The Organ of Hearing and Equilibrium

Abstract

The human ear is an evolutionary derivative of the lateral line canals of early aquatic vertebrates such as fishes. Both the organs of hearing and equilibrium are based on an “internalized” system of fluid-containing membrane-bound spaces embedded in the petrous part of the temporal bone. Movements of fluid within these ducts owing either to oscillations of atmospheric air (hearing) or to postural changes (balance, equilibrium) elicit specific sensations through the action of highly specialized receptors termed hair cells. The anatomical structures representing both sensory modalities develop from a common ectodermal primordium, the otic placode, surrounded by mesenchyme of the otic capsule. The ectodermal anlage gives rise to a vesicle (otocyst), which later is subdivided into an upper portion (labyrinth, organ of balance) and a lower portion (cochlea, organ of hearing), forming parts of the inner ear. Whereas the former is fully operational alone, the latter requires additional systems for transduction of mechanical energy (sound waves) into bioelectric signals. These systems are situated in the anatomical units called external and middle ear.