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The greater petrosal nerve spreads from the seventh facial cranial nerve near the middle ear. In turn, it also runs into nerves that go to the lacrimal gland, which produces tears for the eye, as well as mucous membranes in the nose and hard palate. These spread out from the pterygopalatine ganglion, one of four large nerve structures in the head and neck. This ganglion also controls the blood flow through the nasal cavity to help regulate heating and cooling in the nose. Passing through a bone space called the middle cranial fossa, the greater petrosal nerve passes the carotid artery and is part of a nerve anatomically identified as the pterygoid canal.
Various functions are regulated by the greater petrosal nerve, including salivation. The nerve typically controls the mucous glands above the mouth, and damage to it can cause these glands to stop producing mucous and saliva. It cannot, however, control the salivary glands in the jaw and throat. Damage to the greater petrosal nerve, therefore, usually does not affect mucous production from the under tongue and other glands nearby.
This nerve also affects tear production. Damage can affect the lubrication of the eyes. Sometimes nerve signals can be altered such that hunger can trigger tears to flow instead of stimulation of the salivary glands. Function of the nerve can also be affected by middle ear infections, but the disruption in activity generally does not totally shut down the nerve. It splits from the facial nerve before the middle ear cavity, but is close enough to be affected if infections are severe enough.
In a space called the auditory meatus, the greater petrosal nerve runs as a branch from the seventh facial nerve. This nerve controls the facial muscles, several glands in the head, and taste in the back of the tongue. It also links to sensory components in the ear drum and other parts of the ear. The greater petrosal nerve also passes through an area called the pterygoid canal along with the deep petrosal nerve until it reaches the pterygopalatine ganglion. It is here where different branches lead to areas such as the nose and mouth.
On the other end, the facial nerve travels to the brainstem, which is the destination for some taste nerve fibers. The greater petrosal nerve is the general link between several non-motor but significant facial functions to the brain. It is also part of the complex network of nerves that pass through various cavities in the skull and other facial bones.