modiolus through the small openings in the bony spiral lam-the spiral ganglion cells form the cochlear nerve, which enina and enter the spiral organ. About 90% of dendrites orig-ters the bony cochlea through the modiolus to appear in the inating from the spiral ganglion cells synapse with the inner internal acoustic meatus (see Fig. 25.23). From the internal hair cells; the remaining 10% of dendrites synapse with acoustic meatus, the cochlear nerve enters the brain stem and outer hair cells of the spiral ganglion of Corti. The axons of terminates in the cochlear nuclei of the medulla. Nerve fibers superior part of ventral vestibular nerve cochlear nuclei inferior part of vestibular nerve
FIGURE 25.23 • Diagram illustrating the innervation of the sensory regions of the membranous labyrinth. Note the two parts of the vestibulocochlear nerve. The cochlear nerve carries the hearing impulses from the cochlear duct; the vestibular nerve carries balance information from the semicircular canals. The cell bodies of these sensory fibers are located in the spiral ganglion (for hearing) and vestibular ganglion (for equilibrium). (Modified from Hawke M, Keene M, Alberti PW. Clinical Otoscopy: A Text and Colour Atlas. Edinburgh: Churchill Livingstone, 1984.) from these nuclei pass to the geniculate nucleus of the thalamus and then to the auditory cortex of the temporal lobe.
It is interesting that the organ of Corti also receives small amounts of efferent fibers conveying impulses from the brain pass parallel to the afferent nerve fibers of the vestibulocochlear nerve (olivocochlear tract, cochlear efferents of Rasmussen). Efferent nerve fibers from the brain stem pass through the vestibular nerve. They synapse either on afferent endings of the inner hair cell or on the basal aspect of an outer hair cell. Efferent fibers are thought to affect control of auditory and vestibular input to the central nervous system, presumably by enhancing some afferent signals while suppressing other signals. Damage to the organ of Corti, cochlear nerve, nerve pathways, or auditory cortex is responsible for permanent sensorineural hearing loss.
Blood Vessels of the Membranous Labyrinth
Arterial blood is supplied to the membranous labyrinth by the labyrinthine artery; venous blood drainage is to the venous dural sinuses.
The blood supply to the external ear, middle ear, and bony labyrinth of the internal ear is derived from vessels associated with the external carotid arteries. The arterial blood supply to tissues of the membranous labyrinth of the internal ear is derived intracranially from the labyrinthine artery, a common branch of the anterior inferior cerebellar or basilar artery (Fig. 25.24). The labyrinthine artery is a terminal artery: It has no anastomoses with other surrounding arteries. Branches of this artery exactly parallel the distribution of the superior and inferior parts of the vestibular nerve.
FIGURE 25.24 • Diagram of the arterial supply of the membranous labyrinth of the internal ear. The blood supply to the membranous labyrinth of the internal ear is derived from the labyrinthine artery, a branch of the anterior inferior cerebellar or basilar artery. (Modified from Schuknecht HF. Pathology of the Ear. Cambridge, MA: Harvard University Press, 1974.)