Pairwise coupling of hair cell transducer channels links auditory sensitivity and dynamic range.

Hair cells in the inner ear provide the basis for the exquisite hearing capabilities of mammals. These cells transduce sound-induced displacements of their mechanosensitive hair bundle into electrical currents within a fraction of a millisecond and with nanometer fidelity. Excitatory displacements of the hair cell's bundle tense tip links that open transducer channels. These channels are located either at one or at both ends of the links, where the latter possibility was thought to compromise sensitivity via negative cooperativity, and discarded for quantitatively describing the transduction process. Here, we show instead that this series mode of activation accurately explains measured transduction in hair cells. It enhances both sensitivity and dynamic range of hair cell transduction, by one channel that is extremely sensitive at small displacements while the other responds best to larger stimuli. Our results provide a new framework for exploring the dynamics of hair cell activation.

Susceptibility of aging human bone to mixed-mode fracture increases bone fragility.

Since bone-mass-based measurements have demonstrated limited success in predicting age-related increase in fracture incidence, recent emphasis has been placed on quantification of bone quality. Specifically, material parameters such as strength, stiffness, and toughness have been quantified to characterize bone quality through mechanical testing. This study is the first one to report multiaxial failure characteristics of bone (quality) by conducting fatigue tests on human cortical bone specimens under physiologically relevant loading involving simultaneous application of axial and torsional loading to produce previously reported in vivo shear/normal stress ratios. Our results show that, compared to uniaxial tests, multiaxial fatigue tests show up to a 20-fold reduction in the fatigue life of human cortical bone. More significantly, the susceptibility of mixed-mode failure increases bone fragility in aging human bone. Furthermore, since the magnitude of mixed-mode loading varies with physiological activities, results of this study also suggest that a reduction in the activities involving significant mixed-mode loading may lower the overall fracture incidence among older individuals.

Damage mechanisms and failure modes of cortical bone under components of physiological loading.

Fatigue damage development in cortical bone was investigated in vitro under different mechanical components of physiological loading including tension, compression, and torsion. During each test, stress and strain data were collected continuously to monitor and statistically determine the occurrence of the primary, secondary, and tertiary stages associated with fatigue and/or creep failure of bone. The resultant microdamage and failure modes were identified by histological and fractographic analysis, respectively. The tensile group demonstrated Mode I cracking and the three classic stages of fatigue and creep suggesting a low crack initiation threshold, steady crack propagation and final failure by coalescence of microcracks. In contrast, the compressive group displayed Mode II cracking and a two-stage fatigue behavior with limited creep suggesting a high crack initiation threshold followed by a sudden fracture. The torsion group also displayed a two-stage fatigue profile but demonstrated extensive damage from mixed mode (Modes II and III) microcracking and predominant time-dependent damage. Thus, fatigue behavior of bone was found to be uniquely related to the individual mechanical components of physiological loading and the latter determined the specific damage mechanisms associated with fatigue fracture.

Influence of phase angle between axial and torsional loadings on fatigue fractures of bone.

Fatigue fractures of cortical bone involve combined axial-torsional loading yet it is unknown how the relationship between axial and torsional loadings affects the fatigue behavior of bone. In this study the effect of superimposing in-phase and out-of-phase torsional on axial loading on the fatigue behavior of bone was investigated by conducting in vitro tests involving 0 degrees and 90 degrees phase shift between cyclic torsional and axial loadings. Results obtained indicate that fatigue life, patterns of moduli loss, microcracking and modes of fractures are dependent on the phase angle between axial and torsional loadings. Specimens subjected to in-phase torsional on axial loading demonstrated greater mixed mode interaction, underwent proportionate stiffness losses in tension, compression, and torsion, and consequently had a shorter fatigue life. In contrast, specimens subjected to out-of-phase loading regime displayed a smaller contribution of mixed mode failure, underwent a disproportionately large stiffness loss in torsion, and had a longer fatigue life. Furthermore, increase in phase angle provided additional planes on which damage was diffused delaying the final failure. Change in phase angle, seen in vivo during a number of physiological activities including walking, running and sprinting, will therefore affect fatigue behavior and contribute to pathogenesis of fatigue fractures.