The Facial Muscles
The face has many muscles, each with its own unique function. Some, but not all, are controlled by CN-VII. These muscles are known as “the muscles of facial expression”. Unlike other muscles, the facial muscles insert directly into the skin. Contraction of the muscles causes the skin to move. Signals from the complex array of nerves to the various muscles instruct the muscles to move in combinations as well as individually. Bell’s Palsy temporarily prevents the nerve from transmitting signals to the muscles, causing weakness or paralysis. Another way the facial muscles differ from skeletal muscles is that they do not immediately begin to atrophy from lack of use. Estimates of the time it takes for significant atrophy to begin varies, but it is now believed to be years before this occurs.
CN-VII is one of 12 pairs of cranial nerves. This explains why not all the facial muscles are affected. The muscles that close the eyelid are controlled by CN-VII, but the muscles that control other eye movements and the ability to focus are not. Hence, the dry and wide open, but otherwise functioning eye. The sense of taste is affected, but tongue motion is not. Skin sensation may be affected near the ear, but sensation over the rest of the face usually remains normal. Chewing and swallowing are other examples of functions controlled by cranial nerves that are not involved with 7th nerve disorders.