Vestibulocochlear Nerve(VIII) Dr. Nimir Dr. Safaa

Objectives Describe the structures making the external and middle ear. Discuss the features of the tympanic membrane. Describe the ossicles and related muscles. Describe the auditory tube, its openings and structure. Understand the mastoid air cells and their connection to the middle ear. Describe the structure of cochlea, semicircular canals, utricle and saccule Describe the course of vestibulocochlear nerve and its central connections

Middle ear(tympanic cavity) Internal ear( labyrinth) External Ear: The Ear The ear consists of: External ear Middle ear(tympanic cavity) Internal ear( labyrinth) External Ear: The external ear has an Auricle: It consists of a thin plate of elastic cartilage covered by skin. External auditory meatus: Its outer third is elastic cartilage and inner two thirds is bone. The meatus is lined by skin, with hairs,sebaceous and ceruminous glands (secrete wax). Nerve supply by auriculotemporal vagus nerves.

Middle Ear (Tympanic Cavity): It is an air-containing cavity in petrous bone lined with mucous membrane. It contains the auditory ossicles. It communicates infront with nasopharynx through auditory tube and behind with the mastoid antrum. The tympanic membrane: Is a thin circular fibrous membrane. The tympanic membrane is extremely sensitive to pain and is innervated by auriculotemporal and vagus.

Auditory Ossicles They are the malleus, incus, and stapes The malleus is attached to tympanic membrane The stapes is attached to oval window. Muscles of the Ossicles: These are tensor tympani supplied by mandibular nerve and stapedius muscle supplied by facial nerve.

Auditory Tube: It connects tympanic cavity to th nasal pharynx. Its posterior third is bony, and its anterior two thirds is cartilaginous. It serves to equalize air pressures in tympanic cavity and nasal pharynx.

Mastoid Air Cells: They are a series of communicating cavities within the process that are continuous above with mastoid antrum and middle ear . They are lined with mucous membrane.

Vestibulocochlear Nerve (Cranial Nerve VIII) This nerve consists of two distinct parts: Vestibular nerve. Cochlear nerve. They are concerned with transmission of afferent information from internal ear to central nervous system.

Vestibular Nerve: It conducts nerve impulses from : Utricle and saccule. Semicircular canals. Its fibers are central processes of nerve cells located in vestibular ganglion (in internal acoustic meatus). They enter vestibular nuclear complex by passing between lower border of pons and upper part of medulla oblongata . Fibers then divide into short ascending and long descending fibers A small number of fibers bypass the vestibular nuclei & enter cerebellum through the inferior cerebellar peduncle.

The Vestibular Nuclear Complex: This complex consists of a group of nuclei situated beneath the floor of the fourth ventricle. Four nuclei may be recognized: (1) Lateral vestibular nucleus. (2) Superior vestibular nucleus. (3) Medial vestibular nucleus (4) Inferior vestibular nucleus.

Vestibular nuclei receive afferent fibers from : Utricle ,saccule and semicircular canals through vestibular nerve Cerebellum through inferior cerebellar peduncle. Efferent fibers from the nuclei pass to: Efferent fibers also descend uncrossed to spinal cord from lateral vestibular nucleus and form vestibulospinal tract. Nuclei of oculomotor, trochlear, and abducent nerves through medial longitudinal fasciculus.These connections enable maintainance visual fixation on an object. In addition, information received from the internal ear can assist in maintaining balance by influencing muscle tone of the limbs and trunk.

Ascending fibers also pass upward from vestibular nuclei to cerebral cortex, to vestibular area in postcentral gyrus just above lateral fissure. These fibers are thought to relay in ventral posterior nuclei of thalamus. The cerebral cortex probably serves to orient the individual consciously in space.

Cochlear Nerve: The cochlear nerve conducts nerve impulses concerned with sound from hair cells in organ of Corti in cochlea. Fibers of cochlear nerve are central processes of nerve cells located in spiral ganglion of cochlea. They enter anterior surface of brainstem at lower border of pons. In pons nerve fibers divide: One branch entering posterior cochlear nucleus. Other branch entering anterior cochlear nucleus.

Cochlear Nuclei: The anterior and posterior cochlear nuclei are situated on the surface of inferior cerebellar peduncle. They receive afferent fibers from cochlea through the cochlear nerve. Cochlear nuclei send axons (2nd order neuron fibers) that relay in the same or opposite side of: Trapezoid body (posterior nucleus). Olivary nucleus (superior olivary nucleus) .

Axons pass through posterior part of pons and midbrain form lateral lemniscus which consists of third-order neurons from both sides. As fibers of lateral lemniscus ascend, some of them relay in nucleus of lateral lemniscus. In the midbrain fibers of lateral lemniscus terminate in: Inferior colliculus. Medial geniculate body and pass to auditory cortex of cerebral hemisphere through acoustic radiation of internal capsule.

Course of Vestibulocochlear Nerve: The vestibular and cochlear parts of the nerve leave anterior surface of the brain between the lower border of pons and medulla oblongata. They run laterally in posterior cranial fossa and enter internal acoustic meatus with facial nerve. The fibers are then distributed to different parts of the internal ear .