How Caffeine Affects the Nervous System

Caffeine, chemically known as 1,3,7-trimethylxanthine, is considered to be the most popular, largely unregulated drug in the world. Caffeine is consumed in coffee, tea, cocoa, chocolate, many soft drinks and some drugs. Natural sources of caffeine include coffee beans, tea leaves, kola nuts, guarana berries and cacao pods. Like every drug, caffeine affects the nervous system in multiple ways, some of which are desirable, but many of which are unwanted.

A cup of coffee on a table. Credit: nuiiko/iStock/Getty Images

Brief History

Since caffeine naturally occurs within some plants, it is likely that indigenous people of the Middle East, Africa, Asia and South America intentionally consumed it for its affects for thousands of years. According to "Contemporary Nutrition," it wasn't until the early 1820s that caffeine was chemically isolated from coffee by German and French chemists. The name caffeine was derived from the French word for coffee, or café. Caffeine is an odorless, but bitter, white crystalline powder that has the ability to stimulate the central nervous system.

Modes of Action

According to "Nutrition and Public Health," caffeine is the world's most widely consumed psychoactive substance, with 90 percent of Americans consuming some of it daily. Caffeine enters the bloodstream through the stomach and small intestine and then quickly passes through the blood-brain barrier, where it causes effects in as little as 15 minutes. Caffeine interferes with the actions of adenosine, a naturally occurring neurotransmitter in the brain. The actions of adenosine include suppressing neural activity in the brain, increasing blood flow throughout the body and contributing to energy metabolism. The actions of caffeine counter these mechanisms. The half-life of caffeine is between five and six hours within the human body.

Nervous System Effects

By suppressing the actions of adenosine, caffeine increases neural activity in the brain, which leads to a temporary increase in mental alertness and thought processing, while reducing drowsiness and fatigue, according to "Biochemical, Physiological and Molecular Aspects of Human Nutrition." These are the primary benefits of caffeine and why many people drink coffee and soda pop. Contrary to popular belief, caffeine does not directly increase energy metabolism in the body; in fact, long-term consumption actually suppresses it, which can lead to adrenal fatigue. Further, by counteracting adenosine, caffeine also significantly reduces blood flow to the brain, which leads to headaches, dizziness and reduced fine motor coordination, according to "Human Biochemistry and Disease." However, caffeine can help with migraine headaches that are caused by over-dilation of blood vessels in the brain. Other nervous system effects of caffeine include increased heart rate, increased thirst and hunger, anxiety, nervousness, dilation of air passages, anal sphincter relaxation and insomnia.

Caution with Dosages

The U.S. Food and Drug Administration states that caffeine is generally recognized as a safe food substance in moderation. According to "Meyler's Side Effects of Drugs," caffeine can be lethal at dosages over 10 g for the average adult, which is equivalent to drinking at least 80 cups of coffee in rapid succession. Some people can develop a physical dependence on caffeine and require more over time to experience the same affects. Typical withdrawal symptoms include headache, fatigue, irritability and muscle pain.

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