Have you ever woken up and run straight to your coffee maker because you had something important to do that morning and you needed to be especially alert? Or have you ever poured an extra evening cup of joe because the sun was setting but your list of to-do’s was nowhere near completion? Or have you ever scotch-taped your eyes open and stared at your hand to see how bad it was shaking because you were hoping you could still drink another cup of coffee without having a heart attack because god help you, you just wanted to finish writing that dissertation? …ok, that last one might have just been me. Anyway, if you’ve ever found yourself leaning on the world’s favorite beverage to give you that extra kick, you’ve been tinkering with your brain’s adenosine system.
Adenosine is a very common chemical found throughout your brain. Just about every kind of neuron in your brain has receptors that adenosine can fit into and activate. Caffeine, the active and stimulating ingredient in coffee, works by blocking the adenosine in your brain from reaching those adenosine receptors. And actually adenosine is common outside of your brain too; that’s why drinking too much coffee can cause effects outside of your brain, like heart problems. But why would preventing a chemical from sliding into a receptor make you feel awake and alert?
Adenosine is a strange little neurotransmitter because taking a drug (like caffeine) that prevents it from stimulating its receptor is almost always performance enhancing. With every other neurotransmitter I can think of, blocking its actions might help in a very specific situation or disease but is going to come with lots of bad consequences. Because usually you need those little chemicals to keep doing their jobs. But if blocking adenosine is universally performance enhancing, that implies that adenosine’s role in the brain is to reduce our performance. Why in the world would we have an entire system in place in our brain that generally reduces performance?
To explain why having a performance reducing neurotransmitter is a very good thing for your brain just think of the brake system in your car. The entire purpose of the automobile is to move forward quickly, so why in the world would we develop an entire system with hundreds of moving parts with the sole purpose of slowing down and stopping the car? Ah, right. Because without brakes it would be virtually impossible to use the vehicle without it ending in a terrible accident. The same goes for your brain, it works by having hundreds of billions of neurons furiously processing information as fast as they can. But sometimes they can work too hard and without a way to put on the brakes and slow everything down there could be terrible consequences.
And that is adenosine’s job; it slows your neurons down. Specifically it makes them stop firing. Neurons work by receiving lots of information, processing that information, and then sending the new, processed information on to other neurons down the chain. We call that process of sending the new information down to other neurons “firing”. So whenever a situation arises when a group of neurons start firing too much for their own good, the brain quickly releases lots of adenosine to make it stop.
In what situation would the brain need to apply the brakes to a group of neurons? There are several examples of when this is a helpful thing. One such example comes from studies that have shown that the levels of adenosine slowly increase throughout the day. That means the longer you’re awake, the more adenosine is floating around to slow your neurons down. This makes adenosine a kind of timer; by slowly ramping up over the course of the day it can make you feel less and less alert and therefore help you feel like it’s bedtime. That’s why drinking coffee makes it hard to fall asleep; you’re pulling your brain’s foot off the brakes allowing everything to run at full speed.
Another time your brain needs to put on the brakes is to prevent seizures. Seizures are caused by a group of neurons firing too much. If one region gets over-active, it can cause other regions connected to it to start firing too much as well. This spreads like wildfire across your brain until eventually you have a full blown seizure with neurons all over your brain firing as fast as they can. There is a mechanism in the brain that causes the release of adenosine whenever neurons are firing so much that they use energy faster than they can get it supplied by the blood stream. So if you have a group of neurons going crazy, that over-firing will cause adenosine to get released and thus help slow the neurons back down.
Adenosine will also get released when energy is not getting fed to the neuron properly, like during a stroke. In that case the neuron might be firing at a normal, healthy rate but if something happens to the blood flow feeding the neuron then adenosine will get released and slow the neuron down. This is a good thing because it helps the neuron conserve the little energy it has remaining and thus keep it alive longer. Adenosine helps keep neurons from dying in the hopes that the energy supply will resume soon.
So why does coffee help you feel energetic and alert? Because coffee, that is caffeine, is a chemical that takes the foot off the brake in your brain. It prevents adenosine from binding with its receptor and thus prevents it from slowing things down. Without the adenosine brake, the neurons in your brain are free to fire without inhibition and thus you feel more energetic and alert.
At this point you might be thinking “I wouldn’t drive my car without brakes, so should I be drinking those cups of coffee every morning?” Fortunately the health research seems to suggest that drinking some every day is ok, as long as you’re not drinking too much. So feel free to caffeinate your brain, just don’t overdo it.