Physiology of CN-VII

BELL'S palsy affects the seventh cranial nerve, or CN-VII, and causes facial muscles to weaken or become paralysed. Originating in the brain stem, CN-VII passes through the stylomastoid foramen and enters the parotid gland. It divides into its main branches inside the parotid gland. These branches then further divide into 7,000 smaller nerve fibres that reach into the face, the neck, the salivary glands and the outer ear. The nerve controls the muscles of the neck and the forehead, and facial expressions, and also the perceived sound volume. It also stimulates secretions of the lower jaw, the tear glands and the salivary glands in the front of the mouth. The sensation of taste from two-third of the front portion of the tongue and sensations at the outer ear are transmitted by CN-VII.

Bell's palsy is caused by inflammation within a small, extremely narrow, bony tube called the fallopian canal. An inflammation within it is likely to exert pressure on the nerve, thus compressing it. Likewise, if the nerve itself becomes inflamed within the canal, it can encounter pressure and therefore compression. In this case, the nerve has not yet exited the skull and divided into its many branches; the result is the impairment of all functions controlled by CN-VII. If only part of the face is affected, the condition is not Bell's palsy. If, for example, the mouth area is weakened but the forehead moves, Bell's palsy is ruled out. It could be the result of trauma induced by a tumour or by surgery; it can occur at a location where the nerve has already divided into branches. This type of trauma may spare one or more branches and allow some muscles to remain functional.

The face has many muscles, each with its own unique function. Some of them are controlled by CN-VII. These are known as the muscles of facial expression. Unlike others, facial muscles are inserted directly into the skin. Contraction of these muscles causes the skin to move. Signals from the complex array of nerves to the various muscles instruct them to move in combination as well as individually. Bell's palsy temporarily prevents the nerve from transmitting signals to the muscles.

Facial muscles differ from skeletal muscles in that they do not immediately begin to atrophy from lack of use. Estimates of the time it takes for any significant atrophy varies, but for now it is believed to take many years.

CN-VII is one of 12 pairs of cranial nerves. This explains why not all the facial muscles are affected by Bell's palsy. The muscles that close the eyelid are controlled by CN-VII, but those 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 the movement of the tongue 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 functions that are not controlled by CN-VII.