What actually happens when you get a concussion?

The Hook

A concussion isn’t a bruise on the brain — it’s a chemical storm set off when your brain gets rattled.

The Science

Your brain floats in fluid inside your skull. A blow or sudden stop makes it accelerate and twist, stretching its neurons. That stretching triggers a metabolic cascade: neurons dump their chemical signals all at once, the careful balance of ions across their membranes goes haywire, and the brain burns extra energy to reset itself — right as blood flow is temporarily reduced.

During that mismatch — high energy demand, low supply — the brain is in a short-term energy crisis. That’s why symptoms like confusion, headache, and light sensitivity appear even when a normal scan looks completely clean.

At the cellular level, the stretched neurons flood the brain with a signaling chemical called glutamate, and potassium pours out of the cells while calcium floods in. The cell’s tiny pumps then have to work overtime to shove those ions back where they belong, burning through energy the moment blood flow is at its lowest. Until that balance is restored, the neurons simply can’t fire normally.

The good news is that the cascade is usually temporary. For most people the chemistry rebalances over days to a few weeks, and symptoms fade as it does. Recovery isn’t about a wound “healing shut” — it’s about the brain’s chemistry settling back to baseline, which is why rest and time do more than any pill.

Three Wild Facts

• A standard CT or MRI usually looks completely normal after a concussion — the injury is chemical, not a visible tear or bleed.
• You don’t have to be knocked out. Most concussions happen with no loss of consciousness at all.
• Helmets prevent skull fractures, but they can’t fully prevent concussions — the brain still sloshes and twists inside the skull no matter what’s on the outside.

Why It Matters

It explains why rest matters and why a second hit during recovery is so dangerous — the brain stays metabolically vulnerable until it rebalances. It’s also why concussion is an “invisible” injury that ordinary imaging usually can’t see, and why “I feel fine” isn’t proof the brain has caught up.