The brachial plexus is a network of nerves that conucts signals from the spine to the shoulder, arm and hand. Brachial plexus injuries are caused by damage to those nerves. Symptoms may include a limp or paralyzed arm, lack of muscle control in the arm, hand, or wrist, and lack of feeling or sensation in the arm or hand. Although injuries can occur at any time, many brachial plexus injuries happen during birth: the baby's shoulders may become impacted during the birth process causing the brachial plexus nerves to stretch or tear. There are several different classification systems for grading the severity of peripheral nerve and brachial plexus injuries. The most common system divides nerve injuries into five degrees (described by Sir Sydney Sunderland): first degree or neurapraxia, in which the insulation around the nerve called myelin is damaged but the nerve itself is spared, and second through fifth degree, which denotes increasing severity of injury. With fifth degree injuries, the nerve is completely divided. Although several mechanisms account for brachial plexus injuries, the most common is nerve compression or stretch. Infants, in particular, may suffer brachial plexus injuries during delivery and these present with typical patterns of weakness, depending on which portion of the brachial plexus is involved. The most severe form of injury is nerve root avulsion, which results in complete weakness in corresponding muscles. This usually accompanies high-velocity impacts that occurs during motor vehicle or bicycle accidents.