Drug Information - Brain On Drugs
Brain Anatomy
The brain consists of several large regions, each responsible for
some of the activities vital for living. These include the brainstem,
cerebellum, limbic system, diencephalon, and cerebral cortex (Figure 1).
The brainstem is the part of the brain that connects the brain and the spinal
cord. It controls many basic functions, such as heart rate, breathing, eating,
and sleeping. The brainstem accomplishes this by directing the spinal cord,
other parts of the brain, and the body to do what is necessary to maintain
these basic functions.
The cerebellum, which represents only one-eighth of the total weight of the
brain, coordinates the brain's instructions for skilled repetitive movements
and for maintaining balance and posture. It is a prominent structure located
above the brainstem. |
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Figure 1
This drawing of a brain cut in half demonstrates some of the major regions of
the brain. |
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| On top of the brainstem and buried under the cortex,
there is a set of more evolutionarily primitive brain structures called the
limbic system (Figure 2). The limbic system structures are involved in many of
our emotions and motivations, particularly those that are related to survival,
such as fear, anger, and emotions related to sexual behavior. The limbic system
is also involved in feelings of pleasure that are related to our survival, such
as those experienced from eating and sex. Two large limbic system structures
called the amygdala and hippocampus are also involved in memory. One of the
reasons that drugs of abuse can exert such powerful control over our behavior
is that they act directly on the more evolutionarily primitive brainstem and
limbic structures, which can override the cortex in controlling our behavior.
In effect, they eliminate the most human part of our brain from its role in
controlling our behavior.
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| The diencephalon, which is also located beneath the
cerebral hemispheres, contains the thalamus and hypothalamus (Figure 2). The
thalamus is involved in sensory perception and regulation of motor functions
(i.e., movement). It connects areas of the cerebral cortex that are involved in
sensory perception and movement with other parts of the brain and spinal cord
that also have a role in sensation and movement. The hypothalamus is a very
small but important component of the diencephalon. It plays a major role in
regulating hormones, the pituitary gland, body temperature, the adrenal glands,
and many other vital activities. |
| The cerebral cortex, which is divided into right and left
hemispheres, encompasses about two-thirds of the brain mass and lies over and
around most of the remaining structures of the brain. It is the most highly
developed part of the human brain and is responsible for thinking, perceiving,
and producing and understanding language. It is also the most recent structure
in the history of brain evolution. The cerebral cortex can be divided into
areas that each have a specific function (Figure 3). For example, there are
specific areas involved in vision, hearing, touch, movement, and smell. Other
areas are critical for thinking and reasoning. Although many functions, such as
touch, are found in both the right and left cerebral hemispheres, some
functions are found in only one cerebral hemisphere. For example, in most
people, language abilities are found in the left hemisphere. |
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Figure 2
This drawing of a brain cut in half demonstrates some of the brain's internal
structures. The amygdala and hippocampus are actually located deep within the
brain, but are shown as an overlay in the approximate areas that they are
located. |
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| Nerve Cells and Neurotransmission |
| The brain is made up of billions of nerve cells.
Typically, a neuron contains three important parts (Figure 4): a central cell
body that directs all activities of the neuron; dendrites, short fibers that
receive messages from other neurons and relay them to the cell body; and an
axon, a long single fiber that transmits messages from the cell body to the
dendrites of other neurons or to body tissues, such as muscles. Although most
neurons contain all of the three parts, there is a wide range of diversity in
the shapes and sizes of neurons as well as their axons and dendrites. |
| The transfer of a message from the axon of one nerve
cell to the dendrites of another is known as neurotransmission. Although axons
and dendrites are located extremely close to each other, the transmission of a
message from an axon to a dendrite does not occur through direct contact.
Instead, communication between nerve cells occurs mainly through the release of
chemical substances into the space between the axon and dendrites (Figure 5).
This space is known as the synapse. When neurons communicate, a message,
traveling as an electrical impulse, moves down an axon and toward the synapse.
There it triggers the release of molecules called neurotransmitters from the
axon into the synapse. The neurotransmitters then diffuse across the synapse
and bind to special molecules, called receptors, that are located within the
cell membranes of the dendrites of the adjacent nerve cell. This, in turn,
stimulates or inhibits an electrical response in the receiving neuron's
dendrites. Thus, the neurotransmitters act as chemical messengers, carrying
information from one neuron to another. |
Figure 3
This drawing of a brain cut in half demonstrates the lobes of the cerebral
cortex and their functions. |
Figure 4 |
| There are many different types of neurotransmitters, each of which
has a precise role to play in the functioning of the brain. Generally, each
neurotransmitter can only bind to a very specific matching receptor. Therefore,
when a neurotransmitter couples to a receptor, it is like fitting a key into a
lock. This coupling then starts a whole cascade of events at both the surface
of the dendrite of the receiving nerve cell and inside the cell. In this
manner, the message carried by the neurotransmitter is received and processed
by the receiving nerve cell. Once this has occurred, the neurotransmitter is
inactivated in one of two ways. It is either broken down by an enzyme or
reabsorbed back into the nerve cell that released it. The reabsorption (also
known as re-uptake) is accomplished by what are known as transporter molecules
(Figure 5). Transporter molecules reside in the cell membranes of the axons
that release the neurotransmitters. They pick up specific neurotransmitters
from the synapse and carry them back across the cell membrane and into the
axon. The neurotransmitters are then available for reuse at a later time.
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| Figure 5 |
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| As noted above, messages that are received by
dendrites are relayed to the cell body and then to the axon. The axons then
transmit the messages, which are in the form of electrical impulses, to other
neurons or body tissues. The axons of many neurons are covered in a fatty
substance known as myelin. Myelin has several functions. One of its most
important is to increase the rate at which nerve impulses travel along the
axon. The rate of conduction of a nerve impulse along a heavily myelinated axon
can be as fast as 120 meters/second. In contrast, a nerve impulse can travel no
faster than about 2 meters/second along an axon without myelin. The thickness
of the myelin covering on an axon is closely linked to the function of that
axon. For example, axons that travel a long distance, such as those that extend
from the spinal cord to the foot, generally contain a thick myelin covering to
facilitate faster transmission of the nerve impulse. (Note: The axons that
transmit messages from the brain or spinal cord to muscles and other body
tissues are what make up the nerves of the human body. Most of these axons
contain a thick covering of myelin, which accounts for the whitish appearance
of nerves.) |
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Figure 6
This drawing of a brain cut in half demonstrates the brain areas and pathways
involved in the pleasure circuit. |
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Effects of Drugs of Abuse on the Brain
Pleasure, which scientists call reward, is a very powerful biological force for
our survival. If you do something pleasurable, the brain is wired in such a way
that you tend to do it again. Life-sustaining activities, such as eating,
activate a circuit of specialized nerve cells devoted to producing and
regulating pleasure. One important set of these nerve cells, which uses a
chemical neurotransmitter called dopamine, sits at the very top of the
brainstem in the ventral tegmental area (VTA) (Figure 6). These
dopamine-containing neurons relay messages about pleasure through their nerve
fibers to nerve cells in a limbic system structure called the nucleus
accumbens. Still other fibers reach to a related part of the frontal region of
the cerebral cortex. So, the pleasure circuit, which is known as the mesolimbic
dopamine system, spans the survival-oriented brainstem, the emotional limbic
system, and the frontal cerebral cortex. |
All drugs that are addicting can activate the brain's pleasure
circuit. Drug addiction is a biological, pathological process that alters the
way in which the pleasure center, as well as other parts of the brain,
functions. To understand this process, it is necessary to examine the effects
of drugs on neurotransmission. Almost all drugs that change the way the brain
works do so by affecting chemical neurotransmission. Some drugs, like heroin
and LSD, mimic the effects of a natural neurotransmitter. Others, like PCP,
block receptors and thereby prevent neuronal messages from getting through.
Still others, like cocaine, interfere with the molecules that are responsible
for transporting neurotransmitters back into the neurons that released them
(Figure 7). Finally, some drugs, such as methamphetamine, act by causing
neurotransmitters to be released in greater amounts than normal.
Prolonged drug use changes the brain in fundamental and long-lasting ways.
These long-lasting changes are a major component of the addiction itself. It is
as though there is a figurative "switch" in the brain that "flips" at some
point during an individual's drug use. The point at which this "flip" occurs
varies from individual to individual, but the effect of this change is the
transformation of a drug abuser to a drug addict.
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Figure 7
When cocaine enters the brain, it blocks the dopamine transporter from pumping
dopamine back into the transmitting neuron, flooding the synapse with dopamine.
This intensifies and prolongs the stimulation of receiving neurons in the
brain's pleasure circuits, causing a cocaine "high."
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