Anatomy and Physiology of the Auditory System
Outer and middle ear
The pinna and ear canal funnel sound; the canal's resonance provides ~10–15 dB gain near 2–4 kHz. The tympanic membrane transfers energy to the ossicular chain (malleus, incus, stapes). The middle ear matches impedance between air and cochlear fluid via the area ratio of the tympanic membrane to the stapes footplate and the lever action of the ossicles, yielding roughly 25–30 dB of gain. The Eustachian tube equalizes pressure.
Inner ear
The cochlea is a fluid-filled, tonotopically organized spiral: high frequencies near the base, low frequencies near the apex. The organ of Corti sits on the basilar membrane and contains inner hair cells (mostly sensory, ~95% of afferent fibers) and outer hair cells (motile, amplify and sharpen tuning via the cochlear amplifier). The stria vascularis produces endolymph and the endocochlear potential.
Central pathways
Spiral ganglion → cochlear nucleus → superior olivary complex (first binaural convergence) → lateral lemniscus → inferior colliculus → medial geniculate body → auditory cortex. Binaural processing begins at the superior olive, enabling localization via interaural time and level differences.
Traps
Questions swap inner vs outer hair cell roles, or place the first binaural synapse at the cochlear nucleus. Remember the traveling wave peaks at the base for high frequencies. Efferent (olivocochlear) fibers modulate outer hair cells.
Rule of thumb
Base = high frequency, apex = low frequency; inner hair cells = sensing, outer hair cells = amplifying.