Caffeine is the most commonly used stimulant in the world. Caffeine increases your alertness by increasing the active levels of neurotransmitters in the brain. Additionally, caffeine increases the activation of the sympathetic nervous system, which is involved in the fight-or-flight stress response. The sympathetic nervous system acts in balance with the parasympathetic nervous system, which controls the rest-and-digest response. Caffeine also increases the activity of the parasympathetic nervous system.
Sympathetic Nervous System
The autonomic nervous system controls the function of your organs and metabolism. It is divided into two sub-systems: the sympathetic and parasympathetic. Activation of the sympathetic nervous system is associated with increased levels of arousal. The fight-or-flight response releases large amounts of fat and sugar into the bloodstream to fuel your body during a potential struggle. Caffeine inhibits the actions of the neurotransmitter adenosine, which keeps the levels of other neurotransmitters in check. By inhibiting adenosine, caffeine causes levels of other neurotransmitters to rise, including epinephrine and norepinephrine, or adrenaline. The rise in these neurotransmitters activates the sympathetic nervous system.
Parasympathetic Nervous System
The parasympathetic nervous system opposes the actions of the sympathetic nervous system. The actions of caffeine also increase the levels of acetylcholine, a neurotransmitter involved in the parasympathetic nervous system. Parasympathetic activation slows heart rate, reduces blood pressure, decreases energy use and stimulates digestion. The parasympathetic nervous system also increases the production of mucus, saliva, urine and solid waste and promotes storage of nutritients.
Caffeine Effects
By analogy, the sympathetic nervous system acts as your body's gas pedal and the parasympathetic nervous system acts as the brake. By activating both, the physiological effects of caffeine may be variable between people. While some people experience an increase in heart rate and blood pressure, others may not. However, high caffeine intake may cause the sympathetic nervous system to overwhelm the parasympathetic nervous system.
Too Much Caffeine
Daily intake of 200 to 300 mg of caffeine, or about two to four cups of coffee, is generally thought to be safe, according to the Mayo Clinic. However, daily intake of 400 to 600 mg has been associated with a condition called caffeinism. When injesting too much caffeine, you may experience sleep disturbances, nervousness, irritability, dizziness and gastrointestinal disturbances. Frequent and high caffeine intake can also contribute to stress disorders.
References
- "Principles of Neural Science, 4th Edition"; Kandel, Jessel, Schwartz; 2000
- Mayo Clinic: Caffeine: How Much Is Too Much?
- Drugs.com: Caffeine Side Effects
- "The Journal of Nutrition"; Caffeine Enhances Modulation of Parasympathetic Nerve Activity in Humans: Quantification Using Power Spectrum Analysis; Hibino et al; 1997
