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

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Health Psychology­ PSY408
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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
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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
in
transmitting
information
to
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.
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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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Table of Contents:
  1. INTRODUCTION TO HEALTH PSYCHOLOGY:Health and Wellness Defined
  2. INTRODUCTION TO HEALTH PSYCHOLOGY:Early Cultures, The Middle Ages
  3. INTRODUCTION TO HEALTH PSYCHOLOGY:Psychosomatic Medicine
  4. INTRODUCTION TO HEALTH PSYCHOLOGY:The Background to Biomedical Model
  5. INTRODUCTION TO HEALTH PSYCHOLOGY:THE LIFE-SPAN PERSPECTIVE
  6. HEALTH RELATED CAREERS:Nurses and Physician Assistants, Physical Therapists
  7. THE FUNCTION OF NERVOUS SYSTEM:Prologue, The Central Nervous System
  8. THE FUNCTION OF NERVOUS SYSTEM AND ENDOCRINE GLANDS:Other Glands
  9. DIGESTIVE AND RENAL SYSTEMS:THE DIGESTIVE SYSTEM, Digesting Food
  10. THE RESPIRATORY SYSTEM:The Heart and Blood Vessels, Blood Pressure
  11. BLOOD COMPOSITION:Formed Elements, Plasma, THE IMMUNE SYSTEM
  12. SOLDIERS OF THE IMMUNE SYSTEM:Less-Than-Optimal Defenses
  13. THE PHENOMENON OF STRESS:Experiencing Stress in our Lives, Primary Appraisal
  14. FACTORS THAT LEAD TO STRESSFUL APPRAISALS:Dimensions of Stress
  15. PSYCHOSOCIAL ASPECTS OF STRESS:Cognition and Stress, Emotions and Stress
  16. SOURCES OF STRESS:Sources in the Family, An Addition to the Family
  17. MEASURING STRESS:Environmental Stress, Physiological Arousal
  18. PSYCHOSOCIAL FACTORS THAT CAN MODIFY THE IMPACT OF STRESS ON HEALTH
  19. HOW STRESS AFFECTS HEALTH:Stress, Behavior and Illness, Psychoneuroimmunology
  20. COPING WITH STRESS:Prologue, Functions of Coping, Distancing
  21. REDUCING THE POTENTIAL FOR STRESS:Enhancing Social Support
  22. STRESS MANAGEMENT:Medication, Behavioral and Cognitive Methods
  23. THE PHENOMENON OF PAIN ITS NATURE AND TYPES:Perceiving Pain
  24. THE PHYSIOLOGY OF PAIN PERCEPTION:Phantom Limb Pain, Learning and Pain
  25. ASSESSING PAIN:Self-Report Methods, Behavioral Assessment Approaches
  26. DEALING WITH PAIN:Acute Clinical Pain, Chronic Clinical Pain
  27. ADJUSTING TO CHRONIC ILLNESSES:Shock, Encounter, Retreat
  28. THE COPING PROCESS IN PATIENTS OF CHRONIC ILLNESS:Asthma
  29. IMPACT OF DIFFERENT CHRONIC CONDITIONS:Psychosocial Factors in Epilepsy