatic nervous systemis the main way the body deals with the “outer world.”

The autonomic nervous system sends signals to makechanges to the organs in our bodies. These organs aremade up of a type of muscle called smooth muscle.Smooth muscle is found in organs like your heart andblood vessel walls, and in your glands, such as thethyroid gland, adrenal gland, pancreas, and sweat glands.The autonomic nervous system sends signals to thesmooth muscle cells that cause changes in their muscletone. All of this happens automatically, all day and night,to keep your body functioning. The target organ of thesomatic nervous system is skeletal muscle.

Nerves that go to skeletal muscle to regulate movementcome directly from the central nervous system, butnerves of the autonomic nervous system come indirectlyfrom the central nervous system, by way of clumps ofcells called ganglia. Ganglia are like transformers on theutility pole outside your house. The transformer relaysthe electricity that comes in from the thick trunk lines tothe thin cables that go to the house. The ganglia arearranged like pearls on a string along each side of thespinal cord.

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DistributionCenter

GeneratorPlant

Terminal Box

Brain

Ganglion

Nerve Terminal

Transformer

SpinalCord

Ganglia are like transformers that transferthe electricity from the utility pole to theterminal box outside your house.

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Ganglia

AutonomicNervousSystem

("Automatic")

SomaticNervousSystem

("Voluntary")

SkeletalMuscle

Smooth Muscle& Glands

CentralNervousSystem

("The Inside World") ("The Outside World")

Preganglionic Nerve

Postganglionic Nerve

Ganglia are clumps of cells that relaycontrol signals to the “inner world.” Nervesto the ganglia are preganglionic and fromthe ganglia to the organs postganglionic.

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Spinal Cord

from the Brain

Ganglia

to the Head & Neck

to the Heart

to the Limbs

to the Gut

to the Adrenals

to the Kidneys

to the Sex Organs

Ganglia are arranged like pearls on a stringon each side of the spinal cord.

In the autonomic nervous system, control signals from thebrain and spinal cord go to the ganglia (singularganglion) in the preganglionic nerves, and nerves from

-- 31 --

the ganglia, called postganglionic nerves, deliver thosesignals to the nerve terminals near or in the target tissues.

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What are theParts of theAutonomicNervous System?As first described a littleover a century ago, theautonomic nervous systemincludes theparasympathetic nervoussystem and the sympatheticnervous system.

A third part, the relatively less wellunderstood “enteric” nervous system, isin the gastrointestinal tract.

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Ganglia

AutonomicNervousSystem

("Automatic")

SomaticNervousSystem

("Voluntary")

ParasympatheticNervousSystem

SympatheticNervousSystem

SkeletalMuscle

Smooth Muscle& Glands

CentralNervousSystem

("The Inner World") ("The Outer World")

EntericNervousSystem

The autonomic nervous system includes thesympathetic, parasympathetic, and entericnervous systems, which help regulate the“inner world.”

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Spinal Cord

from the Brain

to the Adrenals

Kidney

EPINEPHRINE(Adrenaline)

Epinephrine (adrenaline) is released fromthe adrenal glands, which sit on top of thekidneys.

Also about a century ago, the hormone, adrenaline(epinephrine), was discovered. Adrenaline is releasedinto the bloodstream from the medulla (from the Latinword for “marrow”) of the adrenal glands, which sit atthe tops of the kidneys.

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With these discoveries, scientists began to understandmore about the autonomic nervous system and what rolesit plays in our daily lives. The combination of thesympathetic nervous system and adrenal glands came tobe understood as a single “emergency” system for thebody. This emergency system was called the“sympathoadrenal” system, or “sympathico-adrenal”system. You may have heard of the “fight-or-flight”response that you experience in distressing situations.The sympathoadrenal system would help you to surviveemergencies, by adjusting several body processes toenhance your ability to protect yourself (fight) or toescape (flight).

The parasympathetic nervous system in many ways actslike the opposite of an emergency system. “Vegetative”behaviors, activities that increase instead of use upenergy, are associated with increased activity of thissystem. Examples of this are sleeping, eating, digesting,and excreting waste.

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SympatheticNervousSystem

Ganglia

AutonomicNervousSystem

("Automatic")

ParasympatheticNervousSystem

Smooth Muscle & Glands("The Inside World")

AdrenomedullaryHormonalSystem

Sympathoadrenal System

Scientists have thought the sympatheticnervous system and adrenal medulla are asingle “emergency” system, the“sympathoadrenal system.”

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CranialNerves

SacralSpinalCord

Head & Necke.g., Pupi ls, Salivary Glands

Heart

Gut Organs

Sex OrgansLower GI Tract

Ganglia in or near the target organs

from the Brain

The parasympathetic nervous system hastwo parts, at opposite ends of the nervoussystem.

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The upper part of the parasympathetic nervous system isthe nerves that come from a portion of your brain calledthe brainstem. The brainstem connects the brain to thespinal cord. Most of the nerves of the parasympatheticnervous system come from the brainstem. These nervestravel to many parts of your body, including the eyes,face, tongue, heart, and most of the gastrointestinalsystem.

The nerves that come from the brainstem are called thecranial nerves (the word, “cranial,” refers to the skull).The parasympathetic nerve fibers travel in major cranialnerves that have specific names. The oculomotor nerveconnects to the eye, the facial nerve to the face, theglossopharyngeal nerve to the tongue and musclesinvolved with swallowing and talking, and the vagusnerve to the heart and most of the abdominal organs. Youmay have heard your doctor talk about some of thesenerves, especially the vagus nerve.

The lower part of your parasympathetic nervous systemis the group of nerves that travel from the bottom level ofthe spinal cord, which is called the sacral spinal cord.These nerves travel to the genital organs, urinary bladder,and lower gastrointestinal tract.

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MidbrainOculomotor Nerve (Eyes)

Pelvic Nerve (Genito-Urinary Organs)

Medulla Vagus Nerve (Most Organs)

Glossopharyngeal Nerve (Throat)

PonsFacial Nerve (Face)

Brainstem

Spinal Cord

SacralLevel

HeartStomachPancreasetc.

Parasympathetic nerves come from thebrainstem and from the bottom of the spinalcord. Scientists have thought that theparasympathetic nervous system regulates“vegetative” body functions.

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The nerves of the sympathetic nervous system come fromthe spinal cord at the levels of the chest and upperabdomen (thoracolumbar spinal cord). The sympatheticnerves to most organs are postganglionic, coming fromcell bodies in the ganglia. Remember that the ganglia arethe clusters of nerve cells like a transformer on the utilitypole that supplies the electricity to your house. From theprefix, “post” meaning “after,” the postganglionic nervescome from the ganglia, like the electric line that comesfrom the utility pole to your house.

You also have sympathetic nerves that travel to youradrenal glands (the glands that sit on top of your kidneysand release the hormone adrenaline). These nerves arecalled preganglionic, from the prefix, “pre”, meaningbefore, the preganglionicnerves come from cell bodies inyour spinal cord and then pass through the ganglia. It islike a direct wiring connection from the electricaldistribution center to the terminal box on your house.Most of the sympathetic nerves to the adrenal medullaare preganglionic, coming from cell bodies in the spinalcord and passing through the ganglia.

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Kidneys

Head & Necke.g. , Pupi ls, Salivary Glands

Heart

Adrenals

Gut

Sweat Glands

Genital OrgansLower GI Tract

Spina

l Cor

d

from the Brain

Ganglia

The sympathetic nerves come from gangliaattached to the spinal cord at the levels ofthe chest and upper abdomen.

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How Does theAutonomicNervous SystemWork?The autonomic nervous system works by

releasing messenger chemicals inside the

body. These chemicals act on receptors on

target cells, such as heart muscle cells, and

this changes body functions.

For example, when you exercise on a hot day, activationof a part of the autonomic nervous system releasesacetylcholine, a chemical messenger, from the nerveterminals, activating receptors on the cells of sweatglands. Activation of the receptors causes the glands torelease sweat.

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There are two types of chemical

messengers that can be released in your

body. The first type is a messenger that

is released directly from the nerves in

body organs. Chemicals released from

nerve terminals in body organs are

called neurotransmitters.

The second type of messenger is

released directly into the bloodstream.

This type of messenger is called a

hormone. One famous hormone is

adrenaline, released into the

bloodstream by the adrenal gland, the

gland that sits on top of each kidney.

Chemicals released into the

bloodstream are called hormones.

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A Quick Review

Now is a good time for us to review the information sofar. It can be a bit confusing, because of the several“nervous systems.”

Remember, you have a central nervous system (yourbrain and spinal cord) and a peripheral nervous system(the rest of your nerves). Your peripheral nervous systemhas two divisions, the somatic nervous system and theautonomic nervous system. The somatic nervous systemis concerned with the “outer world,” and the nerves inthis system travel to skeletal muscle.Your autonomicnervous system is concerned with the “inner world” ofyour body, and it usually works automatically, so thatyou can think of the autonomic nervous system as the“automatic nervous system.”

The control signals of the autonomic nervous systemtravel indirectly from your central nervous systemthrough ganglia (clusters of nerve cells) to smoothmuscle, found in areas like your blood vessels, heart, andglands throughout your body. Nerves coming from theganglia are called postganglionic. Some nerves, such asthose to the adrenal glands, pass through the gangliawithout relaying within the ganglia, so that there is adirect connection from the central nervous system to thetarget organs, and these nerves are called preganglionic.

-- 45 --

You have also learned that there are two divisions of theautonomic nervous system, called the sympatheticnervous system and the parasympathetic nervous system.

And you have learned that the autonomic nervous systemworks by releasing chemical messengers that act onreceptors located in organs throughout the body. Thesemessengers either come from nerves (neurotransmitters)in body organs or are released into the bloodstream(hormones). The adrenal glands located on the tops ofthe kidneys are where the hormone adrenaline isreleased. The adrenal glands, combined with thesympathetic nervous system, has been called the“sympathoadrenal system,” which can function as anemergency system to help protect you in “fight-or-flight”situations.

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Nerve Traffic

NeurotransmitterRelease

Receptor

Nerve Terminal Target Cell

HormoneRelease

Endocrine Cell Bloodstream

Receptor

Target Cell

Chemical Neurotransmission

Hormone Release

Body Function

Body Function

The autonomic nervous system releaseschemical messengers and hormones.

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What are theChemical

Messengers of theAutonomic

Nervous System?There are two main chemical messengers(neurotransmitters) of the autonomicnervous system and one main hormone.Acetylcholine and norepinephrine (alsocalled noradrenaline) are theneurotransmitters. Adrenaline (also calledepinephrine) is the hormone (remember thatthis is the hormone released from theadrenal glands).

These chemical messengers are useddifferently by your autonomic nervoussystem. Acetylcholine (ACh) is theneurotransmitter that is used by theparasympathetic nervous system.

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Norepinephrine (NE, noradrenaline) is themain neurotransmitter used by thesympathetic nervous system. Epinephrine(EPI, adrenaline) is the hormone used by theadrenomedullary hormonal system. (Theadrenal medulla is the center part of theadrenal gland. It is surrounded by theadrenal cortex, which releases otherhormones.)

Acetylcholine is the main chemical messenger that relayscontrol signals from the preganglionic to thepostganglionic cells in the ganglia. Acetylcholine does soby binding to a specific type of receptor called a nicotinicreceptor. As the name suggests, nicotine stimulatestransmission of signals within the ganglia.

Acetylcholine is important for "vegetative"activities, like eating, digesting, sweating,and getting rid of waste.

Acetylcholine is also used to help signal the release ofepinephrine (adrenaline) from the adrenal gland. Thesympathetic nerves that trigger the release of adrenalinepass through the ganglia and directly supply the cells inthe adrenal gland that produce and release adrenaline.Acetylcholine is released from the sympathetic nerveterminals in the center of the adrenal gland, which is

-- 49 --

called the adrenal medulla. The acetylcholine then bindsto nicotinic receptors on these cells, which stimulatesthem to release adrenaline into the bloodstream. Thebloodstream delivers the adrenaline to organs throughoutthe body. This is how adrenaline is able to produce somany different effects in the body.

The bloodstream delivers adrenalinethroughout the body.

We all know that cigarettes contain nicotine. The acuteeffects of nicotine in the body, such as fast pulse rate,increased blood pressure, sweating, and increasedproduction of saliva, result from the release ofepinephrine and from the increased transmission of nervesignals in ganglia supplying the sympathetic nervoussystem and parasympathetic nervous system.

The effects of nicotine in the body resultfrom increased release of adrenaline intothe bloodstream and from increasedtransmission of nerve impulses through theganglia.Norepinephrine is the main chemicalmessenger, or neurotransmitter, of thesympathetic nervous system.

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As a neurotransmitter, norepinephrine released fromsympathetic nerve terminals acts locally on nearby cells.For instance, norepinephrine released from sympatheticnerve terminals in the heart acts on heart muscle cells. Asmall amount of released norepinephrine makes its wayinto the bloodstream, but usually the amount is too smallfor norepinephrine to produce effects as a hormone.Nevertheless, specialized laboratories can measure theamount of norepinephrine in the bloodstream, and thiscan provide an index of the activity of the sympatheticnervous system. The chapter about testing goes into detailabout the source and meaning of plasma norepinephrinelevels.

Although the main chemical messenger of thesympathetic nervous system is norepinephrine(noradrenaline), an exception to this rule is in the sweatglands, where sympathetic nerves release acetylcholine asthe signal for sweating. This means that sympatheticcholinergic nerves cause changes in sweating such as inresponse to changes in environmental temperature.

Norepinephrine (noradrenaline) andepinephrine (adrenaline) are in a chemicalfamily called catecholamines.

Catecholamines are a small family of body chemicalswhose members are norepinephrine, epinephrine, anddopamine. Norepinephrine and epinephrine are keychemical messengers of the autonomic nervous system.

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Dopamine is an important chemical messenger in thebrain that helps to regulate movement and mood.Suprisingly, most of the dopamine made in the body isproduced outside the brain, and the functions ofdopamine outside the brain are still poorly understood.

You may have heard your physician talk about testingyour catecholamine levels. This is done to help determinehow your autonomic nervous system is working. We willtalk more about this in the chapter about autonomicfunction testing.

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What are theFunctions of theDifferent Parts ofthe AutonomicNervous System?The parasympathetic nervous systemregulates what are known as“vegetative” processes. These includebody functions like digestion andurination. Remember that acetylcholineis the neurotransmitter used by theparasympathetic nervous system.Acetylcholine released fromparasympathetic nerves acts tostimulate the gut, increase urinarybladder contractions, increasesalivation, and decrease the pulse rate.

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The parasympathetic nervous systemregulates “vegetative” processes. Thesympathetic nervous system keeps bodynumbers like temperature and bloodpressure in check. The adrenomedullaryhormonal system regulates “emergency”processes such as in distress.

The sympathetic nervous system regulatesunconscious “housekeeping” processes, suchas tightening blood vessels when you standup and increasing the rate and force of theheartbeat when you exercise. When you areexposed to cold, norepinephrine releasedfrom sympathetic nerves in the skin causespallor, goosebumps, and hair standing out.The sympathetic nervous system thereforehelps keep body numbers like temperatureand blood pressure in check.

When you are exposed to heat, eat spicyfoods, or experience distress, acetylcholinereleased from sympathetic nerves in the skinstimulates production of sweat.

-- 54 --

The adrenomedullary hormonal systemplays a key role in “emergencies” and“distress,” when all organs of the body arethreatened, such as by low blood sugar(hypoglycemia), low blood temperature(hypothermia), choking (asphyxiation),shock, and fear. Adrenaline increases bloodglucose levels, increases pulse rate andblood pressure, stimulates metabolism,quiets the gut, and dilates blood vessels inskeletal muscle.

Psychologists and researchers have differed about themeanings of “stress” and “distress” as medical scientificideas. For the purposes of this book, “stress” is acondition where the brain senses a challenge to physicalor mental stability that leads to altered activity of systemsto meet that challenge; and “distress” is a form of stresswhere there is a sense that you can’t cope with thesituation, you want to avoid or escape it, you show built-in, instinctively communicated signs, and the adrenalgland is activated. According to these definitions, neitherstress nor distress is necessarily harmful or causesdisease.

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The Sympathetic Norepinephrine System andthe Parasympathetic Acetylcholine Systemusually antagonize each other…

Sympathetic Parasympathetic≠ Pulse rate Ø Pulse rate

Ø Gut tone ≠ Gut tone

Ø Bladder tone ≠ Bladder tone

Dilated pupils Constricted pupils

…but not always…

Sympathetic Parasympathetic≠ Saliva (thick) ≠ Saliva (mucus)

Ejaculation Erection

-- 56 --

The Sympathetic Norepinephrine System and

the Adrenomedullary Hormonal System

usually work together,

SNS AHSPulse rate ≠ ≠

Systolic blood pressure ≠ ≠Gut tone Ø ØSweating ≠ ≠Dilated pupils ≠ ≠

…but not always…

SNS AHSPeriph. resistance ≠ ØSkeletal m. blood flow Ø ≠Fainting Ø ≠

-- 57 --

Sympathetic Norepinephrine EffectsTightens blood vessels

Relaxation of the gutEmotional sweatingGoosebumps and hair standing out

Increased pulse rateIncreased force of heartbeat

Increasedblood pressure

Salt retention by the kidneysIncreased salivation (mucus)Dilation of the pupils

Adrenomedullary Hormonal EffectsIncreased pulse rateIncreased force of heartbeatConstriction of skin blood vessels (pallor)Dilation of the pupilsRelaxation of the gutIncreased blood sugarDecreased serum potassium levelIncreased respirationEmotional sweatingAnti-fatigue effectIncreased emotional intensity

Sympathetic Acetycholine EffectsSweating from altered temperatureSweating from eating spicy foods

Parasympathetic Acetylcholine Effects

Increased salivation (watery)Stimulation of the gutStimulation of the urinary bladderPenile erectionConstriction of the pupils

Decreased pulse rate

-- 58 --

Summing Up

You now have a basic understanding of your autonomicnervous system. It plays a critical role both inemergencies and in our daily activities, working to keepus going and helping our bodies make adjustmentsthroughout the day.

Much of what you have learned may seem a bitcomplicated, but if you remember the basics, this willhelp you understand what can happen in your body if theautonomic nervous system is not working like it should.

You will probably need to refer back to this chapter forreview as you continue to learn about the disorders andtesting of the autonomic nervous system and thetreatments of dysautonomias in the following chapters.