The most important physiological effect of caffeine is that it stimulates nerve cells, particularly those in the brain. It appears that caffeine molecules bind to neurotransmitter receptor sites in nerve cells, causing the continual stimulation of those cells. This property explains the most common clinical symptoms of caffeine ingestion: wakefulness, excitability, increased mental awareness, and restlessness.
Caffeine affects nerve tissue in the brain much more quickly than it does nerve tissue anywhere else in the body. As a result, it will bring about muscular changes such as convulsions only with very high doses of the drug—10 g or more, the equivalent of drinking 70-100 cups of coffee in a short time. Death from caffeine overdose is, therefore, extremely unlikely.