The Science of Mental Illness
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National Institute of Mental Health

The Science of Mental Illness

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Teacher's Guide

Information about Mental Illness and the Brain (Page 1 of 3)

1 Defining Mental Illnessa

We can all be “sad” or “blue” at times in our lives. We have all seen movies about the madman and his crime spree, with the underlying cause of mental illness. We sometimes even make jokes about people being crazy or nuts, even though we know that we shouldn’t. We have all had some exposure to mental illness, but do we really understand it or know what it is? Many of our preconceptions are incorrect. A mental illness can be defined as a health condition that changes a person’s thinking, feelings, or behavior (or all three) and that causes the person distress and difficulty in functioning. As with many diseases, mental illness is severe in some cases and mild in others. Individuals who have a mental illness don’t necessarily look like they are sick, especially if their illness is mild. Other individuals may show more explicit symptoms such as confusion, agitation, or withdrawal. There are many different mental illnesses, including depression, schizophrenia, attention deficit hyperactivity disorder (ADHD), autism, and obsessive-compulsive disorder. Each illness alters a person’s thoughts, feelings, and/or behaviors in distinct ways. In this module, we will at times discuss mental illness in general terms and at other times, discuss specific mental illnesses. Depression, schizophrenia, and ADHD will be presented in greater detail than other mental illnesses.

Not all brain diseases are categorized as mental illnesses. Disorders such as epilepsy, Parkinson’s disease, and multiple sclerosis are brain disorders, but they are considered neurological diseases rather than mental illnesses. Interestingly, the lines between mental illnesses and these other brain or neurological disorders is blurring somewhat. As scientists continue to investigate the brains of people who have mental illnesses, they are learning that mental illness is associated with changes in the brain’s structure, chemistry, and function and that mental illness does indeed have a biological basis. This ongoing research is, in some ways, causing scientists to minimize the distinctions between mental illnesses and these other brain disorders. In this curriculum supplement, we will restrict our discussion of mental illness to those illnesses that are traditionally classified as mental illnesses, as listed in the previous paragraph.

2 Mental Illness in the Populationb

Many people feel that mental illness is rare, something that only happens to people with life situations very different from their own, and that it will never affect them. Studies of the epidemiology of mental illness indicate that this belief is far from accurate. In fact, the surgeon general reports that mental illnesses are so common that few U.S. families are untouched by them.44

group of four human figures, with one standing out because of its more intense color
Figure 1. Scientists estimate that one of every four people is affected by mental illness either directly or indirectly.
Few U.S. families are untouched by mental illness.

2.1 Mental Illness in Adults

Even if you or a family member has not experienced mental illness directly, it is very likely that you have known someone who has. Estimates are that at least one in four people is affected by mental illness either directly or indirectly. Consider the following statistics to get an idea of just how widespread the effects of mental illness are in society: 4, 25, 44

2.2 Mental Illness in Children and Adolescents

Mental illness is not uncommon among children and adolescents. Approximately 12 million children under the age of 18 have mental disorders.4 The National Mental Health Association33 has compiled some statistics about mental illness in children and adolescents:

3 Warning Signs for Mental Illnessc

Each mental illness has its own characteristic symptoms. (See Section 10 for information about some specific illnesses.) However, there are some general warning signs that might alert you that someone needs professional help.4 Some of these signs include

A person who shows any of these signs should seek help from a qualified health professional.

4 Diagnosing Mental Illnessc

4.1 Mental Health Professionals

To be diagnosed with a mental illness, a person must be evaluated by a qualified professional who has expertise in mental health. Mental health professionals include psychiatrists, psychologists, psychiatric nurses, social workers, and mental health counselors. Family doctors, internists, and pediatricians are usually qualified to diagnose common mental disorders such as depression, anxiety disorders, and ADHD. In many cases, depending on the individual and his or her symptoms, a mental health professional who is not a psychiatrist will refer the patient to a psychiatrist. A psychiatrist is a medical doctor (M.D.) who has received additional training in the field of mental health and mental illnesses. Psychiatrists evaluate the person’s mental condition in coordination with his or her physical condition and can prescribe medication. Only psychiatrists and other M.D.s can prescribe medications to treat mental illness.

4.2 Mental Illnesses are Diagnosed by Symptoms

Unlike some disease diagnoses, doctors can’t do a blood test or culture some microorganisms to determine whether a person has a mental illness. Maybe scientists will develop discrete physiological tests for mental illnesses in the future; until then, however, mental health professionals will have to diagnose mental illnesses based on the symptoms that a person has. Basing a diagnosis on symptoms and not on a quantitative medical test, such as a blood chemistry test, a throat swab, X-rays, or urinalysis, is not unusual. Physicians diagnose many diseases, including migraines, Alzheimer’s disease, and Parkinson’s disease based on their symptoms alone. For other diseases, such as asthma or mononucleosis, doctors rely on analyzing symptoms to get a good idea of what the problem is and then use a physiological test to provide additional information or to confirm their diagnosis.

When a mental health professional works with a person who might have a mental illness, he or she will, along with the individual, determine what symptoms the individual has, how long the symptoms have persisted, and how his or her life is being affected. Mental health professionals often gather information through an interview during which they ask the patient about his or her symptoms, the length of time that the symptoms have occurred, and the severity of the symptoms. In many cases, the professional will also get information about the patient from family members to obtain a more comprehensive picture. A physician likely will conduct a physical exam and consult the patient’s history to rule out other health problems.

Mental health professionals evaluate symptoms to make a diagnosis of mental illness. They rely on the criteria specified in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV; currently, the fourth edition), published by the American Psychiatric Association, to diagnose a specific mental illness.5 For each mental illness, the DSM-IV gives a general description of the disorder and a list of typical symptoms. Mental health professionals refer to the DSM-IV to confirm that the symptoms a patient exhibits match those of a specific mental illness. Although the DSM-IV provides valuable information that helps mental health professionals diagnose mental illness, these professionals realize that it is important to observe patients over a period of time to understand the individual’s mental illness and its effects on his or her life. We present the DSM-IV criteria for the specific diseases discussed in this module in Section 10, Information about Specific Mental Illnesses.

5 Mental Illness and the Braind

The term mental illness clearly indicates that there is a problem with the mind. But is it just the mind in an abstract sense, or is there a physical basis to mental illness? As scientists continue to investigate mental illnesses and their causes, they learn more and more about how the biological processes that make the brain work are changed when a person has a mental illness.

5.1 The Basics of Brain Function

Before thinking about the problems that occur in the brain when someone has a mental illness, it is helpful to think about how the brain functions normally. The brain is an incredibly complex organ. It makes up only 2 percent of our body weight, but it consumes 20 percent of the oxygen we breathe and 20 percent of the energy we take in. It controls virtually everything we as humans experience, including movement, sensing our environment, regulating our involuntary body processes such as breathing, and controlling our emotions. Hundreds of thousands of chemical reactions occur every second in the brain; those reactions underlie the thoughts, actions, and behaviors with which we respond to environmental stimuli. In short, the brain dictates the internal processes and behaviors that allow us to survive.

How does the brain take in all this information, process it, and cause a response? The basic functional unit of the brain is the neuron. A neuron is a specialized cell that can produce different actions because of its precise connections with other neurons, sensory receptors, and muscle cells. A typical neuron has four structurally and functionally defined regions: the cell body, dendrites, axons, and the axon terminals.

Figure 2
Figure 2. The neuron, or nerve cell, is the functional unit of the nervous system. The neuron has processes called dendrites that receive signals and an axon that transmits signals to another neuron.

The cell body is the metabolic center of the neuron. The nucleus is located in the cell body and most of the cell’s protein synthesis occurs here.

A neuron usually has multiple fibers called dendrites that extend from the cell body. These processes usually branch out somewhat like tree branches and serve as the main apparatus for receiving input from other nerve cells.

The cell body also gives rise to the axon. The axon is usually much longer than the dendrites; in some cases, an axon can be up to 1 meter long. The axon is the part of the neuron that is specialized to carry messages away from the cell body and to relay messages to other cells. Some large axons are surrounded by a fatty insulating material called myelin, which enables the electrical signals to travel down the axon at higher speeds.

Near its end, the axon divides into many fine branches that have specialized swellings called axon terminals or presynaptic terminals. The axon terminals end near the dendrites of another neuron. The dendrites of one neuron receive the message sent from the axon terminals of another neuron.

Figure 3
Figure 3. Diagram of a synapse.

The site where an axon terminal ends near a receiving dendrite is called the synapse. The cell that sends out information is called the presynaptic neuron, and the cell that receives the information is called the postsynaptic neuron. It is important to note that the synapse is not a physical connection between the two neurons; there is no cytoplasmic connection between the two neurons. The intercellular space between the presynaptic and postsynaptic neurons is called the synaptic space or synaptic cleft. An average neuron forms approximately 1,000 synapses with other neurons. It has been estimated that there are more synapses in the human brain than there are stars in our galaxy. Furthermore, synaptic connections are not static. Neurons form new synapses or strengthen synaptic connections in response to life experiences. This dynamic change in neuronal connections is the basis of learning.

Figure 4
Figure 4. Neurons relay their information using both electrical signals and chemical messages in a process called neurotransmission.

Neurons communicate using both electrical signals and chemical messages. Information in the form of an electrical impulse is carried away from the neuron’s cell body along the axon of the presynaptic neuron toward the axon terminals. When the electrical signal reaches the presynaptic axon terminal, it cannot cross the synaptic space, or synaptic cleft. Instead, the electrical signal triggers chemical changes that can cross the synapse to affect the postsynaptic cell. When the electrical impulse reaches the presynaptic axon terminal, membranous sacs called vesicles move toward the membrane of the axon terminal. When the vesicles reach the membrane, they fuse with the membrane and release their contents into the synaptic space. The molecules contained in the vesicles are chemical compounds called neurotransmitters. Each vesicle contains many molecules of a neurotransmitter. The released neurotransmitter molecules drift across the synaptic cleft and then bind to special proteins, called receptors, on the postsynaptic neuron. A neurotransmitter molecule will bind only to a specific kind of receptor.

The binding of neurotransmitters to their receptors causes that neuron to generate an electrical impulse. The electrical impulse then moves away from the dendrite ending toward the cell body. After the neurotransmitter stimulates an electrical impulse in the postsynaptic neuron, it releases from the receptor back into the synaptic space. Specific proteins called transporters or reuptake pumps carry the neurotransmitter back into the presynaptic neuron. When the neurotransmitter molecules are back in the presynaptic axon terminal, they can be repackaged into vesicles for release the next time an electrical impulse reaches the axon terminal. Enzymes present in the synaptic space degrade neurotransmitter molecules that are not taken back up into the presynaptic neuron.

The nervous system uses a variety of neurotransmitter molecules, but each neuron specializes in the synthesis and secretion of a single type of neurotransmitter. Some of the predominant neurotransmitters in the brain include glutamate, GABA, serotonin, dopamine, and norepinephrine. Each of these neurotransmitters has a specific distribution and function in the brain; the specifics of each are beyond the scope of this module, but a few of the names will arise in reference to particular mental illnesses.

a Relevant to Lessons 1, 2, and 5.

b Relevant to Lesson 3.

c Relevant to Lessons 2, 3, 4, and 5.

d Relevant to Lessons 1, 2, and 4.

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