Nervus Intermedius

Also called the nerve of Wrisberg, the nervus intermedius contains general visceral afferent, special visceral afferent, and general visceral efferent fibers that convey sensation from the posterior region of the external auditory canal and concha, taste from the anterior two thirds of the tongue; and secretomotor function for the lacrimal glands; submandibular and sublingual glands; and minor salivary glands in the nasal cavity, paranasal sinuses, and palate ( .Fig 11-2 ). In the cerebellopontine angle, the nervus intermedius travels from the pons to the internal auditory meatus between the motor root and CN VIII. The geniculate ganglion, found within the temporal bone along the course of the facial nerve, contains the primary cell bodies of the special visceral afferent taste fibers and general somatic afferent sensory fibers. The preganglionic general visceral efferent fibers and special visceral efferent fibers pass through the geniculate ganglion without synapsing. The chorda tympani and the greater superficial petrosal nerve are subdivisions of this nerve.

SENSORY NUCLEUS

The general somatic afferent fibers from the posterior aspect of the external auditory canal, lateral pinna, and mastoid region (carried within the posterior auricular

Figure 11-1 Facial nerve projections within the pons(From LaRouere MJ, Lundy LB: Anatomy and physiology of the facial nerve. In Jackler RK, Brackmann DE [eds]: Neurotology. St. Louis, Mosby-Year Book, 1994, pp 1271-1281.)

nerve) travel centrally in the nervus intermedius to the dorsal part of the primary sensory nucleus of CN V within the pons. Hypesthesia of the posterior external auditory canal has been found to be an early sign of tumors within the internal auditory canal (e.g., acoustic neuroma). It is speculated that sensory fibers are more susceptible to the effects of pressure than motor axons. y

PARASYMPATHETIC NUCLEUS

The general visceral efferent fibers originate from the superior salivatory nucleus and the lacrimal nucleus in the dorsal pons and are carried distally within the nervus intermedius. After joining the motor root of the facial nerve, two main branches carry the parasympathetic fibers

Figure 11-2 Diagram of facial nerve anatomy. GVE, general visceral efferent; SVA, special visceral afferent; GSA, general somatic afferent; SVE, special visceral efferent; IAC, internal auditory canal.

to their destination. At the geniculate ganglion, the greater superficial petrosal nerve branches at the hiatus of the facial canal and travels anteromedially through the middle fossa within the pterygoid canal. Contained within this nerve are preganglionic fibers destined for the lacrimal gland and the minor salivary glands of the nasal cavity, paranasal sinuses, and palate. The deep petrosal nerve, carrying postganglionic sympathetic fibers, joins the greater superficial petrosal nerve to form the nerve of the pterygoid canal (vidian nerve). At the pterygopalatine ganglion (found within the pterygopalatine fossa), the parasympathetic fibers synapse with postganglionic neurons, which continue toward their target organs.

The preganglionic fibers for the submandibular and sublingual salivary glands pass through the geniculate ganglion and then leave the mastoid segment of the facial nerve within the chorda tympani. The chorda tympani travels through the temporal bone within the canal of Huguier and enters the infratemporal fossa via the petrotympanic fissure, where it joins the lingual nerve (CN V). Within the submandibular ganglion, the parasympathetic fibers synapse with the postganglionic cells that innervate the submandibular and sublingual salivary glands.

GUSTATORY NUCLEUS

The special visceral afferent fibers from the taste buds of the anterior two thirds of the tongue (i.e., tongue anterior to the sulcus terminalis) also travel within the chorda tympani. As stated earlier, the cell bodies for taste fibers are located within the geniculate ganglion. The gustatory afferent fibers are carried centrally by the nervus intermedius to cells in the rostral part of the nucleus of tractus solitarius in the medulla. Also synapsing within this nucleus are the taste fibers from other areas of the oral cavity, pharynx, and larynx carried by CN IX and CN X. Intermingling of these three cranial nerves occurs here. y

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