Concussion, or mild traumatic brain injury (MTBI), is the most common and least serious type of traumatic brain injury. A milder type of diffuse axonal injury, concussion involves a transient loss of mental function. It can be caused by acceleration or deceleration forces, or by a direct blow. Concussion is generally not associated with penetrating injuries.

The brain floats within the skull surrounded by cerebrospinal fluid (CSF), one of the functions of which is to protect the brain from normal light "trauma", e.g., being jostled in the skull by walking, jumping, etc., as well as mild head impacts. More severe impacts or the forces associated with rapid acceleration/deceleration may not be absorbed by this cushion.

Concussion is considered a type of diffuse brain injury (as opposed to focal brain injury), meaning that the dysfunction occurs over a more widespread area of the brain.

Excitatory neurotransmitters are released as the result of the traumatic injury and cause the brain to enter a state of hypermetabolism which can last for 7 to 10 days.[1] During this time, the brain needs extra nutrients and is especially sensitive to inadequate blood flow.

Areas of the brain whose functions are commonly disturbed in concussion include the reticular formation or the deep structures of the brain, the brainstem or cerebral cortex.[2] Damage to cranial nerves and other white matter tracts may be temporary or permanent.[3] Other theories hold that concussion is a diffuse injury affecting all parts of the brain, caused by physical trauma that alters neuronal metabolism and excitability through molecular commotion. Having a concussion does not mean that the patient does not have another brain injury as well; in fact, more serious brain trauma is almost always accompanied by concussion.[4]