This paper reviews the known physical origins of hearing and equilibrium in vertebrates, focusing on the results of studies in the 1970s and 80s particularly on the role of hair bundles in converting sound into electrical potential in the nervous system. The contemporary understanding of structural details of the ear are summarized, including the structure of hair cells and mechanoreceptive hair bundles, transduction channels, adaptation to a range of frequencies, and the possibilities for direct mechanoelectrical transduction, driven directly by hair motion without secondary messengers.
Particular attention is paid to mechanisms for transduction and frequency tuning, areas of active research and study at the time. Both positive and negative discoveries are covered, noting areas where further research is needed. Some new micrographs and figures from the author's work are included to tie the review together. Over 100 related papers are cited and synthesized into the review, most by other authors.