Tympanic nerve

The tympanic nerve (Jacobson's nerve) is a branch of the glossopharyngeal nerve found near the ear. It gives sensation to the middle ear, the Eustachian tube, the parotid gland, and mastoid air cells. It gives parasympathetic to supply to the parotid gland via the otic ganglion and the auriculotemporal nerve.

Structure
The tympanic nerve contains sensory axons to the middle ear (including the internal surface of the tympanic membrane) whose cell bodies are lodged in the superior ganglion of the glossopharyngeal nerve.

It also contains parasympathetic axons which continue as the lesser petrosal nerve to the otic ganglion, which itself gives off postganglionic parasympathetic neurons.

Origin
The tympanic nerve arises from the inferior ganglion of the glossopharyngeal nerve (CN IX) in the jugular fossa. 

Course
It passes through the petrous part of the temporal bone within the tympanic canaliculus that is situated within the bony ridge separating the carotid canal and the jugular foramen to reach the middle ear. 

In the tympanic cavity of the middle ear, it ramifies upon the promontory of tympanic cavity to form the tympanic plexus. 

Sensory
The tympanic nerve provides sensation to the middle ear (tympanic cavity). This includes the internal surface of the tympanic membrane. It also supplies the Eustachian tube, the parotid gland, and mastoid air cells.

Parasympathetic autonomic
The tympanic nerve is also the parasympathetic root of the otic ganglion. These neurons then provide secretomotor innervation of the parotid gland via the auriculotemporal nerve. It is involved in the salivatory reflex to increase salivation during chewing.

Variation
The tympanic nerve usually arises from the inferior ganglion of the glossopharyngeal nerve. Rarely, it may arise from a higher part. Rarely, it may provide no parasympathetic fibres to the otic ganglion.

Clinical significance
The tympanic nerve is involved in a reflex, where stimulation of the ear canal increases salivation.

Cancer
The tympanic nerve may be involved by paraganglioma, in this location referred to as a glomus tympanicum tumour. This causes a soft mass in the middle ear (tympanic cavity). There may also be pulsatile tinnitus, hearing loss or hearing problems, and some cardiac abnormalities.

History
The tympanic nerve is also known as the nerve of Jacobson, or Jacobson's nerve.