20.11: Auditory Pathway
Auditory pathways constitute the complex neural circuits responsible for transmitting and interpreting auditory information from the peripheral auditory system to the brain. Sound waves are initially captured by the outer ear, funneled through the ear canal, and reach the tympanic membrane (eardrum). These vibrations are transmitted via the middle ear's ossicles to the inner ear's cochlea.
When viewed cross-sectionally, the cochlea reveals the scala vestibuli and scala tympani flanking the cochlear duct. Numerous Corti organs reside within the cochlear duct, which converts the scala's wave motion into neural impulses. Positioned atop the basilar membrane - which resides between the Corti organs and the scala tympani within the cochlear duct - these organs respond to fluid waves traveling through the scala vestibuli and scala tympani. Locations on the basilar membrane react selectively to wave frequencies; areas proximal to the cochlea base respond to higher frequencies, and areas closer to the cochlea tip react to lower frequencies.
Interspersed within the Corti organs are hair cells, christened for the stereocilia (resembling hair) that project from their apical surfaces. These stereocilia, organized in a gradient from tallest to shortest, are interconnected by protein fibers within each array. These protein tethers facilitate the collective bending of these arrays in response to basilar membrane movement. Extending towards the tectorial membrane - which is affixed medially to the Corti organ - these stereocilia undergo lateral movement as pressure waves from the scala stimulate the basilar membrane. The bending of stereocilia either towards or away from the tallest in the array causes a shift in protein tether tension, opening ion channels within the hair cell membrane if bent towards the tallest and closing them if bent towards the shortest. In the absence of sound, standing stereocilia exert a small degree of tension on the tethers, resulting in a slight depolarization of the hair cell membrane.
The hair cells convert mechanical vibrations into electrical signals, activating the auditory nerve fibers. These signals travel through the auditory nerve to the brainstem, specifically the cochlear nuclei, and ascend through multiple relays, including the superior olivary complex and the inferior colliculus.
The auditory signals continue their journey to the thalamus and ultimately arrive at the auditory cortex in the brain's temporal lobe. This region processes the information, distinguishing various sound attributes such as pitch, intensity, and localization, enabling the perception and interpretation of auditory stimuli.
