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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

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