Bell's palsy is a weakness of the muscles on one side of the face caused by problems with a facial nerve. The nerve becomes inflamed and swollen and stops functioning properly.
There are two facial nerves, one for the right side of the face and one for the left. Each has several branches. The main branch controls most of the muscles on one side of the face, including the muscles that control facial expression and the muscles that close and open the eyes and the lips. Other smaller branches go to the tongue and ear.
To continue reading this article, you must login
Subscribe to Harvard Health Online for immediate access to health news and information from Harvard Medical School.