As you down that third beer a familiar, warm sensation might begin to sweep over you. The loving embrace of alcohol has you, and your squishy human brain is at its mercy. Consuming alcohol does a lot of things, some good and some not so good, but the most prominent short-term effect is that it gets us drunk. But what causes drunkenness?
There are many kinds of alcohol if you’re speaking in chemistry lingo — it’s just a carbon compound with a hydroxyl group (-OH) bound to one of the carbon atoms. When we talk about the kind of alcohol humans drink, or the alcohol content of a bottle of whisky, we’re talking about ethanol. This is a small two-carbon chain with a hydroxyl (or alcohol group) stuck on one end. There are six hydrogens in the mix, making the molecular formula of that odious suppressor of commonsense C2H6O (or C2H5OH if you want to get specific).
The key to ethanol’s effect is that it’s water-soluble, which means it mixes readily with good old H2O. That’s why you can drink a beer and find no difference in the concentration of alcohol at different points in the bottle. When you guzzle that drink, the ethanol diffuses through the stomach and into the water in your bloodstream, ending up literally everywhere in your body. It seeps into your muscles, leaks from the skin, and aspirates into the lungs. Coincidentally, that is how a breathalyzer is able to estimate your blood-alcohol content.
Of course, your liver begins breaking down ethanol through a normal metabolic pathway as soon as it shows up. An enzyme called alcohol dehydrogenase (seen below) breaks ethanol down to acetaldehyde, which does not cause intoxication, but is actually toxic. Acetaldehyde and other byproducts of alcohol metabolism are partially responsible for hangovers, but ethanol itself is cleared more slowly than it is absorbed. So the concentration of alcohol inevitably goes up and it finds its way to the brain through blood vessels. This is where things get interesting.
Your neurons, like other cells in your body, rely on receptor proteins embedded in the cell membrane to help them respond to conditions in the body. Receptors have one or more molecules that cause them to become activated. You can think of it like a key fitting into a lock — no key, no activation. Ethanol at high enough concentrations will stick to receptors non-specifically, and this is what’s going on in your brain as you drink.
While alcohol is classified as a central nervous system depressant, it activates and suppresses several types of receptors. The receptors that handle a neurotransmitter called GABA are strongly activated by ethanol molecules sticking to them. GABA is an inhibitory neurotransmitter, so the activation of this receptor slows the brain down — it makes you feel calm and relaxed. This is why drunkenness tends to make people talkative and less stressed.
Other receptors in the brain are simply blocked by ethanol without being activated. This includes the NMDA receptor, which normally responds to a molecule called glutamate. With the receptor blocked, it can’t do its job, which is to increase neuronal activity and control memory function. This contributes to the sluggishness we feel while drunk, and also the memory issues excessive drinking can cause.
The more alcohol is consumed, the more the nervous system is depressed, and that can take things from pleasant to dangerous. For most people a blood alcohol content of 0.1% is where nervous system impairment becomes severe enough to affect motor control and speech. At double that level, unconsciousness is common.
Alcohol concentrations of 0.4-0.5% are usually high enough that brain activity becomes so depressed that the nervous system cannot keep the lungs working or the heart beating. This is beyond drunk — it’s life threatening. More commonly, the gag reflex disappears, making it possible for a person to aspirate on their own vomit and die. But this is an uncommon state to be in. Most folks get drowsy after a sufficient number of drinks and decide to hit the hay.
The next time you have a couple beverages and your senses begin to blur and the world seems a little less scary, reflect on the intricate molecular dance going on in your brain that makes it all happen. Well, you might be too drunk to reflect on anything, but give it a shot. Consume your ethanol responsibly!
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