THE FUNCTION OF NERVOUS SYSTEM:Prologue, The Central Nervous System

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Health Psychology PSY408

LESSON 07

THE FUNCTION OF NERVOUS SYSTEM

Prologue

When Tom was born 20 years ago, his parents were thrilled. Here was their first child-- a delightful baby

with such promise for the future. He seemed to be healthy. His parents were pleased that he began to

consume large amounts of milk, often without becoming satiated. They took this as a good sign. But, in this

case, it wasn't.

As the weeks went by, Tom's parents noticed that he wasn't gaining as much weight as he should, especially

since he was still consuming lots of milk. He started to cough and wheeze often and developed one

respiratory infection after another. They became concerned, and so did his pediatrician. After a series of

tests, the devastating diagnosis was clear: Tom had Cystic Fibrosis, a chronic, progressive, and eventually fatal

disease. Cystic fibrosis is an inherited disease of the respiratory system for which there is no cure and no

effective treatment.

Tom has had a difficult life, and so has his family. The respiratory infections he had in infancy were just the

beginning. His disease causes thick, sticky secretions that constantly block airways, trap air in the lungs, and

help bacteria to thrive. Other body systems also become affected, causing additional problems, such as

insufficient absorption of food and vitamins. As a result, he was sick often and remained short,

underweight, and weak compared with other children. His social relationships have always been limited and

strained, and the burden of his illness has taken its toll on his parents.

When Tom was younger and people asked him, "what do you want to be when you grow up'? he would

answer, I'm going to be an angel when I grow up." What other plans could he have had, realistically? At 20,

he has reached the age by which half of the victims of cystic fibrosis die. Physical complications, such as

heart damage, that generally afflict several body systems in the last stages of this disease have begun to

appear.

We can see in Tom's story that biological factors, such as heredity, can affect health; illness can alter social

relationships; and all interrelated physiological systems of the body can be affected.

An understanding of health requires that we have a working knowledge of human physiology. This

knowledge makes it possible to understand, among others, such issues as how good health habits make

illness less likely, how genetic or environmental factors influence the physical body of human beings etc.

In this lecture we will talk about the nervous system. In our coming lectures we will outline the other major

physical systems of the body. Our discussions will focus on the normal functions of these systems, but we

will consider some important problems, too. For example, what determines the degree of paralysis a person

suffers after injury to the spine? How does stress affect our body systems? What is a heart attack, and what

causes it?

THE NERVOUS SYSTEM

How the Nervous System Works

We all know that the nervous system, particularly the brain, in human beings and other animals controls the

way we initiate behavior and respond to events in our world. The nervous system receives information

about changes in the environment from sensory organs, including the eyes, ears, and nose, and it transmits

directions that tell our muscles and other internal organs how to react. The brain also stores information--

being a repository for our memory of past events--and provides our capability for thinking, reasoning, and

creating.

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The nervous system is constantly integrating the actions of our internal organs--although we are not

generally aware of it. Many of these organs, such as the heart and digestive tract, are made of muscle tissues

that respond to commands. The nervous system provides these commands through an intricate network of

billions of specialized nerve cells, called neurons.

Although neurons in different parts of the nervous system have a variety of shapes and sizes, the diagram

on your screen shows their general features. Projecting from the cell body are clusters of branches called

dendrites. Generally, dendrites function as receivers for messages from adjacent neurons. These messages

then travel through a long, slender projection called the axon, which splits into branches at the Far end. The

tips of these branches have small swellings called synaptic knobs that connect to the dendrites of other

neurons, usually through a Fluid-filled gap. This junction is called a synapse.

Messages From the knobs cross the gap to adjacent neurons and in this way eventually reach their

destination.

What changes occur in the nervous system as a person develops? By the time the typical baby is born, a

basic structure has been formed for almost all the neurons this person will have. But the nervous system is

still quite immature--For instance, the brain weighs only about 25% of the weight it will have when the

child reaches adulthood. Most of the growth in brain size after birth results from an increase in the number

of Glial cells and the presence of a white fatty substance called myelin. The Glial cells are thought to service

and maintain the neurons.

A myelin sheath surrounds the axons of most, but not all, neurons. This sheath is responsible for increasing

the speed of nerve impulses and preventing them from being interfered with by adjacent nerve impulses,

much the way insulation is used on electrical wiring. The importance of myelin can be seen in the disease

called multiple sclerosis, which results when the myelin sheath degenerates and nerves become severed

(Trapp, 1998). People afflicted with this disease have weak muscles that lack coordination and move

Health Psychology PSY408

spastically (AMA, 1989).

As the infant grows, the network of dendrites and synaptic knobs to carry messages to and from other

neurons expands dramatically, as you can see in the diagram.

The myelin sheath covering the neurons is better developed initially in the upper regions of the body than in

the lower regions. During the first years of life, the progress in myelin growth spreads down the body--

from the head to shoulders, to the arms and hands, to the upper chest and abdomen, and then the legs and

feet. This sequence is reflected in the individual's motor development: the upper parts of the body are

brought under control at earlier ages than the lower parts. Studies with animals have found that chronic

poor nutrition early in life impairs brain growth by retarding the development of myelin, Glial cells, and

dendrites. Such impairment can produce long-lasting deficits in a child's motor and intellectual. Although

researchers had thought that the brain forms few, if any, new neurons after birth, it is now known that new

cells do form in some areas of the brain, but it is not yet clear how extensive this growth is.

Beginning in early adulthood, the brain slowly loses weight with age. Although the number of brain cells

does not change very much, the synapses do, leading to a decline in ability to send nerve impulses. These

alterations in the brain are associated with the declines people often notice in their mental and physical

functions after they reach 50 or 60 years of age.

The nervous system is enormously complex and basically has two major divisions--the central nervous

system and the peripheral nervous system--that connect to each other. The central nervous system consists

of the brain and spinal cord. The peripheral nervous system is composed of the remaining network of neurons

throughout the body. Each of these major divisions consists of interconnected lower-order divisions or

structures. We will examine the nervous system, beginning at the top and working our way down.

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The Central Nervous System

People's brain and spinal cord race toward maturity early in life. For example, the brain weighs 75% of its

adult weight at about 2 years of age, 90% at 5 years, and 95% at 10 years (Tanner, 1970, 1978). The brain

may be divided into three parts: the forebrain, the cerebellum, and the brainstem. Each of these parts has special

functions.

The Forebrain

The forebrain is the uppermost part of the brain. As you can see the diagram on the screen, the forebrain

has two main subdivisions: the telencephalon, which consists of the cerebrum and the limbic system, and the

diencephalon, which includes the thalamus and hypothalamus. As a general rule, areas toward the top and

outer regions of the brain are involved in our perceptual, motor, learning, and conceptual activities. Regions

toward the center and bottom of the brain are involved mainly in controlling internal and automatic body

functions

and

transmitting

information

and

from

the

telencephalon.

The cerebrum is the upper and largest portion of the human brain and includes the cerebral cortex, its

outermost layer. The cerebrum controls complex motor and mental activity. It develops rapidly in the first

few years of life, becoming larger, thicker, and more convoluted. The cerebrum has two halves--the left

hemisphere and the right hemisphere--each of which looks like the left hemispheres shown here.

Health Psychology PSY408

Although the left and right hemispheres are physically alike, they control different types of processes. For

one thing, the motor cortex of each hemisphere controls motor movements on the opposite side of the

body. (You can see the next diagram of brain showing the motor cortex on your screen now). This is why

damage to the motor cortex on, say, the right side of the brain may leave part of the left side of the body

paralyzed. The two hemispheres also control different aspects of cognitive and language processes. In most

people, the left hemisphere contains the areas that handle language processes, including speech and writing.

The right hemisphere usually processes such things as visual imagery, emotions, and the perception of

patterns, such as melodies (Tortora & Grabowski, 2000).

You probably noticed in our previous diagram that each hemisphere is divided into a front part, called the

frontal lobe, and three back parts: the temporal, occipital, and parietal lobes. The frontal lobe is involved in a

variety of functions, one of which is motor activity. The back part of the frontal lobe contains the motor

cortex, which controls the skeletal muscles of the body. If a patient, who is undergoing brain surgery,

receives stimulation to the motor cortex, some part of the body will move. The frontal lobe is also involved

in important mental activities, such as the association of ideas, planning, self-awareness, and emotion. As a

result, injury to areas of this lobe can produce personality and emotional reactions.

The temporal lobe is chiefly involved in hearing, but also in vision and memory. Damage to this region can

impair the person's comprehension of speech and ability to determine the direction from which a sound is

coming.

The occipital lobe contains the principal visual area of the brain. Damage to the occipital lobe can produce

blindness or the inability to recognize an object by sight.

The parietal lobe is involved mainly in body sensations, such as of pain, cold, heat, touch, and body

movement.

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The second part of the telencephalon--called the limbic system--lies along the innermost edge of the

cerebrum, and adjacent to the diencephalon (refer back to diagram 3). The limbic system is not well

understood yet. It consists of several structures that seem to be important in the expression of emotions,

such as fear, anger, and excitement. To the extent that heredity affects a person's emotions, it may do so by

determining the structure arid function of the limbic system.

The diencephalon includes two structures--the thalamus and hypothalamus--that lie below and are

partially encircled by the limbic system. The thalamus is a truly pivotal structure in the flow of information

in the nervous system. It functions as the chief relay station for directing sensory messages, such as of pain

or visual images, to appropriate points in the cerebrum, such as the occipital or parietal lobe. The thalamus

also relays commands going out to the skeletal muscles from the motor cortex of the cerebrum.

The hypothalamus, a small structure just below the thalamus, plays an important role in people's emotions

and motivation.

Its function affects eating, drinking, and sexual activity, for instance (Tortora & Grabowski, 2000). For

example, when the body lacks water or nutrients, the hypothalamus detects this and arouses the sensation of

thirst or hunger, which is relieved when we consume water or food. Research with animals has shown that

stimulation of specific areas of this structure can cause them to eat when they are full and stop eating when

they are hungry. A rare disease that affects this structure can cause people to become overweight. Another

important function of the hypothalamus is to maintain homeostasis--a state of balance or normal function

among our body systems.

Our normal body temperature and heart rate, which are characteristic of healthy individuals, are examples of

homeostasis. When our bodies are cold, for instance, we shiver, thus producing heat. When we are very

warm, we perspire, thus cooling the body. The hypothalamus controls these adjustments. We will see later

that the hypothalamus also plays an important role in our reaction to stress.

The Cerebellum

The cerebellum lies at the back of the brain, below the cerebrum. The main function of the cerebellum is in

maintaining body balance and coordinating movement. This structure has nerve connections to the motor

cortex of the cerebrum and most sense organs of the body. When areas of the cerebrum initiate specific

movements, the cerebellum makes our actions precise and well coordinated.

How does the cerebellum do this? There are at least two ways. First, it continuously compares our intent

with our performance, ensuring that a movement goes in the right direction, at the proper rate, and with

appropriate force. Second, it smoothes our movements. Because of the forces involved in movement, there

is an underlying tendency for our motions to go quickly back and forth, like a tremor. The cerebellum

damps this tendency. When injury occurs to the cerebellum, the person's actions become jerky and

uncoordinated--a condition called ataxia. Simple movements, such as walking or touching an object,

become difficult and unsteady.

Diagram 3 (above) shows the location of the cerebellum relative to the brainstem, which is the next section

of the brain we will discuss.

The Brainstem

The lowest portion of the brain--called the brainstem--has the form of an oddly shaped knob at the top of

the spinal cord. The brainstem consists of four parts midbrain, pons, reticular system, and medulla.

The midbrain lies at the top of the brainstem. It connects directly to the thalamus above it, which relays

messages to various parts of the forebrain. The midbrain receives information from the visual and auditory

systems and is especially important in muscle movement. The disorder called Parkinson's disease results from

degeneration of an area of the midbrain (Tortora & Grabowski, 2000). People severely afflicted with this

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disease have noticeable motor tremors, and their neck and trunk postures become rigid, so that they walk in

a crouch. Sometimes the tremors are so continuous and vigorous that the victim becomes crippled.

The reticular system is a network of neurons that extends from the bottom to the top of the brainstem and

into the thalamus. The reticular system plays an important role in controlling our states of sleep, arousal,

and attention. When people suffer a coma, often it is this system that is injured or disordered. Epilepsy a

condition in which a victim may become unconscious and begin to convulse seems to involve an

abnormality in the reticular system. One type of epileptic seizure called grand mal may result from

reverberating cycles" in the reticular system:

That is, one portion of the system stimulates another portion, which stimulates a third portion, and this in

turn re-stimulates the first portion, causing a cycle that continues for 2 to 3 minutes, until the neurons of

the system fatigue so greatly that the reverberation ceases. Following a grand mal seizure, the person often

sleeps at least a few minutes and sometimes for hours.

The pons forms a large bulge at the front of the brainstem and is involved in eye movements, facial

expressions, and chewing. At the bottom of the brain- stem is the medulla, which contains vital centers that

control breathing, heartbeat rate, and the diameter of blood vessels (which affects blood pressure). Because

of the many vital functions it controls, damage to the medulla can be life threatening. Polio, a crippling

disease that was once epidemic, sometimes damaged the center that controls breathing. Patients suffering

such damage needed constant artificial respiration to breathe (McClintic, 1985).

The Spinal Cord

Extending down the spine from the brainstem is the spinal cord, a major neural pathway that transmits

messages between the brain and various parts of the body. It contains neurons that carry impulses away

from (the efferent direction) and toward (afferent) the brain. Efferent commands travel down the cord on their

way to produce muscle action; afferent impulses come to the spinal cord from sense organs in all parts of

the body.

The organization of the spinal cord parallels that of the body--that is, the higher the region of the cord, the

higher the parts of the body to which it connects. Damage to the spinal cord results in impaired motor

function or paralysis: the duration and extent of the impairment depends on the amount and location of

damage. If the damage does not sever the cord, the impairment is less severe, and may be temporary. If the

lower portion of the cord is severed, the lower areas of the body are paralyzed--a condition called paraplegia.

If the upper portion of the spinal cord is severed, paralysis is more extensive. Paralysis of the legs and arms

is called quadriplegia.

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