Caffeine
What
Caffeine Actually Does To Your Brain
Roughly 20 percent to 30 percent of Americans consume more than 600
milligrams — considered a high dose of the drug — on a typical
day [source: Kovacs].
If you consume more than four cups of coffee a day, you're probably
among that number. Heavy daily caffeine use — more than 500 to
600 mg a day — may cause:
- Insomnia
- Nervousness
- Restlessness
- Irritability
- Stomach upset
- Fast heartbeat
- Muscle tremors
How caffeine works (from Caffeine
and Migraine)
Caffeine works by occupying and blockading adenosine receptors embedded
in the surface membranes of neurons (nerve cells). The caffeine molecule
is similar in shape to adenosine. Caffeine molecules fit into adenosine
receptors without activating them, so prevent adenosine from binding
to adenosine receptors, thereby blocking the action of adenosine.
Adenosine does a lot of things. It's called a neuromodulator, because
it modulates, or controls, the activity of neurotransmitter molecules
including serotonin, norepinephrine, dopamine, and acetylcholine. The
effect adenosine has depends on where in the body the adenosine is;
adenosine can have opposite effects in different parts of the nervous
system. Overall, adenosine acts as a tranquilizer, inhibiting nerve
firing by inhibiting the release of excitatory neurochemicals. Centrally,
in the brainstem and spinal cord, adenosine is a painkiller, but peripherally,
in the outer reaches of the nervous system, it causes pain. Adenosine
applied to the skin causes localized pain and vasodilation. Adenosine
dilates blood vessels in the head and neck.
Caffeine, because it blocks adenosine, has the opposite effects of
adenosine. Caffeine constricts blood vessels in the head and neck, and
increases the release of excitatory neurochemicals, so increases the
rate of nerve firing. That's why caffeine is stimulating.
How the body compensates for caffeine's interference with adenosine
The human nervous system compensates for caffeine's interference with
adenosine by becoming more sensitive to adenosine. It does so by releasing
more adenosine, increasing the number of adenosine receptors on the
surfaces of neurons, increasing the affinity of those receptors for
adenosine (increasing the amount of time adenosine molecules remain
bound to adenosine receptors), and decreasing the rate at which adenosine
is removed from the synapse (the gap between neurons) by transporter
molecules that retrieve and ferry adenosine back inside neurons. All
these changes tend to increase adenosine receptor activation, to compensate
for adenosine receptors clogged by caffeine.
The nervous system also adapts to caffeine by becoming less sensitive
to the neurotransmitters adenosine typically inhibits. The nervous system
of a caffeine user adapts by decreasing the number of serotonin, dopamine,
acetylcholine, and norepinephrine receptors, decreasing the affinity
of those receptors for their corresponding neurotransmitter molecules,
and speeding up destruction or reuptake of those neurotransmitters.
Without these adaptations the caffeine user would suffer nervousness,
anxiety, restlessness, anorexia, sleeplessness, the jitters, muscle
twitches, and nervous tics.
Absent caffeine, however, a caffeine user is oversensitive to adenosine
and undersensitive to excitatory neurotransmitters. A caffeine user
is adapted to exposure to caffeine but ill-adapted to its absence.
Caffeine and migraine
Caffeine use causes a wide range of long- and short-lasting adaptive
changes to the nervous system, changes that render the nervous system
dependent on caffeine and set the nervous system up for a disturbance
when it goes too long without it. Caffeine increases the affinity and
density (number) of type A1 and type A2A adenosine receptors. A migrainer
is excessively sensitive to adenosine and excessively insensitive to
excitatory neurotransmitters. These are exactly the adaptations caffeine
causes. Caffeine, in other words, alters the nervous system in just
the right way to make a person a migrainer. No wonder, therefore, that
exposure to caffeine can be followed by a withdrawal syndrome indistinguishable
from a migraine episode.
More on Adenosine (from How
Caffeine Works)
As adenosine is created in the brain, it binds to adenosine receptors.
This binding causes drowsiness by slowing down nerve cell activity.
In the brain, this also causes blood vessels to dilate, most likely
to let more oxygen into that organ during sleep. To a nerve cell, caffeine
looks like adenosine: Caffeine binds to the adenosine receptor. However,
caffeine doesn't slow down the cell's activity like adenosine would.
As a result, the cell can no longer identify adenosine because caffeine
is taking up all the receptors that adenosine would normally bind to.
Instead of slowing down because of the adenosine's effect, the nerve
cells speed up. Caffeine also causes the brain's blood vessels to constrict,
because it blocks adenosine's ability to open them up. This effect is
why some headache medicines like Anacin contain caffeine -- constricting
blood vessels in the brain can help stop a vascular headache.
Caffeine's effect on the brain causes increased neuron firing. The
pituitary gland senses this activity and thinks some sort of emergency
must be occurring, so it releases hormones that tell the adrenal glands
to produce adrenaline (epinephrine). Adrenaline is the "fight or
flight" hormone, and it has a number of effects on your body:
- Your pupils dilate.
- The airway opens up (this is why people suffering from severe asthma
attacks are sometimes injected with epinephrine).
- Your heart beats faster.
- Blood vessels on the surface constrict to slow blood flow from cuts
and increase blood flow to muscles.
- Blood pressure rises.
- Blood flow to the stomach slows.
- The liver releases sugar into the bloodstream for extra energy.
- Muscles tighten up, ready for action.
This explains why, after consuming a big cup of coffee, your hands
get cold, your muscles grow tense, you feel excited and your heart beats
faster.
Cardiovascular
Caffeine increases cardiac arrhythmia (improper heart rate) by increasing
stress hormone (e.g.adrenaline) secretions. It has been shown there
is an increase in brachial diastolic blood pressure, but not in brachial
systolic blood pressure. However, both aortic systolic and diastolic
blood pressures increase significantly during caffeine consumption.
It has been noted that long term consumption leads to increasing aortic
systolic pressure which leads to chronic arterial stiffness.
Gastrointestinal distress
Caffeine can stimulate the secretion of stress hormones (such as epinephrine
and norepinephrine), which can increase blood pressure. Moreover, stress
hormones activate the body's "fight or flight" reactions,
causing the body to redirect blood supply from the digestive system
to muscles. In this way, decreased blood flow to the gastrointestinal
tract will slow down the absorption rate and lead to indigestion. Moreover,
the additional epinephrine increases the secretion of the main gastric
hormone gastrin, which will speed up gastric peristalsis and hypersecretion
of gastric juice. Additional gastric acid will lead to acidic chyme
going into the small intestine and cause intestinal injury. Therefore,
it is not recommended for ulcer patients to drink too much coffee.
Withdrawal
The most frequently seen withdrawal symptoms are headache and fatigue.
In prolonged caffeine drinkers, symptoms such as increased depression
and anxiety, nausea, vomiting and intense desire for caffeine containing
beverages are also reported. Withdrawal symptoms begin after 12–24
hours and peaks at 20–48 hours after abstinence from caffeine.
One patient reported: I had symptoms such as the flu, slept day and
night for several days. After that, I felt as walking in a fog, far
away from life, and had difficulty doing anything. After about three
months, the withdrawal symptoms had almost completely declined.
Caffeine and Dopamine (from How
Caffeine Works)
Dopamine is a neurotransmitter that activates pleasure centers in certain
parts of the brain. Heroin and cocaine manipulate dopamine levels by
slowing down the rate of dopamine reabsorption. Caffeine increases dopamine
levels in the same way. Its effect is much weaker than heroin's, but
the mechanism is the same. Researchers suspect that this dopamine connection
is what contributes to caffeine addiction.
You can see why your body might like caffeine in the short term, especially
if you are low on sleep and need to remain active. Caffeine blocks adenosine
reception so you feel alert. It injects adrenaline into the system to
give you a boost. And it manipulates dopamine production to make you
feel good. But caffeine can cause a vicious cycle of problems in the
long term. For example, once caffeine-induced adrenaline wears off,
you face fatigue and depression. Another cup of coffee or energy drink
can get the adrenaline flowing again, but having your body in a state
of emergency, jumpy and irritable all day long, isn't very healthy.
The most important long-term problem with caffeine is its effect on
your sleep. The half-life of caffeine in your body is about six hours.
That means that drinking a big cup of coffee containing 200 milligrams
of caffeine at 3:00 p.m. will leave about 100 milligrams of that caffeine
in your system at 9:00 p.m. Adenosine reception, which is affected by
caffeine, is important to sleep, and especially to deep sleep. You may
be able to fall asleep hours after that big cup of coffee, but your
body will probably miss out on the benefits of deep sleep.
That sleep deficit adds up fast. The next day you feel worse, so you
need caffeine as soon as you get out of bed. The cycle continues day
after day. Once you get into this cycle, you have to keep consuming
the drug to put off an inevitable comedown. Trying to quit can leave
you tired and depressed, fighting splitting headaches as blood vessels
in the brain dilate. These negative effects can be enough to force caffeine
addicts back onto the drug
Notes
