Anatomy of the facial nerve
The facial nerve starts in an area of the brain called the brainstem and more specifically in the pons.
It has two roots. The biggest root is called the motor root (it is responsible for the control of movement) and the smaller one is called the sensory root (it is responsible for sensation). The size difference relates to the fact that the facial nerve is primarily a nerve which controls movement and to a lesser extent a nerve with very specific sensory functions. The two roots travel through a channel within the brain called the internal auditory meatus (IAM). It is 1 cm long and opens out into the temporal bone. From here the two roots enter the facial canal, a ‘z’ shaped structure contained within the temporal bone.
Several important things happen here:
- The roots join together to form the facial nerve.
- Nerve cell bodies form called ganglions. The medical name for this group of nerve cell bodies is the geniculate ganglion.
- From the geniculate ganglion, the greater petrosal nerve forms and produces parasympathetic nerve fibres which control mucus production and tear production. The parasympathetic nervous system is involuntary so not under our control. It constantly controls bodily functions, such as blood pressure, the size of our pupils, heart rate amongst other things. These specific parasympathetic nerve fibres control how much mucous we produce and how much lubrication and tears are produced to keep the eye healthy.
- Nerve fibres which supply the stapedius muscle in the ear also sprout at this point and its role is to diminish loud sounds to protect our hearing.
- Specialist sensory nerve fibres which also originate along this pathway are called the Chorda Tympani and control taste and saliva production in the mouth. The facial nerve then leaves the skull to enter the face which forms the beginning of the extracranial section. This can be seen in the diagram below. The extracranial pathway begins with the branching out of the posterior auricular nerve and ends with the terminal motor branches.
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