A concussion stretches brain cells hard enough to open their ion channels. Potassium floods out, calcium floods in, and glutamate is released. The brain hits an energy crisis just as its blood flow drops. The cells stay structurally intact, which is why the CT is normal.
The single most useful thing to understand about concussion: the injury is chemical and metabolic before it is anything else. Once you know what is actually happening in the tissue, almost everything else about this injury stops being mysterious. That includes why the scan is normal.
What happens to the brain in the first seconds of a concussion?
Mechanical force stretches neuronal membranes and axons (the long fibers that carry signals between brain cells). That stretch opens ion channels indiscriminately. The carefully kept separation of ions across the cell membrane collapses. Potassium floods out of cells and calcium floods in. The excitatory neurotransmitter glutamate is released in bulk, which drives more ionic flux in neighboring cells.
None of this destroys tissue in a way a scanner can resolve. The cells are intact. What has failed is their electrochemical order.
Why does a concussion starve the brain of energy?
Restoring that order is expensive. The sodium-potassium pumps that push ions back where they belong run on ATP, the cell’s fuel. So the brain’s demand for glucose spikes sharply. That happens at exactly the moment when cerebral blood flow is typically reduced. The result is a mismatch between energy needed and energy that can be delivered. The literature calls this period a metabolic crisis.
Calcium building up inside cells also impairs mitochondria. Those are the very structures that would otherwise make the ATP required. The injury degrades the machinery needed to recover from the injury.
Giza CC, Hovda DA. The New Neurometabolic Cascade of Concussion. Neurosurgery. 2014;75(Suppl 4):S24–S33. doi:10.1227/NEU.0000000000000505
Why does a concussion cause headaches, fatigue and brain fog?
The clinical picture follows directly from the biology. That is why it is so consistent across patients who have never spoken to each other:
- Headache and light sensitivity — glutamate release and cortical spreading depression share mechanisms with migraine. That is why post-concussive headache so often behaves like one
- Fatigue and cognitive fog — a brain in an energy deficit cannot fund sustained attention. Thinking becomes expensive, so it becomes slow
- Symptoms provoked by cognitive or visual load — load raises energy demand that the tissue currently cannot meet. That is why reading a screen brings symptoms on
- Vulnerability to a second injury — while the metabolic crisis is unresolved, a further impact lands on tissue with no reserve. That is the physiological basis for return-to-play caution
Why is the CT scan normal after a concussion?
This is the whole point. A CT scan detects structural pathology: blood, fracture, displaced tissue. At that resolution, the neurometabolic cascade is an invisible injury. There is no bleeding, no mass, no lesion. On a CT, cells that are electrochemically deranged and energetically starved look exactly like healthy cells.
So a normal scan is not evidence against a concussion. It is the expected finding. Any reasoning that treats it as exoneration has misunderstood what the scan measures. See diagnosis and testing.
What does this mean for concussion treatment?
If the problem is an energy supply-and-demand mismatch, then the treatment logic writes itself. Do not add demand the tissue cannot fund. But do not withdraw all demand either, because prolonged inactivity has its own costs. That is precisely the graded, symptom-guided return to load that replaced dark-room rest.
It also explains why metabolic health is not a side issue here. Take a patient whose glucose handling, sleep and vascular function are already impaired. That patient is trying to resolve an energy crisis with a compromised energy system. See treatment.
Common questions
If my brain cells were not destroyed, why do I feel so bad?
Because the problem is functional rather than structural. Ionic balance collapses and glutamate floods out. Restoring order uses more energy than the injured brain can supply. Intact cells that cannot meet their energy demands produce exactly the symptoms you are describing.
Why do my symptoms get worse when I concentrate or look at a screen?
Cognitive and visual work raises energy demand in tissue that is already in an energy deficit. The symptom is the tissue reporting that it cannot fund the task. That is why graded return to load works better than either total rest or pushing through.
Why is a second concussion more dangerous before I have recovered?
While the metabolic crisis is unresolved, the tissue has no reserve. A second impact during that window lands on a brain that cannot absorb it. That is the physiological reasoning behind return-to-play restrictions.
Does this show up on any scan?
Not on routine CT or MRI. Those resolve structure rather than metabolism. Advanced research imaging can observe aspects of it non-invasively. But in ordinary clinical practice, the cascade is inferred from the examination and the symptom pattern. It is not imaged directly.
Does a concussion cause brain damage?
Not the kind a scanner can see. In the first seconds, stretched brain cells lose their ionic balance and glutamate floods out. The brain enters an energy crisis just as its blood flow drops. Calcium building up inside the cells also impairs the mitochondria that make energy. The cells stay structurally intact. That is why the CT is normal while the brain is not working normally.
How does a concussion happen?
Mechanical force stretches the brain’s nerve cells and their axons. That stretch opens ion channels indiscriminately. Potassium pours out of the cells, calcium pours in, and glutamate is released in bulk. That spreads the disturbance to neighboring cells. Putting the ions back takes a surge of energy. It comes at the very moment blood flow to the brain is reduced.
What does a concussion feel like?
The symptoms follow the biology. Headache and light sensitivity often behave like migraine, because glutamate release shares mechanisms with it. Fatigue and brain fog come from an energy deficit that makes thinking slow and costly. Reading or looking at a screen brings symptoms on. That is because the load raises an energy demand the injured tissue cannot yet meet.
Why do noise, crowds and busy screens overwhelm me after a concussion?
Because filtering is work, and the injured brain is short of the energy to do it. A healthy brain quietly discards most of what reaches the eyes and ears and fuses vision, balance and body position into one steady picture. After a concussion that sorting slows, so a grocery aisle, a restaurant or a scrolling feed arrives as raw input, which is what people mean by sensory overload. Treatment adds load in measured steps; weeks in a quiet room lower the ceiling instead of raising it.
Learn more: how a graded return to activity is built.