Marijuana is the dried
leaves and flowers of the cannabis plant. Tetrahydrocannabinol (THC) is the
main ingredient in marijuana that causes people who use it to experience a
calm euphoria. Marijuana changes brain messages that affect sensory
perception and coordination. This can cause users to see, hear, and feel
stimuli differently and to exhibit slower reflexes.
THC, the main active ingredient in
marijuana, binds to and activates specific receptors, known as cannabinoid
receptors. There are many of these receptors in parts of the brain that
control memory, thought, concentration, time and depth perception, and
coordinated movement. By activating these receptors, THC interferes with the
normal functioning of the cerebellum, the part of the brain most responsible
for balance, posture, and coordination of movement. The cerebellum
coordinates the muscle movements ordered by the motor cortex.
Nerve impulses alert the cerebellum
that the motor cortex has directed a part of the body to perform a certain
action. Almost instantly, impulses from that part of the body inform the
cerebellum as to how the action is being carried out. The cerebellum
compares the actual movement with the intended movement and then signals the
motor cortex to make any necessary corrections. In this way, the cerebellum
ensures that the body moves smoothly and efficiently.
The hippocampus, which is involved
with memory formation, also contains many cannabinoid receptors. Studies
have suggested that marijuana activates cannabinoid receptors in the
hippocampus and affects memory by decreasing the activity of neurons in this
area. The effect of marijuana on long-term memory is less certain, but while
someone is under the influence of marijuana, short-term memory can be
compromised. Further, research studies have shown chronic administration of
THC can permanently damage the hippocampus of rats, suggesting that
marijuana use can lead to permanent memory impairment.
Marijuana also affects receptors in
brain areas and structures responsible for sensory perception. Marijuana
interferes with the receiving of sensory messages (for example, touch,
sight, hearing, taste, and smell) in the cerebral cortex. For many years, it
was known that THC acted on cannabinoid receptors in the brain.
Long-term use of marijuana (THC)
produces changes in the limbic system that are similar to those that occur
after long-term use of other major drugs of abuse such as cocaine, heroin,
and alcohol. These changes are most evident during withdrawal from THC.
During withdrawal, there are increases in both the levels of a brain
chemical involved in stress and certain emotions and the activity of neurons
in the amygdala. These same kinds of changes also occur during withdrawal
from other drugs of abuse, suggesting that there may be a common factor in
the development of drug dependence.