Why does injury cause brief unconsciousness?
Loss of consciousness is usually caused by a lack of oxygen to the brain - in the case of injury it could be directly cause by the injury or it may be because they are shocked or scared - this is called vasovagal syncope. Basically, severe emotional stress leads to a burst of activity in particular nerves that lead to reduced heart rate and dilation of vessels throughout the body, reducing blood pressure suddenly. The effect is temporarily reduced blood flow to the brain, which causes temporary loss of consciousness.

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