A concussion happens when a force hits your head or body hard enough to make your brain shift or rotate inside your skull. That movement stretches and disrupts brain cells, triggering a cascade of chemical changes that temporarily impair normal brain function. You don’t need to lose consciousness or even hit your head directly for a concussion to occur.
What Happens Inside the Skull
Your brain floats in cerebrospinal fluid inside the skull, which cushions it during normal movement. But a sudden impact, whether from a fall, a car crash, or a collision on the field, can accelerate the skull faster than the fluid can absorb. The brain slides, bounces, or twists within that space. Two types of force do the damage, and they work differently.
Linear force, the kind from a straight-on hit, creates sudden pressure changes inside the skull. That pressure gradient tends to cause more localized injury at the point of impact and the opposite side of the brain. Rotational force, which comes from angular or twisting blows, is generally considered more dangerous. It causes the brain to move relative to the skull, creating shearing strain across wide areas of tissue. Most real-world impacts involve a combination of both.
How Brain Cells Get Disrupted
The mechanical force of a concussion doesn’t typically kill brain cells outright. Instead, it stretches their outer membranes, creating tiny defects in the cell walls. This lets charged particles flood in and out of cells in the wrong direction: potassium rushes out, while sodium and calcium rush in. The brain’s main signaling chemical, glutamate, gets released in a massive, indiscriminate burst. All of this triggers a wave of uncontrolled electrical activity that spreads across the brain, somewhat like a power surge through a circuit.
That surge is likely the biological basis for the immediate symptoms of concussion: confusion, dizziness, blurred vision, and that foggy “not right” feeling. The brain then scrambles to restore normal balance, burning through its energy reserves at an accelerated rate while blood flow is simultaneously reduced. This mismatch between high energy demand and low energy supply creates a vulnerable metabolic state that can persist for days.
Damage to the Brain’s Wiring
The long nerve fibers that connect different brain regions, called axons, are especially vulnerable to rotational forces. These fibers act like transmission cables, carrying signals and transporting proteins across the brain’s white matter. Shearing forces can snap the tiny internal scaffolding structures that support these cables, somewhat like breaking the rails inside a train tunnel. When that happens, proteins pile up at the break points, creating visible swellings along the length of the fiber. Transport doesn’t fail completely in most concussions, but it slows down and becomes unreliable.
This type of diffuse injury helps explain why concussion symptoms are so varied. Depending on which connections are affected, you might have trouble with memory, reaction time, balance, emotional regulation, or sleep, sometimes all at once.
The Most Common Causes
Falls are the single most common cause of traumatic brain injuries, and they disproportionately affect the youngest and oldest age groups. For adults 65 and older, falls are the leading reason for TBI-related hospitalization and death. Being struck by or against an object, particularly in sports, is the second most common cause. Vehicle-related injuries, including those involving pedestrians and cyclists, round out the top three.
In younger people, sports like football, soccer, hockey, and basketball carry significant concussion risk, not just from dramatic collisions but from routine contact during play. You can also get a concussion without a direct blow to the head. A hard hit to the chest or a whiplash-type motion can transmit enough force through the neck to shake the brain inside the skull.
Why Hits Below the Threshold Still Matter
Not every impact that affects the brain produces obvious symptoms. Subconcussive hits, those that fall below the threshold for a clinical concussion, cause no immediate confusion, headache, or dizziness. But they happen far more frequently than full concussions, especially in contact sports, and the evidence increasingly suggests they accumulate over time.
Studies of high school football players have found measurable changes in brain activation over the course of a single season from subconcussive impacts alone, even without any clinically observable impairment. Animal research has confirmed that these smaller hits cause acute neuroinflammation despite producing no behavioral symptoms. Post-mortem studies point to a cumulative effect that may accelerate cognitive aging and alter brain biology later in life. Because these impacts don’t produce symptoms, they go undiagnosed and unmanaged, meaning athletes can accumulate hundreds or thousands of them over a career without anyone flagging a concern.
How a Concussion Is Recognized
There’s no blood test or brain scan that reliably diagnoses a concussion. Standard imaging like CT and MRI typically looks normal because the injury is chemical and microstructural, not a visible bleed or fracture. Diagnosis relies on a combination of the mechanism of injury (a blow to the head, face, neck, or body) and the presence of characteristic signs or symptoms afterward.
Headache, dizziness, and difficulty concentrating are the three most commonly reported symptoms. The standardized assessment tool used in sports settings evaluates 22 symptoms on a severity scale, including pressure in the head, neck pain, blurred vision, nausea, confusion, and a general feeling of being “off.” On-field signs that raise immediate concern include visible disorientation, loss of consciousness, balance problems, and amnesia for events before or after the impact. You don’t need all of these to have a concussion. A single symptom following a plausible mechanism of injury is enough to warrant evaluation.
The Brain’s Recovery Window
After the initial chemical disruption, the brain enters an altered metabolic state. Potassium levels outside cells spike, triggering a period of hypermetabolism that can last up to ten days. During this window, protein production slows, the brain’s ability to use oxygen drops, and energy reserves remain depleted. Research using brain imaging has shown that metabolic markers generally return to normal by about 30 days post-injury in athletes who recover uneventfully.
This recovery window is the reason rest matters so much in the early days after a concussion. Pushing your brain to perform, whether through intense studying, screen time, or physical exertion, increases energy demand during a period when supply is already compromised. Gradual return to activity, guided by symptom monitoring, gives the brain time to restore its chemical balance without additional strain.
Why a Second Hit Is So Dangerous
During the metabolic recovery window, the brain is significantly more vulnerable to a second injury. After the initial concussion, the brain limits its own blood flow as a protective measure, which leads to a buildup of lactic acid and internal acidity in cells. This compensatory mechanism prevents massive swelling but leaves the system fragile. If a second impact occurs before recovery is complete, even one of lesser intensity than the original, the brain can lose its ability to regulate internal pressure and blood flow. In severe cases, this leads to rapid, catastrophic swelling that can compress the brainstem. This scenario, known as second impact syndrome, is rare but can be fatal, and it’s the primary reason that returning to play or activity too soon after a concussion carries real risk.