[d_phaneuf]

http://www.businessinsider.com/your-...ussion-2013-10

A concussion immediately paralyzes the nervous function of the brain — you can't sense anything for a moment. But this paralysis is reversible, and goes away not too long after the impact. You don't actually have to be knocked unconscious to have a concussion.

There are essentially three levels of severity, according to Missouri University Health Care:

The person is confused and unable to think clearly, but still conscious. This usually lasts less than 15 minutes.

The person is still conscious, but is confused and experiences memory loss. This lasts at least 15 minutes.

The person is actually knocked unconscious. When they regain consciousness, they will noticeably have trouble moving and thinking clearly.