Chapter 11: Hearing & Equilibrium

Hearing & Equilibrium delineates the anatomical functional divisions of the ear, starting with the external ear's role in sound capture and the middle ear's impedance-matching function, where the tympanic membrane and auditory ossicles (malleus, incus, and stapes) transmit vibrations to the oval window. The text details the fluid dynamics of the inner ear, specifically how sound waves travel through the perilymph of the cochlea to displace the basilar membrane and stimulate the organ of Corti. A significant portion of the chapter focuses on the mechanotransduction process in hair cells, explaining how the shearing motion of stereocilia against the tectorial membrane stretches tip links to open cation channels, allowing potassium ions to enter from the endolymph and generate receptor potentials. The discussion includes the physical properties of sound, such as frequency (pitch) and amplitude (loudness/decibels), and traces the central auditory pathways from the cochlear nerve to the dorsal and ventral cochlear nuclei, inferior colliculus, medial geniculate body, and finally the auditory cortex. Clinical correlations are highlighted through the distinction between conductive and sensorineural hearing loss, diagnostic assessments like the Rinne and Weber tuning fork tests, and the pathophysiology of tinnitus and presbycusis. Shifting to the vestibular system, the summary explains how the semicircular canals utilize the crista ampullaris and cupula to detect rotational acceleration, while the otolith organs (utricle and saccule) employ the macula and otoconia to sense linear acceleration and gravitational changes. The integration of these sensory inputs for spatial orientation is discussed alongside the vestibulo-ocular reflex (VOR) and vestibular pathologies, including nystagmus, vertigo, benign paroxysmal positional vertigo (BPPV), and Ménière disease.