the amazing human body nervous system

The human nervous system is made up of the brain, the spinal cord, and the networks of nerves running throughout the body... The spinal cord is a long, bundle of nerves that carries nerv

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T H E A M A Z I N G H U M A N B O D Y

NERVOUS SYSTEM

GE O R GE C A PA C C I O

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2nd Proof Title : The Amazing Human Body 2-Nervous System : 28147

: CPL909-20/4028

Marshall Cavendish Benchmark

99 White Plains Road Tarrytown, New York 10591 www.marshallcavendish.us Text copyright © 2010 by Marshall Cavendish Corporation All rights reserved No part of this book may be reproduced or utilized in any form or by any means electronic or mechanical including photocopying, recording, or by any information storage

and retrieval system, without permission from the copyright holders.

All websites were available and accurate when this book was sent to press

Editor: Karen Ang Publisher: Michelle Bisson Art Director: Anahid Hamparian Series Design by: Kay Petronio Series Designer: Elynn Cohen Library of Congress Cataloging-in-Publication Data

Capaccio, George.

The nervous system / by George Capaccio

p cm (The amazing human body) Includes bibliographical references and index.

Summary: “Discusses the parts that make up the human nervous system, what can go wrong, how to treat those

ill-nesses and diseases, and how to stay healthy” Provided by publisher.

1 Nervous system Juvenile literature 2 Nervous system Diseases Juvenile literature I Title.

QP361.5.K56 2010 612.8 dc22 2008037255

Front cover: The brain and the spinal cord Back Cover: Nerve cells

Photo research by Tracey Engel Front cover photo: Derek Berwin / Getty The photographs in this book are used by permission and through the courtesy of: Getty Images: Dr Fred Hossler, 1;

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10, 14, 19; Dr David M Phillips, 12; Scientifica, 13; artpartner-images, 16; Carol and Mike Werner/Visuals Unlimited, Inc.,

20; Barbara Singer, 22; Nucleus Medical Art, Inc., 24, 27, 29, 35, 36, 40, 55, 59, 65; James Worrell, 26; Dr John D

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123456

= nerve cells from the brain stem

This book is not intended for use as a substitute for advice, consultation, or treatment by a licensed medical practitioner

The reader is advised that no action of a medical nature should be taken without consultation with a licensed medical practitioner, including action that may seem to be indicated by the contents of this work, since individual circumstances vary and medical standards, knowledge, and practices change with time The publisher, author, and medical consultants disclaim all liability and cannot be held responsible for any problems that may arise from use of this book

ISBN 978-0-7614-4489-3

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C H A P T E R 1

What Is the Nervous System? 5

C H A P T E R 2 How Does the Nervous System Work? 21

C H A P T E R 3 Diseases and Disorders 47

C H A P T E R 4 A Healthy Nervous System 63

GLO S S A RY 73

F IN D OU T MOR E 76

IN DE X 78

CONTENTS

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The human nervous system can be divided into two separate systems—the central nervous system (CNS) and the peripheral nervous system (PNS) The central nervous system includes the brain and the spinal cord The peripheral nervous system is everything else This

What Is the Nervous System?

The human nervous system is made up of the brain, the spinal cord, and the networks of nerves running throughout the body

1

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N E R V OU S S Y S T E M

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“everything else” includes all the nerves that connect the central nervous

system with the muscles, glands, and organs of our bodies

T H E BR A IN

An adult brain weighs about 3 pounds (1.4 kilograms) It is roughly the

size of a large grapefruit and kind of looks like a cauliflower The human

brain is probably the most complex organ in the body

It has about 100 billion nerve cells, or neurons, and trillions of support cells called glia If you could remove all 100 billion nerve cells

and arrange them in a straight line, they would stretch for a distance of

about 600 miles (1,000 kilometers) Of course, you would not be able to

see this line since nerve cells are only visible under a microscope Many

scientists think that if you tried to count all of the neurons in the human

brain, it would take you more than three thousand years

The activities of the billions of nerve cells in the brain are responsible for many of the important processes in the body

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W hat Is the Ner v o us S ys tem ?

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The spinal cord is a long, bundle of nerves that carries nerve impulses

back and forth between the brain and the rest of the body It is about 17

inches (43 centimeters) long in women and 18 inches (45 cm) in men It is

about 5 inch (1.3 cm) wide

The top of the spinal cord connects with the base of the brain at the brain stem The lower end of the spinal cord is about two-thirds of the

way down the spinal column The spinal cord is safely contained within

the spinal column, which is also called the spine or the vertebral column

The spine is made up of thirty-three bones called vertebrae Each vertebra

is separated from the one above and the one below by a disc of flexible,

shock-absorbent tissue called cartilage The spinal column is an important

part of the body’s skeletal system It supports the skull and connects with

the hips

The spinal cord has thirty-one pairs of spinal nerves With the exception of the top pair of nerves, the spinal nerves exit the spinal

column through the spaces between the vertebrae Spinal nerves conduct

information to and from the spinal cord to the rest of the body

T H E P E R IP H E R A L N E R V O U S S Y S T E M

The peripheral nervous system is made up of all the nerves outside the

central nervous system These include the nerves that leave the brain

and the spinal cord and extend to all parts of the body Part of the job

of the peripheral nervous system is to gather information from sensory

receptors and relay, or send out, this information to the central nervous

system The messages are transmitted in the form of electrical signals, or

nerve impulses.The central nervous system then interprets the messages

it receives and sends out the appropriate responses to the body’s glands,

muscles, and organs

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The spinal cord starts at the base of the brain and stretches down to the bottom of the spine

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down into a sensory nervous system and a motor nervous system The

sensory nervous system carries information from sensory receptors to the

central nervous system The motor nervous system gathers information

Nerves (blue and green) stretch throughout the body to connect the different organs and glands to the brain and spinal cord.

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from the central nervous system and relays it to the appropriate organs and muscles

The motor nervous system can be further divided into the autonomic and the somatic nervous systems The autonomic system is also called the involuntary nervous system It is responsible for all those movements and activities we have no conscious control over This includes activities like sweating when we are hot or shivering when we are cold

The somatic, or voluntary nervous system, handles those movements

we consciously decide to make Getting out of bed in the morning, running down the football field for a touchdown, or playing a video game are activities that require conscious choice Our somatic nervous system makes it possible for us to make such choices in order to move our bodies

in certain ways

N E U R O N S

Our body is made up of many different kinds of cells Each type has its own unique function or purpose Most cells are microscopic in size, which means they can only be seen with the aid of a microscope Neurons are the most basic part of the nervous system A neuron is a nerve cell Nerve cells are highly specialized cells, and are different from all the other cells

in our bodies

Neurons are the only cells in the human body that can send

“messages” from part of the body to another part These messages are

in the form of electrochemical changes In other words, nerve cells can communicate with each other This exchange of information results

in either voluntary or involuntary actions in our muscles, glands, and organs

Many neurons look like squashed stars that have been stretched in different directions While there is much variety in the size and shape of

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neurons, all nerve cells have certain characteristics in common They have

a central part called a nucleus The parts that extend from the nucleus are the axons and dendrites Axons carry information—in the form of

an electrical signal—away from the neuron to another neuron Dendrites receive electrical signals from other neurons

A typical neuron is about 10 microns wide, which is smaller than

a period at the end of a sentence or the dot on top of a small letter “i.”

Though they are not very wide, neurons are the longest cells in the human body The axon a neuron in the spinal cord that connects to a foot muscle can be about 3 feet (1 meter) long Nerves are thin threads of neurons

These nerves create a kind of electrical circuitry in the body Without

it, we could not feel or respond to changes inside our body and in the world outside

All neurons, or nerve cells, have a main cell body and long extensions called axons and dendrites

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There are different types of neurons One way to classify neurons is by the number of extensions they have Another way is by the direction in which the neurons send information Motor, or efferent, neurons send information away from the central nervous system toward muscles and glands Sensory, or afferent, neurons send information from sensory receptors toward the central nervous system Interneurons send information between sensory and motor neurons and are mostly located

in the brain and spinal cord

Sensory neurons respond to stimuli from the environment and transmit an electrical signal to the spinal cord or the brain where the signal is interpreted The brain then sends out an appropriate response through a pathway of motor neurons to a muscle or gland This results

in some kind of action or response For example, we pull our hand away from something hot, or we smile with delight from the taste or smell of a favorite food

N E R V E IM P U L S E S

Suppose you get up in the middle of the night to get a drink of water On the way to the kitchen, you stub your big toe on a table leg Right away, sensory neurons in your big toe go into action

Have you ever seen baseball fans do a “wave” during a game? That is when one section after another stands up and then sits down This creates

a wavelike motion that travels around the entire stadium Or maybe you have stacked up a line of dominos and then knocked over the first one

What happens next? All the dominos fall, one by one

The act of stubbing your toe has set off an electrical charge or pulse

Like the baseball stadium wave or the falling dominos, the pulse travels all the way down the axon of one neuron to the dendrites of the next

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neuron in line, and so on until it reaches your spinal cord From there, this

nerve impulse travels up to your brain where the signal is received and

interpreted In a flash, your brain recognizes the sensation and you yell,

“Ouch!” Of course, a similar process takes place if the original stimulus

is something pleasurable like taking a hot shower on a cold morning Then

your response will be more like “Ahhh!”

Information in the form of electrical pulses travels at different speeds depending upon what types of neurons are activated The speed of

transmission can range from about 1.5 feet (0.5 m) per second to 394 feet

(120 m) per second The faster speed is equivalent to driving 268 miles

(431 kilometers) per hour

C R O S S IN G T H E G A P

At the end of every axon there is a tiny gap or space called a synapse

Synapses occur between neurons (They also occur between a neuron and

a muscle or gland.) When a nerve impulse reaches the end of an axon, it

Nerve cells are always found alongside other nerve cells so that they can pass along signals and

messages

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Messages are transmitted from neuron to neuron through chemicals called neurotransmitters that cross the synaptic gap between the neurons

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has to cross the synapse in order to continue on its way and to get picked

up by the dendrites of the next neuron in line So how does it do this?

Imagine yourself trying to cross from one side of a river to the other side

You could try swimming But a better choice would be to take a boat or

use a bridge if there is one available

Nerve impulses cannot do any of these things to cross a synapse

They have to rely on special chemicals called neurotransmitters These

chemical messengers are stored in the body of the nerve cell When an

action potential, or nerve impulse, reaches the end of an axon, the cell

releases these chemicals Molecules of neurotransmitters travel across

Chemical and electrical signals are constantly moving through the body and transmitting messages to the brain

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the synapse to the dendrites of the next neuron in line These chemicals give the next neuron two choices It can either increase or decrease the likelihood that the next neuron will send the impulse along For example,

if you go to the dentist to have a cavity filled, you will sometimes get an injection of medication that numbs your mouth The numb feeling occurs because you have lost sensation in part of your mouth That is because the medication blocks the nerve impulses that would otherwise make you feel the pain of having a cavity filled You do not feel the pain because the nerve impulse does not travel from neuron to neuron to your brain

Teeth have nerves running through them A toothache occurs when the nerves inside are exposed or damaged.

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How Does the Nervous

System Work?

2

T he brain is what makes it possible for us to do all the things we

do in our lives It is also responsible for regulating our glands and organs It receives, interprets, and responds to nerve impulses It enables our five senses to give us the world in all its incredible richness and complexity

Besides being in charge of our body’s primary functions, the brain

is a very sensitive organ It is easily bruised or injured Fortunately,

it is enclosed in five layers of protection The outermost layer is the skull The next three layers are tough membranes called meninges

Without the transmission of electrical and chemical signals between neurons, the nervous system would not function

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In the space between the second and third meninges, a layer of clear

cerebrospinal fluid acts as a liquid cushion for added protection between

the brain and the skull

Before a child is born, the brain develops at a very fast rate Each minute about 250,000 neurons are added By the time a child is born, the

brain has almost all of the neurons the person will need for the rest of

his or her life But the brain does not stop growing By the age of two, the

child’s brain will be about 80 percent of the size of an adult brain Much

of the additional brain matter comes from specialized cells called glia or

neuroglia Glial cells are necessary for normal brain functions They also

support nerve cells in the brain and spinal cord by producing myelin, a

The fragile brain is protected by the hard and bony skull (white outline) Between the brain and skull are layers of meninges and cerebrospinal fluid that cushion the brain

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Ho w Do e s the Ner v o us S ys tem Wor k ?

The nerves and tissues of the brain stay healthy and active because of the nutrients provided by the many blood vessels (red).

protein-based substance that insulates nerve axons This insulation acts

as a protective covering that allow nerve impulses to travel more smoothly from neuron to neuron

A healthy brain is like a smoothly functioning team A football team, or any team for that matter, is made up of players with specific jobs

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to do When all the players are doing their job and working well with

each other, the team is in good shape It is the same way with the brain,

which consists of different parts and structures Each part of the brain

has a specific structure and a unique set of functions to perform

T W O H E MI S P H E R E S

The brain has two halves, or hemispheres The right hemisphere controls

the left half of the body The left hemisphere controls the right half of the

body Scientists used to think that the two hemispheres were extremely

different in terms of their functions But current research suggests the

There is a definite physical division that creates two hemispheres, or halves, of the brain, but

scientists continue to explore how these two halves are used

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differences may not be that huge Still, these differences do exist, and may cause different styles of thinking or interpreting and processing information

The left hemisphere is more oriented toward logical, rational thinking, the right hemisphere is more sensitive to colors, shapes, and sounds, and better able to recognize larger patterns A student who is considered “left-brained” might do well in math and science A student who is more “right-brained,” on the other hand, might be more interested

in creative or artistic pursuits A “whole-brained” student would likely feel comfortable exercising both hemispheres of his brain

The two halves of the brain are able to communicate with each other through the corpus callosum The corpus callosum is a thick bundle

of nerve fibers just above the brain stem

T H E C E R E BR U M

The cerebrum is the largest part of the brain It makes up about 85 percent

of the brain’s weight The cerebrum’s job is to handle our thoughts and feelings, our capacity for language and reasoning, and our performance

of voluntary actions

The outer covering of the cerebrum is called the cerebral cortex

The word “cortex” is from a Latin word which means “bark.” The cortex itself looks somewhat like the grooved bark of a tree In humans and in other highly evolved mammals, the cerebral cortex has many folds The folding of the cerebral cortex produces many bumps and grooves The bumps are called gyri (singular is gyrus) The grooves are sulci (singular

is sulcus) The folds of the cerebral cortex increase the amount of cortex that can fit inside the skull

In terms of evolution, the cerebral cortex is the most recent development More folds mean a larger area of the brain is available

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for complex brain functions like thinking, reasoning, planning, and imagining If you unfolded the cerebral cortex from a human brain, it would take up about 324 square inches, which is about the size of a page from a newspaper The cortex has about 10 billion neurons with about 50 trillion synapses

Each hemisphere of the cerebrum has four regions called lobes

Some of the longer grooves (sulci) mark the boundaries between the lobes

Each lobe has its own set of functions

The frontal lobe handles speech, emotion, voluntary movements, and higher level functions like reasoning, planning, and judgment The parietal lobe is involved with the sense of touch and the sensations of

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warmth and cold, and of pain The occipital lobe manages our vision The

temporal lobe is mainly concerned with hearing

T H E C E R E BE LL U M

The cerebellum is the second-largest part of the brain The name means

“little brain” in Latin It is an eighth of the size of the cerebrum Like

the cerebrum, the cerebellum has two hemispheres and a surrounding

cortex

The cerebellum is located in back of the brain, just below the cerebrum The cerebellum is responsible for balance, movement, and

coordination It controls all of our voluntary movements Without your

“little brain,” you would not be able to walk upright for very long You

also could not ice skate, windsurf, skateboard, or play any other sports

A section of the cerebellum.

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Amazingly, Gage survived, but his personality underwent a dramatic change Before the accident, people saw him as hard working, even-tempered, and sensible in all of his dealings However, after the accident, his old friends hardly recognized him He had become insensitive to others, unpredictable, and indecisive The story of Phineas Gage gave scientists in the nineteenth century their first look into how different parts of the brain perform different functions and influence personality and emotion.

THE STORY OF PHINEAS GAGE

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The brain stem is sort of like the brain’s personal postal service It handles

the millions of messages shuttling back and forth between the brain and

the rest of the body That is a pretty big job by itself, but it is not all that

the brain stem does This part of the brain also manages all the activities

that keep us alive

Thanks to the brain stem, we do not have to think about digesting our food, circulating our blood, or controlling our breathing Imagine

if we had to think about doing all of these things There would be no

time for anything else The brain stem does it all for us by controlling

our involuntary muscles These include the muscles that keep our hearts

beating and our stomachs and intestines converting food into energy and

waste

When we are born, our brains have most of the neurons we will need for the rest of our lives But many of these nerve cells are not connected with each other As we learn new things, connections begin to form As you kept repeating or practicing the new action, it became easier to do many them, often at the same time This is because as you learned the new action and tried to do it, your nerve cells sent similar messages back and forth along the same routes in your brain Gradually, your brain created connections

As these connections grew stronger, your skills improved A similar process takes place whenever we are learning something new

HOW DO WE LEARN?

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Ho w Do e s the Ner v ous S ys tem Wor k ?

It is the brain stem that instructs your heart to pump more blood when you are running down the soccer field or chasing after the school bus Like other parts of the brain, the brain stem is very complex and is made up of several smaller structures like the medulla oblongata, pons, and midbrain

T H E D IE N C E P H A LO N

The diencephalon is located between the two cerebral hemispheres and above the midbrain of the brain stem It includes two very important structures—the thalamus and the hypothalamus The thalamus sends incoming sensory nerve impulses to appropriate regions of the brain where they are interpreted The thalamus tells your brain, through the messages

Some of the structures that make up the diencephalon include the hippocampus, thalamus, hypothalamus, and pituitary gland

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it sends, what you are seeing, hearing, tasting, smelling, and touching It

also receives outgoing messages from the cerebral cortex and passes this

information to other parts of the brain or to the spinal cord

The hypothalamus is only about the size of a pea But for something

so small, it has a very important role to play Its major responsibility

involves making sure your body’s needs are taken care of It tells you

when you are hungry or thirsty, for example, and does its best to keep

your body temperature within a normal range When you are cold, the

hypothalamus makes you shiver to raise your temperature When you

The pituitary gland (colored pink in this scan called an MRI) is often called the master gland because

it sends out instructions to many glands and organs responsible for important life processes

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are overheated, it comes to your rescue once again by making you sweat

in order to lower your body temperature It also tells the blood vessels in your skin to expand so your blood can start to cool

T H E LIM BI C S Y S T E M

Without emotions and feelings, we would only be partially human Our emotional responses are forever changing as we deal with the many different situations and challenges that confront us every day So where

do our emotions come from? What is the source of our feelings?

Many scientists think the brain’s limbic system holds the answer to these questions This system is really the interplay between two structures

of the brain—the hippocampus and the amygdala The amygdala is where

we feel different emotions in ourselves and perceive different emotions in other people Located between the corpus callosum and the temporal lobe, the hippocampus resembles a tiny seahorse It is important for memory and learning

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Like the brain, the spinal cord is part of the central nervous system It looks

like a slender white cable, somewhat like the cable that a television uses

A television cable transmits audio and video signals from outside sources

to the parts inside the television In a similar way, the spinal cord carries

other kinds of electrical signals to and from the brain Sensory nerves

carry incoming signals that tell your brain what you are experiencing

at any given moment Outgoing signals from the brain pass through the

spinal cord and tell your muscles, glands, and organs what to do about

this information

An illustration of a portion of the spinal cord shows the outermost meninges (the dura), the roots

of the spinal nerves, and the subarachnoid space which is a cerebrospinal fluid-filled area

between meninges

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If you put your hand too close to a flame, for instance, pain receptors

in your hand send nerve impulses along neural pathways that extend up through the spinal cord to parts of your brain that recognize the sensation

of pain The brain responds by telling you to pull your hand to get away from the fire In a person with a healthy nervous system, the entire process happens in the blink of an eye thanks to the work of your central and peripheral nervous systems

In adults, the spinal cord is about 17 inches (43 cm) long in women and 18 inches (45 cm) in men It weighs about 1 ounce (35 to 40 grams) and

is about half an inch (1.27 cm) wide The top of the spinal cord connects with the base of the brain at the brain stem The lower end stops about two-thirds of the way down the spinal column

The most important function of the spinal cord is to carry information

to and from the brain Because of its extreme sensitivity, the spinal cord must be protected from injury Three tough membranes called meninges surround it A layer of cerebrospinal fluid between the inner and middle membrane nourishes the spinal cord and serves as a shock absorber The spinal cord resides in the spinal column, or spine, which is composed of thirty-three bones called vertebrae Flexible discs of cartilage separate and cushion the vertebrae This allows the spine to stretch and bend

The entire spine has five distinct regions The cervical or neck region has seven vertebrae The cervical vertebrae are often referred to

as C1 to C7 The thoracic or upper back region has twelve vertebrae (T1 to T12) The lumbar or lower back region has five vertebrae (L1 to L5) There are a total of nine fused vertebrae in the sacral and coccygeal regions The sacral region is also called the sacrum The coccygeal region at the base of the spine is also called the coccyx, or tailbone

Like the spinal column, the spinal cord is divided into segments

These segments correspond to the different parts of the spine (cervical, thoracic, lumbar, and sacral) While the spine has thirty-three bones, the

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spinal cord has thirty-one pairs of spinal nerves There is one pair for each segment of the spinal cord These nerves reach out from the spinal cord to all parts of the body and carry impulses to and from the brain

They control the movement of our muscles and carry sensations from our body’s five senses They also regulate our body’s production of various fluids

The Gray and the White

Scientists also describe the spinal cord in terms of what it is made of

If you look at a cross-section of a spinal cord, you can see two kinds of matter One kind is gray and the other is white The gray matter resembles the wings of a butterfly This is where nerve cell bodies are located These neurons transmit messages between the spinal cord and the pairs of spinal nerves

The white matter is made of axons, the extensions that carry impulses away from the nerve cell body Most of the white matter is

in the cervical region where numerous axons travel from the brain

to the different segments of the spinal cord and from these segments up to the brain The fewest number of axons is in the sacral region So there is much less white matter in this region of the spinal cord

SPINAL NERVES THE AREAS OF THE BODY THAT THEY SERVE

Cervical spinal nerves Back of the head, neck, shoulders, arms, hands, and

dia-phragmThoracic spinal nerves Chest and parts of abdomenLumbar spinal nerves Lower back and parts of thighs and legsSacral spinal nerves Buttocks, most of legs and feet, anal and genital area

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We need our senses in order to experience what is happening inside and

outside our bodies Without the nervous system, our senses would not

work

Touch

Our organs, muscles, and bones are contained in one big, washable, extra

sensitive covering, we call skin The skin is the largest organ in the human

body Sweat glands, hair follicles, blood vessels, and nerve endings are

located in the skin Unlike the other senses, the sense of touch is not

limited to one area of the body However, some parts are more sensitive

than others It all depends on how many receptors, or nerve endings are

present in that area of skin

An illustration of a cross-section of the skin shows the blood vessels and nerves that run through the inner layers

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The tongue, lips, hands, and fingertips are among the most sensitive areas The face, neck, and feet are also extra sensitive to the sense of touch

Our skin contains an assortment of touch receptors The most common ones are for pressure, pain, temperature, vibration, and movement There are more nerve endings for pain than for any other type of touch receptor

Without a sense of touch, we could not tell where our feet are when

we walk We would not know if something were hot or cold, wet or dry, soft or hard We could not feel the keys on a piano or the keyboard of a computer Our sense of touch enables us to find our way through the world around us, to handle objects, and coordinate our movements

Nerve endings in the skin carry information to the spinal cord, which sends it to the brain What happens after touch signals reach the brain? The brain decides what all these signals mean Usually, the brain receives a wide range of signals at the same time It quickly processes this information and tells you all the qualities of whatever you are touching

So if you are holding a hot, cooked potato, you know that in addition to being hot, the potato is round, has a dry covering, and is soft enough to squish in your hand

Sight

Our eyes make it possible for us to see the world around us Some neuroscientists think of the eye as an extension of the brain For us to see the world in its many colors and shapes, our eyes must first absorb light

Light rays pass through the pupil, which is the opening in the eye

Small muscles attached to the iris widen or narrow the pupil depending

on how much light is available Located behind the iris, the lens focuses the light The lens directs the light to the back of the eye where the retina

is located The retina in each eye contains about 120 million rods and

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