19.5: Hearing
When we hear a sound, our nervous system is detecting sound waves—pressure waves of mechanical energy traveling through a medium. The frequency of the wave is perceived as pitch, while the amplitude is perceived as loudness.
Sound waves are collected by the external ear and amplified as they travel through the ear canal. When sounds reach the junction between the outer and middle ear, they vibrate the tympanic membrane—the eardrum. The resulting mechanical energy causes the attached ossicles—a set of small bones in the middle ear—to move.
The ossicles vibrate the oval window, the outermost part of the inner ear. In the labyrinth of the inner ear, the sound wave energy is transferred to the cochlea—a coiled structure in the inner ear—causing the fluid within it to move. The cochlea contains receptors that transduce mechanical sound waves into electrical signals that can be interpreted by the brain. Sounds within the hearing range vibrate the basilar membrane in the cochlea and are detected by hair cells on the organ of Corti, the site of transduction.
Along the primary auditory pathway, the signals are sent through the auditory nerve to the cochlear nuclei in the brainstem. From here, they travel to the inferior colliculus of the midbrain and up to the thalamus, and then to the primary auditory cortex. Along this pathway, information about the sound is maintained such that once the signal reaches the primary auditory cortex, basic characteristics (like pitch) can be identified and perceived. From the primary auditory cortex, sound information is sent to nearby areas of the cerebral cortex for higher-level processing—such as Wernicke’s area, which is critical for understanding speech.
