Hair-Bundle Mechanics

Vestibular and non-mammalian auditory hair bundles (left) differ in shape from mammalian cochlear hair bundles (right). (Right) In the mammalian cochlea, the structure of inner-hair-cell bundles (top) differs from outer-hair-cell bundles (bottom).

• The hair bundle is the organelle that converts mechanical input into electrical output for transmission to the brain in the auditory, vestibular, and lateral line systems of all vertebrates ^{1, 2}. We study how a bundle functions as a detector of different types of signals in divergent sensory organs.
• In response to the quietest sounds we can hear, the hair bundle moves by less than one-billionth of a meter. Hair bundles of many auditory and vestibular organs oscillate spontaneously in the absence of sensory input owing to an active mechanical process. Mathematical modeling predicts that this process can, in some situations, amplify an auditory bundle’s response to periodic stimulation arising from sound and can, under different conditions, allow a vestibular bundle to detect step stimulation owing to head movement³. The mechanosensory function of a hair bundle is controlled by its mechanical environment and can be manipulated in experiments to adjust the bundle’s response to stimulation^4,5. Modeling suggests how vestibular systems changed function through evolution to become auditory organs (see figure below). We further propose that despite environmental and developmental variation, a hair-bundle’s function can be ensured by a homeostatic mechanism within sensory organs⁴. We are presently studying the influence of intrinsic noise on the signal-detection properties of active systems ^{7, 8}.
• A hair bundle is comprised of a set of stereocilia, hair-like projections extending from the surface of the sensory hair cell. Stereocilia are attached to each other by links that allow the bundle to move as a unit when stimulated. One crucial type of link stems from the interaction between the proteins and lipids coating each stereocilium⁹. Hair bundles extracted from the auditory systems of mammals do not move cohesively, however, and differ in shape from those in other vertebrate organs (see figure above). To understand how a hair-bundle’s shape contributes to its function, we determined the morphology of a living auditory hair bundle from a mammal for the first time¹⁰. How a hair bundle's cohesiveness and shape contribute to its function are topics of current investigation.

The mechanical load on a hair bundle controls its function.