Chapter 1: The Brain
Dementia is the decline of a person's memory, knowledge, thinking, and other mental activities. It helps to know something about how the brain works. The brain stores and acts on information. It also regulates activities of systems throughout the body.
The main types of cells that store and process information in the brain are neurons. In the adult, once neurons are lost, they cannot be replaced.
The central part of the neuron is the cell body. Small branches that come off the cell body are called dendrites. These dendrites carry information to the cell body. The axon, a large branch, carries information away from the cell body and to the next neuron (see Figure 1).
The neurons make chemicals called neurotransmitters. These neurotransmitters help carry information from one neuron to another (see Figure 2). Several neurotransmitters are important for the function of the memory systems. In dementias (decline of mental function) neurotransmitters may be lost.
Different parts of the brain store different types of information and control different functions. When neurons and the neurotransmitters made by these neurons do not work properly or are lost, the brain cannot store new information and loses information already stored.
The largest part of the brain is the cerebrum. The cerebrum is divided into two halves called the left and right cerebral hemispheres. The left and right cerebral hemispheres have very different functions.
Basic Functions of Cerebral Hemispheres
Thinking involves both cerebral hemispheres. The cerebral hemispheres work together to learn new information, form thoughts, make judgments, and store memories.
Left Cerebral Hemisphere - For most people, the left cerebral hemisphere is important in language (including speech, reading, and writing) and math (see Figure 3). People with brain injuries in the left hemisphere sometimes have a disturbance in verbal communication. This disorder is called aphasia.
Right Cerebral Hemisphere - The right cerebral hemisphere is important for paying attention and understanding as well as expressing emotions (see Figure 4). Also, it is important for visual spatial skills, such as having a sense of direction. A person with good visual spatial skills can find the family car in a large parking lot.
The Four Cerebral Lobes
Within each cerebral hemisphere are four major areas called lobes: the 1) frontal, 2) parietal, 3) temporal, and 4) occipital lobes. Each lobe is important and has special functions (see Figure 5).
The Frontal Lobes - The frontal lobes are the largest of the four lobes responsible for many different functions. Some of the important ones discussed here are motor, speech, and intellectual functions.
1. Motor - In the back part of the frontal lobes are the motor areas which control voluntary motor movements, such as moving an arm to reach for a book.
Nerve cells in the motor area cross to the other side of the brain. As a result the right hemisphere of the brain controls movement on the left side of the body and the left hemisphere controls movement on the right side of the body. Therefore, damage in the motor area on one side of the brain affects the opposite side of the body.
2. Speech - Another area of the frontal lobe controls the ability to form words. Damage in this area results in difficulty expressing words. People with damage in this area are often unable to speak in sentences. Each word spoken may require a great deal of effort. They may be able to give only "yes" and "no" answers. They understand speech and know what they want to say but cannot speak normally. As a result they sometimes become very frustrated or embarrassed.
3. Intellectual - The frontal lobes also have a strong influence on personality and "higher" intellectual functions. These functions include judgment, motivation, abstract thinking, and emotional control. People with problems in the frontal lobes are easily distracted. They may not be able to keep up with a conversation because they look away and are distracted every time there is a noise from another direction.
Other problems associated with frontal lobe injury include the inability to concentrate and to develop and carry out plans and goals. Also there is a loss of "get up and go." People with frontal lobe injury may act quickly without thinking beforehand of the possible consequences of their actions. They may continue an activity after they should have stopped and stop an activity they should have continued.
People with damage to the frontal lobes may be easily frustrated and may act inappropriately. For example, they may lose their temper over minor issues, such as not finding their shoes.
The Parietal Lobes - The anterior (or front) parts of the parietal lobes are associated with the sense of touch. The posterior (or back) part of the parietal lobes and especially the parietal lobe on the right side of the brain control the ability to pay attention.
Another function of the right parietal lobe is perceiving the spatial relationship and location of objects.
The left parietal lobe plays an important role in language and speech. It is also important for reading, writing, calculating numbers, and performing learned skilled acts, such as combing one's hair or repairing a leaky faucet.
The Temporal Lobes - The temporal lobes have many important functions. The temporal lobes can be divided into two parts. One part is on the bottom (ventral) of the hemispheres, and the other part is on the side (lateral) of each hemisphere. One area of the bottom part is important for recognizing objects and peoples' faces. The portion of the temporal lobe on the left lateral (side) of the brain is important for understanding speech and speaking. The rear of the temporal lobe is also important for understanding emotions on faces (facial expressions) and emotional tones of voice (see Figure 3 and Figure 4).
Another portion (toward the middle of the temporal lobes) is important for memory. Normally, a person is able to put together and store the brain information that is seen and heard. In the temporal lobes information that is seen, heard, smelled, felt, or tasted enters, is stored briefly, and soon put into long-term memory storage.
The Occipital Lobes - The occipital lobes are in the back part of the brain and are important to one's ability to see.
Other Parts of the Brain - Deep in the cerebral hemispheres are groups of neurons called basal ganglia (see Figure 6). The basal ganglia consist of the caudate nucleus, the putamen, the pallidum, and the substantia, nigra. The basal ganglia appear to be primarily important in the control of movement. However, some portions of the basal ganglia work together with the frontal lobes. People with Parkinson's disease and Huntington's disease have damage to the basal ganglia.
Deep in the hemispheres, surrounded by the basal ganglia, is another group of neurons called the thalamus. The thalamus is important in relaying sensory information. The thalamus is also connected to the temporal lobes and is important in memory.
Below the hemispheres and just above the spinal cord is the brain stem. Motor and sensory neurons travel through the brain stem. The brain stem controls the nerves that go to the head and also controls vital functions, such as heart activity, blood pressure, and breathing (respiration).
BLOOD SUPPLY FOR BRAIN FUNCTION
A good supply of blood is necessary to deliver the oxygen and nutrients needed for normal brain function. The brain uses about one fourth of the body's oxygen supply. Cells in the brain will die if the blood supply is stopped for more than a few minutes.
The blood supply to the brain is provided by two major pairs of arteries: the vertebral arteries and the carotid arteries (see Figure 7). The vertebral arteries supply blood to the brain stem, the back part of the brain (occipital lobe), and the thalamus. The carotid arteries and their branches supply blood to the rest of the brain (frontal lobes, parietal lobes, most of the temporal lobes, and much of the basal ganglia).
The brain can be damaged by too little blood (cerebral infarction) or too much blood (cerebral hemorrhage). Too little blood can be caused by a blood clot in the vessels or a narrowing of the vessels. Blood clots can also break off from inside the blood vessels or from the heart and go to the brain. Damage to the brain from blood vessel disease-such as clogged arteries is called a stroke.
An important means of preventing blood vessel damage to the brain is to treat high blood pressure (called hypertension). Hypertension is any blood pressure that is over 140/90. Treating high cholesterol-with changes in diet, daily exercise, and medicine-may also reduce the chances of stroke. Smoking increases the chances of stroke. Aspirin, if approved by your doctor, can decrease the risk of stroke.
CEREBROSPINAL FLUID (CSF)
The brain floats in fluid called cerebrospinal fluid (CSF). It acts as a cushion to protect the brain and also to "cleanse" it. The fluid flows through a set of four cavities called ventricles (see Figure 8). Problems in memory may occur when the flow of this CSF is blocked. The block causes pressure to build up and damages the brain cells.
The normal functions of intelligence, language, speech, and body movements involve healthy brain cells working together. The cerebral hemispheres have four pairs of lobes: the frontal lobes, the parietal lobes, the temporal lobes, and the occipital lobes. The basal ganglia lie deep within the cerebral hemispheres. A good blood supply is important for delivering oxygen to all the brain cells. The cerebrospinal fluid bathes the brain cells and flows through a system of ventricles.
In the next chapter, we discuss briefly different kinds of dementia.
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Material taken from
"Helping People with Progressive Memory Disorders: A Guide For You And Your Family, 2nd ed." (University of Florida Health Science Center). Used with permission from the authors: K. M. Heilman, MD, L. Doty, PhD, J. T. Stewart, MD, D Bowers, PhD, & L. Gonzalez-Rothi, PhD. (1999).