Caffeine is a chemical substance found primarily in soft drinks, coffee, tea and chocolate. Caffeine is also found in energy drinks, diet pills and certain medications. Once ingested, caffeine enters the bloodstream and travels to the central nervous system, or CNS. Since it is lipid-soluble, caffeine is readily absorbed by the brain. In the CNS, caffeine exerts its effects by binding to adenosine receptors and blocking the actions of adenosine, a calming agent.
Caffeine is well known as a stimulant that causes people to stay awake longer and sometimes feel jittery, nervous or unable to sleep. In the CNS, caffeine does these things by binding to adenosine receptors on nerve cells. When the caffeine molecule binds to these receptors, it blocks the ability of adenosine to do its job. Since adenosine normally calms activity in the CNS, the presence of caffeine in its place causes nerve cells to fire more rapidly. This accounts for the stimulating effect of caffeine.
In the brain, adenosine has the effect of dilating blood vessels. When caffeine binds to adenosine receptors on nerve cells, it blocks the action of adenosine and causes the brain's blood vessels to constrict, or become more narrow. Due to this action, people with vascular headache often find relief with caffeine.
By working at the pain nerve endings, adenosine has a role in causing muscle pain. When caffeine binds to the adenosine receptor, pain is partially blocked. For this reason, caffeine itself is considered a mild analgesic, or pain reliever. Moreover, caffeine is an analgesic adjuvant, meaning that it helps other pain reliever drugs to work more effectively and more quickly.
References
- Discovery Health: "How Caffeine Works"
- University of Minnesota Department of Psychology: "How Does Caffeine Work in the Brain?"
- Stroke Journal of the American Heart Association: "Caffeine Induced Changes in the Cerebral Circulation"
- The Migraine Headache Center: "Why Does Caffeine Cure My Headache?"
- European Journal of Pharmacology: "Adenosine Receptor Activation and Nociception"
- Journal of the American Medical Association: "Caffeine as an Analgesic Adjuvant"
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