A concussion triggers a rapid chain of chemical, structural, and metabolic disruptions inside the brain that can persist for weeks, even after symptoms fade. The injury isn’t a bruise you can see on a scan. It’s a cellular-level crisis: neurons flood with the wrong chemicals, energy supplies plummet, and the brain’s internal communication lines stretch and fray. Here’s what actually happens, step by step.
The Initial Chemical Surge
The moment your brain absorbs a hit, the force deforms cell membranes and stretches open channels that normally stay tightly regulated. Potassium rushes out of neurons into the surrounding space, and the brain’s main excitatory signaling chemical, glutamate, pours out in a massive, uncontrolled wave. Under normal conditions, glutamate carries signals between neurons in precise amounts. After a concussion, it floods the gaps between cells indiscriminately.
That glutamate surge forces even more potassium out and drives calcium into neurons through newly opened receptor channels. Calcium inside a cell acts like a slow poison at high levels: it damages the cell’s energy-producing machinery (mitochondria), triggers enzymes that break down the cell’s internal scaffolding, and can push the neuron toward death. The process feeds on itself. More potassium outside the cell triggers more depolarization, which releases more glutamate, which lets in more calcium. This self-reinforcing loop is the core of what makes even a “mild” brain injury genuinely dangerous at the cellular level.
The Brain’s Energy Crisis
To restore its chemical balance, the brain has to work overtime. Ion pumps on every affected neuron start burning through enormous quantities of ATP, the cell’s energy currency, trying to push potassium back in and calcium back out. This creates a spike in glucose consumption that lasts roughly six hours in animal models.
But the mitochondria responsible for making that energy are themselves damaged by the calcium flooding. The result is a mismatch: the brain desperately needs fuel while its power plants are half-broken. After the initial spike, glucose metabolism drops well below normal and stays depressed for weeks. Studies comparing brain-injured patients to healthy volunteers found glucose use dropped from about 4.5 mg per 100 grams of brain tissue per minute to roughly 3.4, and the brain shifted toward less efficient, oxygen-free energy production. This energy deficit is a key reason why thinking feels effortful, concentration is poor, and mental fatigue sets in so easily after a concussion.
Stretched and Damaged Wiring
The brain’s long-distance communication depends on axons, thin cables that connect neurons across different regions. These cables are bundled into white matter tracts. When the head accelerates or rotates suddenly, shearing forces stretch those axons, particularly at boundaries where dense gray matter meets lighter white matter.
This stretching rarely snaps axons outright. Instead, it disrupts the internal transport system. Tiny structural rails called microtubules, which carry supplies along the length of the axon, break at the stretch points. Within two to three hours, cargo starts piling up at the damage sites because it has nowhere to go. Meanwhile, the same calcium overload happening everywhere else activates enzymes that chew through the axon’s structural proteins. Over hours to days, some of these partially damaged axons deteriorate and disconnect entirely, a process called secondary axotomy. This delayed damage is why symptoms can worsen in the first 24 to 72 hours after a hit.
Inflammation Ramps Up
The brain has its own immune cells called microglia. They act as first responders, detecting damage signals released by injured neurons. Within hours of a concussion, microglia change shape, pull in their branches, and begin releasing inflammatory molecules, including signaling proteins that recruit additional immune activity to the injured area.
This inflammatory response is a double-edged sword. In its aggressive, pro-inflammatory form, it can worsen tissue damage, producing reactive oxygen species and compounds that are toxic to nearby healthy neurons. The brain also has a repair-oriented form of microglial activation that releases growth factors and calming signals. After a single concussion, the aggressive inflammatory response tends to resolve relatively quickly. After repeated concussions, it persists. Research on repeated head injuries found that microglia remained in their pro-inflammatory state for significantly longer, creating a sustained hostile environment that delays healing and may contribute to lasting consequences.
Reduced Blood Flow
On top of the energy crisis, blood flow to the brain drops after a concussion. A study of concussed athletes who had pre-injury baseline scans found that blood flow to the frontal and insular regions decreased by roughly 9 mL per 100 grams of tissue per minute after injury. Less blood flow means less oxygen and glucose delivery to cells that are already starving for fuel. The autonomic nervous system, which regulates blood flow and heart rate variability, also shows measurable dysfunction that can outlast the resolution of obvious symptoms like headache and dizziness.
Why Recovery Takes Longer Than It Feels
Most concussion symptoms, such as headaches, fogginess, and mood changes, resolve within seven to ten days for adults. But the brain’s physiology tells a different story. Multiple studies have found that the autonomic nervous system remains disrupted beyond the point when athletes are cleared to return to play. A systematic review by the Concussion in Sport Group concluded that physiological dysfunction outlasts current clinical measures of recovery. In practical terms, this means the brain is still healing even when you feel fine. This gap is why a gradual return to activity, rather than an abrupt jump back to full contact or intense work, matters so much.
The Younger Brain Is More Vulnerable
Children and adolescents face additional risks. The developing brain has thinner skull bones, less neck strength to absorb force, and incomplete myelination, the insulating coating around axons that makes signal transmission fast and efficient. These factors don’t just make concussions easier to sustain; they can make recovery slower.
Comparisons of high school and college athletes found that younger athletes took one to two extra days to recover on cognitive testing measures after concussion. Both groups showed symptom improvement by about day five and normalization around day seven on average, but the cognitive lag in younger athletes suggests the developing brain needs more time to restore its processing speed. Children are also at higher risk for a rare but dangerous complication called malignant cerebral edema, a severe swelling response that can occur after even a mild brain injury.
What Repeated Concussions Do Over Time
A single concussion triggers a temporary crisis that, in most cases, fully resolves. Repeated concussions are a different problem. Chronic traumatic encephalopathy (CTE) is a progressive brain disease caused by repetitive head impacts, and it has been identified in individuals as young as 17. The hallmark is an abnormal buildup of a modified protein called phosphorylated tau around blood vessels deep in the brain’s folds. Currently, CTE can only be diagnosed after death by examining brain tissue under a microscope.
Research published in Nature in 2025 found that multiple years of repetitive head impacts are sufficient to cause lasting cellular changes, including neuron loss and chronic inflammation, that appear to precede tau deposits. The inflammatory response from microglia may be the trigger: sustained, unresolved neuroinflammation from repeated injuries creates the conditions for tau to accumulate and spread. This is why the total burden of head impacts over a lifetime, not just the number of diagnosed concussions, is increasingly seen as the relevant risk factor.
How Brain Damage Is Detected
Standard CT scans and MRIs often look normal after a concussion because the damage is at the cellular level, not the structural level visible on imaging. Newer tools are changing this. A blood test measuring two proteins, one released by damaged neurons and one by injured support cells, received FDA clearance in 2021 as a point-of-care diagnostic for mild traumatic brain injury. These proteins are present in only trace amounts in healthy people, so elevated levels after a head impact indicate that brain cells have been damaged. The test helps clinicians determine whether a CT scan is needed, reducing unnecessary radiation exposure while catching injuries that warrant closer monitoring.