Caffeine is a central nervous system stimulant, which is found in many natural sources. These include the kola plant, tea, coffee. However, many beverages contain added caffeine such as energy drinks and sodas. When caffeine is active in the brain, an antagonism of adenosine receptors occurs. Levels of neurotransmitters increase. This leads to the increased focus mentally and alertness the person experiences. However, when it wears off, a person may experience a caffeine crash. Caffeine is thought to antagonize Adenosine A1 receptors which are presynaptic and are located in the cerebral cortex and hippocampus. Here, the release of dopamine, glutamate and acetycholine are inhibited. Caffeine is thought to also antagonize A2 receptors. Withdrawal from caffeine can lead to depression, irratic heartbeat, muscle pain, nausea, headaches, etc.