151. Mechanism of Hearing – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Hearing begins when sound vibrations are converted into electrical signals by cochlear hair cells. Sound waves are mechanical vibrations consisting of alternating compressions and rarefactions and require a medium for transmission. The normal human hearing range is 20–20,000 hertz.
• Pitch is the subjective perception of sound frequency. High-frequency sounds produce high pitch, while low-frequency sounds produce low pitch. Loudness mainly depends on sound-wave amplitude and is expressed in decibels. A 10-decibel increase represents a tenfold increase in sound intensity.
• The normal hearing threshold is approximately 0 decibels at 1,000 hertz. About 20 decibels corresponds to whispering, 60 decibels to normal conversation, and 140 decibels may cause pain and auditory damage.
• Masking occurs when background noise raises the hearing threshold and makes another sound difficult to detect. Low-frequency sounds mask higher-frequency sounds more effectively. White noise is commonly used as a masking stimulus in audiological testing.
• In normal hearing, sound travels through the external auditory canal and vibrates the tympanic membrane. The vibrations pass through the malleus → incus → stapes to the oval window. Ossicular conduction is the principal route for normal hearing. Bone conduction bypasses the external and middle ear and is useful clinically for distinguishing conductive from sensorineural hearing loss.
• Movement of the stapes at the oval window produces pressure waves in the perilymph of the scala vestibuli. These waves move the basilar membrane and are relieved through the round window. Movement of the basilar membrane relative to the tectorial membrane bends hair-cell stereocilia and initiates auditory transduction.
• At rest, the hair cell has a membrane potential of about −60 millivolts, while the endocochlear potential is about +80 millivolts. Because endolymph is rich in potassium, there is a strong electrical gradient favouring potassium entry into the hair cell.
• When stereocilia bend toward the tallest stereocilia, mechanically gated channels open. Potassium enters → hair cell depolarizes → voltage-gated calcium channels open → glutamate is released → cochlear nerve fibres generate action potentials. Bending in the opposite direction closes the channels, producing hyperpolarization and reduced neurotransmitter release.
• The basilar membrane is the major pitch discriminator. It is narrow and stiff at the base and wider and more flexible toward the apex. High-frequency sounds produce maximum displacement near the base, whereas low-frequency sounds produce maximum displacement near the apex. This is called place coding or the place principle.
• The traveling wave moves from the base toward the apex, reaching maximum amplitude at a location determined by sound frequency. The frequency arrangement is preserved throughout the auditory pathway and is called tonotopic organization. Louder sounds produce greater basilar membrane displacement and greater hair-cell activation.
• A shearing force develops because the basilar and tectorial membranes move differently. This bends stereocilia and produces receptor potentials. Outer hair-cell stereocilia are embedded in the tectorial membrane, whereas inner hair-cell stereocilia are deflected mainly by endolymph movement.
• Cochlear electrical responses include the cochlear microphonic, summation potential and auditory nerve action potential. Cochlear microphonic mainly reflects the combined receptor potentials of hair cells and follows the frequency of the sound stimulus. Electrocochleography records these responses and helps assess cochlear and auditory nerve function.
• Sound localization depends on comparison of information from both ears. For low-frequency sounds, interaural differences in arrival time are most important; for high-frequency sounds, differences in sound intensity are more important. The superior olivary complex performs the initial bilateral comparison, while the pinna also contributes to vertical and front–back localization.