The intermediate nerve, also known as the nerve of Wrisberg or the intermediate part of the facial nerve, is a component of the facial nerve (cranial nerve VII). It is a separate bundle of nerve fibers that carries sensory, autonomic, and, to a lesser extent, taste information. It is named “intermediate” because it lies between the larger motor root and the smaller sensory root of the facial nerve as they exit the brainstem.
Structure
Anatomical Origin: The intermediate nerve originates from the brainstem, specifically from the lateral aspect of the pontomedullary junction, close to where the main motor root of the facial nerve (cranial nerve VII) arises.
Course: It exits the brainstem along with the facial nerve and the vestibulocochlear nerve (cranial nerve VIII). All three travel through the internal acoustic meatus, a canal in the temporal bone of the skull.
Geniculate Ganglion: The fibers of the intermediate nerve contribute to the formation of the geniculate ganglion, which is an aggregation of nerve cell bodies (ganglion) located at the facial nerve’s first genu or bend.
Fiber Composition: The intermediate nerve consists of sensory fibers, taste fibers, and autonomic fibers. Each type has a different cellular origin and destination.
Association with Other Nerves: After contributing to the geniculate ganglion, some fibers of the intermediate nerve become part of the chorda tympani, a branch of the facial nerve that eventually joins the lingual nerve.
Functions
Sensory Function: The intermediate nerve carries general sensory information from a region near the external auditory meatus, providing sensation to a part of the ear.
Taste Function: The nerve is responsible for carrying taste sensations from the anterior two-thirds of the tongue. These fibers synapse at the geniculate ganglion and are conveyed via the chorda tympani, which eventually joins the lingual nerve, itself a branch of the mandibular division of the trigeminal nerve (cranial nerve V).
Autonomic Function:
- Salivary Glands: The intermediate nerve provides parasympathetic innervation to the submandibular and sublingual salivary glands, facilitating salivation.
- Lacrimal Gland: It also carries parasympathetic fibers that stimulate tear production in the lacrimal gland. These fibers typically synapse in the pterygopalatine ganglion before proceeding to the gland.
Clinical Significance
- Diagnosis: In conditions affecting the facial nerve, such as Bell’s palsy or tumors of the cerebellopontine angle, the intermediate nerve may also be impacted. This can result in loss of taste sensation in the anterior two-thirds of the tongue or dry mouth and eyes due to loss of autonomic function.
- Surgical Procedures: Understanding the anatomy of the intermediate nerve is crucial during surgical procedures involving the cerebellopontine angle, internal acoustic meatus, or facial nerve to prevent complications like loss of taste or dry mouth and eyes.
- Radiological Imaging: The nerve may sometimes be visible in detailed imaging studies, such as MRI, which can be useful for diagnosis in cases of suspected nerve pathology.
