Anatomy and Physiology of the Peripheral and Central Auditory System

The auditory system is responsible for the hearing sense and it consists of the peripheral auditory system (outer, middle and inner ear) and of the central auditory system (vestibular and cochlear nuclei, auditory and vestibular pathways and vestibular and auditory cortices). The outer ear comprises the auricle and the auditory canal and its function is to guide air pressure waves to the middle ear. The middle ear consists of the tympanic membrane, connected to the inner ear by three ossicles (malleus, incus and stapes), which vibrations allow the transmission of originally airborne sound waves to the perilymph of the inner ear. The middle ear provides a pressure gain as well as enhanced quality of the sound waves transmitted to the inner ear and protects it from high pressure levels produced by loud sounds. The stapes footplate of the middle ear connects to the oval window of the cochlea, an inner ear spiral-shaped bony canal. The stapes of the middle ear connects to the oval window in the cochlea. The vibrations of a flexible membrane (basilar membrane) on which sensory cells (hair cells) reside are responsible for the transduction of the sound waves into electrical impulses. The vestibulocochlear nerve (CN VIII) transmits both hearing and balance information from the inner ear to the brain. The vestibular (balance) and cochlear (hearing) components of the vestibulocochlear nerve target different nuclei. The vestibular component reaches the vestibular nuclei in the pons and medulla oblongata. The cochlear component instead reaches the ventral and dorsal cochlear nuclei, located laterally at the junction between the pons and medulla, in close proximity to the inferior cerebellar peduncle. CN VIII emerges from the brainstem at the cerebellopontine angle and exits the posterior cranial fossa of the neurocranium through the internal acoustic meatus of the temporal bone. Here the vestibulocochlear nerve splits, thus forming the vestibular nerve and the cochlear nerve. The vestibular nerve innervates the vestibular system of the inner ear, which is responsible for detecting balance. The cochlear nerve travels to the cochlea, forming the spiral ganglia of Corti, involved in the sense of hearing. The hearing pathway originates in the cochlear nuclei which receive first-order auditory input from the organ of Corti in the cochlea. The second neuron of this pathway is located in the superior olivary nuclei of the pons where the majority of the auditory fibers synapse, crossing the midline. The fibers ascend, forming the lateral lemnisc and proceed towards the inferior colliculus in the mesencephalus. The last relay, prior to the primary auditory cortex, occurs in the medial geniculate body of the thalamus. A tonotopic organization is evident throughout the hearing pathway, from cochlea to auditory cortices. In the balance pathway, neurons synapsing on the hair cells of maculae and cristae ampullares of the semicircular canals converge in the vestibular ganglion. The sensory fibers originating from here join the sensory fibers from the cochlear ganglion to form the vestibulocochlear nerve and terminate in the vestibular nuclei of the pons and medulla. The axons originated in these nuclei reach different areas of Central Nervous System (CNS), such as the spinal cord, the cerebral cortex, the cerebellum and the nuclei controlling extrinsic eyes muscles. The vestibular nuclei also receive input from proprioceptive neurons, as well as the visual system.