what is a concussion

[Ronald Petrarca]

     I understand that a concussion is a severe blow to the head 
resulting in a temporary change in consciousness. What exactly happens 
when one suffers a concussion? What is the difference between a 
concussion and an ordinary knock on the head? Also, is there a threshold 
below which the brain is completely shielded from mechanical disturbance? 
In other words, how well do the skull and cerebrospinal fluid protect the 
brain?

                             Ron Petrarca