Topography and Pathway
The chorda tympani originates within the facial canal (fallopian canal) of the temporal bone, branching off the main trunk of the facial nerve after the greater petrosal nerve (n. petrosus major) and the nerve to the stapedius (n. stapedius) have already branched off.
- Skull Exit: The nerve exits the cranial cavity and middle ear through the petrotympanic fissure (fissura petrotympanica).
- Joining the Lingual Nerve: Entering the infratemporal fossa, the chorda tympani joins the lingual nerve (n. lingualis, a branch of the mandibular nerve, CN V₃).
- Further Course: The fibers travel inferiorly within the sheath of the lingual nerve, passing superficial to the hyoglossus muscle and reaching the submandibular gland and tongue.
Fiber Composition and Innervation
The chorda tympani carries two distinct functional types of nerve fibers:
- Taste (Special Visceral Afferent) Fibers: Mediate taste sensation from the anterior two-thirds of the tongue. Receptors are located within taste buds (caliculi gustatorii). The impulses travel via the lingual nerve and chorda tympani to the cell bodies of primary sensory neurons located in the geniculate ganglion (ganglion geniculi). From there, central processes travel via the nervus intermedius into the brainstem to synapse in the nucleus of the solitary tract (nucleus tractus solitarii). Second-order neurons ascend via the medial lemniscus (lemniscus medialis) to the thalamus and project to the primary gustatory cortex (insular cortex and inferior postcentral gyrus).
- Secretomotor (General Visceral Efferent) Fibers: Originate from the superior salivatory nucleus (nucleus salivatorius superior) in the brainstem. Passing through the geniculate ganglion without synapsing, these preganglionic fibers travel in the chorda tympani to synapse in the submandibular ganglion (ganglion submandibulare). Postganglionic fibers then innervate the submandibular (glandula submandibularis) and sublingual (glandula sublingualis) salivary glands.
Clinical Significance and Topographic Diagnostics
Damage to the chorda tympani results in ageusia (loss of taste) in the anterior two-thirds of the tongue and reduced salivation on the affected side. Total ageusia is rare because the posterior third and root of the tongue are innervated by the glossopharyngeal (CN IX) and vagus (CN X) nerves.
The anatomy of the chorda tympani is vital for topographic localization of facial nerve lesions:
- Lesions proximal to the branching of the chorda tympani (but distal to the nerve to the stapedius): Cause peripheral facial palsy accompanied by loss of taste on the anterior two-thirds of the tongue and impaired salivation. Hearing and lacrimation remain normal.
- Lesions distal to the chorda tympani (at or below the stylomastoid foramen): Cause isolated facial muscle paralysis without loss of taste or altered salivation.
Chorda tympani dysfunction can also occur due to compression by a vestibular schwannoma (acoustic neuroma, CN VIII tumor) or during idiopathic facial neuropathy (Bell's palsy), where MRI may show abnormal contrast enhancement along the nerve course.
Physiological Experiments
In experimental physiology, the chorda tympani is classically used to study autonomic nervous system regulation. When the central cut end of the vagus nerve (CN X) is surgically anastomosed to the peripheral stump of the chorda tympani, regenerating vagal efferent fibers sprout into the submandibular gland. Consequently, salivary secretion begins to reflect physiological activity related to central vagal hunger and satiety signals.