Molecular Biology of Hearing and Balance
The senses of hearing, balance, and touch all rely on mechanorecep­tors, as do the proprioceptive sensations that indicate an organism how it is situated within the environment. Mechanoreception is probably one of the most ancient of the senses with mechanosensitivity existing in every type of organism in all three domains for example, Bacteria, Archaea, and Eukarya. Hair cells are the sensory cells of the internal ear, essential for the senses of sound and balance. The hair cell's transduc­tion apparatus, the molecular machinery that converts forces and displacements into electrical responses, can respond to mechanical stimuli of less than 1 nm in amplitude and tens of kilohertz in frequency. Indeed, our hearing is ultimately lim­ited by Brownian motion of water molecules impinging on the transduction apparatus. In multicellular organisms, mechanoreceptors are either neurons or neuroepithelial cells with synapses that allows the cells to stimulate other neurons. In vertebrates, hair cells are used in the inner ear's balance and hearing organs; in amphibians and fish, hair cells also are the mechanoreceptors of the lateral line that detects the nearby fluid movement.