Auditory Pathway

Sound waves enter the external acoustic meatus and vibrate the tympanic membrane. The auditory ossicles amplify these vibrations and transmit them to the cochlea, creating pressure waves in the cochlear fluid. These waves travel through the cochlea, vibrate the basilar membrane, and stimulate hair cells in the spiral organ. The resulting neural signals travel via the cochlear nerve to the cochlear nuclei in the medulla oblongata, then ascend through the brainstem to reach the auditory cortex in the temporal lobe.

Stereocilia are hair-like projections on hair cells organized in a gradient from tallest to shortest and connected by protein fibers called tip links. When basilar membrane vibrations bend stereocilia toward the tallest in the array, tension on tip links opens mechanically-gated ion channels, depolarizing the hair cell membrane. This depolarization increases neurotransmitter release, generating action potentials in the cochlear nerve that transmit auditory information to the brain.

Different locations on the basilar membrane respond selectively to specific frequencies. Areas proximal to the cochlea base respond to higher frequencies, while areas closer to the cochlea tip react to lower frequencies. This frequency-based organization allows the auditory system to distinguish pitch and process complex sound information as signals travel from the cochlea through multiple brainstem relays to the auditory cortex.

Depolarization of hair cells increases the release of neurotransmitters at the synapse with cochlear nerve fibers. This generates significantly more action potentials in the cochlear nerve compared to the resting state. These enhanced neural signals carry auditory information through the brainstem, including the superior olivary complex and inferior colliculus, ultimately reaching the auditory cortex for conscious sound perception.

The auditory cortex, located in the temporal lobe of the brain, is the final destination of the auditory pathway and processes auditory information. This region distinguishes various sound attributes including pitch, intensity, and localization, enabling perception and interpretation of auditory stimuli. Signals reach the auditory cortex after ascending through multiple brainstem relays and the thalamus.

The spiral organ, also called the Corti organ, resides within the cochlear duct and contains the hair cells responsible for hearing. The basilar membrane, which lies between the Corti organ and the scala tympani, vibrates in response to pressure waves traveling through the cochlea. These vibrations stimulate the hair cells' stereocilia, converting mechanical motion into electrical signals transmitted via the cochlear nerve.

The tectorial membrane is affixed medially to the Corti organ and extends over the stereocilia of hair cells. As pressure waves from the cochlear fluid stimulate the basilar membrane, the stereocilia undergo lateral movement and bend against the tectorial membrane. This bending either opens or closes ion channels depending on direction, allowing the hair cells to convert mechanical vibrations into neural signals.