Cranial Nerve Reflexes

Taste fibers to the anterior 2/3 of the tonge are carried via the lingual nerve to the chorda tympani, which joins the facial nerve within the facial canal of the petrous temporal bone on its way to the nucleus tractus solitarius in the pons.

Facial nerve lesions distal to the junction of chorda tympani with the facial nerve (extracranial lesions, such as those within the parotid gland) will thus spare taste fibers and taste to the anterior tongue will be retained.

Facial Palsy

The facial nerve is composed of a main motor part and a minor (sensory and parasympathetic) part. The minor part is known as the intermediate nerve of Wrisberg.

The motor part innervates the stapedius muscle in the middle ear, stylohyoid muscle and posterior belly of the digastric muscle in the upper neck and the muscles of the facial expression in the face.

The intermediate nerve is represented by two branches: (1) the greater superficial petrosal nerve to the lacrimal gland through the pterygopalatine ganglion, and (2) the chorda tympani, which provides taste sensation to the anterior two-thirds of the tongue as well as parasympathetic innervation to the submandibular and sublingual salivary glands.

Bell's palsy can involve the facial nerve anywhere along its course, and the site of the lesion can be predicted based on the associated signs and symptoms as follows:

• Decreased lacrimation/tearing or salivation: proximal to the geniculate ganglion, where the greater petrosal nerve originates.
• Hyperacusis: proximal to the origin of the nerve to the stapedius muscle
• Loss of taste sensation: proximal to the origin of the chorda tympani

The labyrinthine segment of the facial nerve is located proximal to the geniculate ganglion.

The tympanic segment is located distal to the geniculate ganglion and proximal to the oval window area.

The mastoid segment is located proximal to the stylomastoid foramen and distal to the oval window region.