Cochlear outer hair cells (OHCs) are fast biological motors that serve to enhance the vibration of the organ of Corti and increase the sensitivity of the inner ear to sound. Exactly how OHCs produce useful mechanical power at auditory frequencies, given their intrinsic biophysical properties, has been a subject of considerable debate. To address this we formulated a mathematical model of the OHC based on first principles and analyzed the power conversion efficiency in the frequency domain. The model includes a mixture-composite constitutive model of the active lateral wall and spatially distributed electro-mechanical fields. The analysis predicts that: 1) the peak power efficiency is likely to be tuned to a specific frequency, dependent upon OHC length, and this tuning may contribute to the place principle and frequency selectivity in the cochlea; 2) the OHC power output can be detuned and attenuated by increasing the basal conductance of the cell, a parameter likely controlled by the brain via the efferent system; and 3) power output efficiency is limited by mechanical properties of the load, thus suggesting that impedance of the organ of Corti may be matched regionally to the OHC. The high power efficiency, tuning, and efferent control of outer hair cells are the direct result of biophysical properties of the cells, thus providing the physical basis for the remarkable sensitivity and selectivity of hearing.

Microvilli (stereocilia) projecting from the apex of hair cells in the inner ear are actively motile structures that feed energy into the vibration of the inner ear and enhance sensitivity to sound. The biophysical mechanism underlying the hair bundle motor is unknown. In this study, we examined a membrane flexoelectric origin for active movements in stereocilia and conclude that it is likely to be an important contributor to mechanical power output by hair bundles. We formulated a realistic biophysical model of stereocilia incorporating stereocilia dimensions, the known flexoelectric coefficient of lipid membranes, mechanical compliance, and fluid drag. Electrical power enters the stereocilia through displacement sensitive ion channels and, due to the small diameter of stereocilia, is converted to useful mechanical power output by flexoelectricity. This motor augments molecular motors associated with the mechanosensitive apparatus itself that have been described previously. The model reveals stereocilia to be highly efficient and fast flexoelectric motors that capture the energy in the extracellular electro-chemical potential of the inner ear to generate mechanical power output. The power analysis provides an explanation for the correlation between stereocilia height and the tonotopic organization of hearing organs. Further, results suggest that flexoelectricity may be essential to the exquisite sensitivity and frequency selectivity of non-mammalian hearing organs at high auditory frequencies, and may contribute to the “cochlear amplifier” in mammals.

The dynamic displacement of the semicircular canal cupula and modulation of afferent nerve discharge were measured simultaneously in response to physiological stimuli in vivo. The adaptation time constant(s) of normal cupulae in response to step stimuli averaged 36 s, corresponding to a mechanical lower corner frequency for sinusoidal stimuli of 0.0044 Hz. For stimuli equivalent to 40–200 deg/s of angular head velocity, the displacement gain of the central region of the cupula averaged 53 nm per deg/s. Afferents adapted more rapidly than the cupula, demonstrating the presence of a relaxation process that contributes significantly to the neural representation of angular head motions by the discharge patterns of canal afferent neurons. We also investigated changes in time constants of the cupula and afferents following detachment of the cupula at its apex—mechanical detachment that occurs in response to excessive transcupular endolymph pressure. Detached cupulae exhibited sharply reduced adaptation time constants (300 ms–3 s, n = 3) and can be explained by endolymph flowing rapidly over the apex of the cupula. Partially detached cupulae reattached and normal afferent discharge patterns were recovered 5–7 h following detachment. This regeneration process may have relevance to the recovery of semicircular canal function following head trauma.

Elastin, a principal structural component of native arteries, has distinct biological and mechanical advantages when used as a biomaterial; however, its low ultimate tensile strength has limited its use as an arterial conduit. We have developed a scaffold, consisting of a purified elastin tubular conduit strengthened with fibrin bonded layers of acellular small intestinal submucosa (aSIS) for potential use as a small diameter vascular graft. The addition of aSIS increased the ultimate tensile strength of the elastin conduits nine-fold. Burst pressures for the elastin composite vascular scaffold (1,396 +/- 309 mmHg) were significantly higher than pure elastin conduits (162 +/- 36 mmHg) and comparable to native saphenous veins. The average suture pullout strength of the elastin composite vascular scaffolds was 14.612 +/- 3.677 N, significantly higher than the pure elastin conduit (0.402 +/- 0.098 N), but comparable to native porcine carotid arteries (13.994 +/- 4.344 N). Cyclic circumferential strain testing indicated that the composite scaffolds were capable of withstanding physiological loading conditions for at least 83 h. Implantation of the elastin composites as carotid interposition grafts in swine demonstrated its superiority to clinically acceptable ePTFE with significantly longer average patency times of 5.23 h compared to 4.15 h. We have developed a biologically based elastin scaffold with suitable mechanical properties and low thrombogenicity for in vivo implantation, and with the potential for cellular repopulation and host integration reestablishing an appropriate elastic artery.

Abstract. The concentrations of free Na ϩ , K ϩ , Ca 2ϩ , and Cl -in endolymph and perilymph from the inner ear of the oyster toadfish, Opsanus tau, were measured in vivo using double-barreled ion-selective electrodes. Perilymph concentrations were similar to those measured in other species, while endolymph concentrations were similar to those measured previously in elasmobranch fish, though significantly different from concentrations reported in mammals. Perilymph concentrations (mean Ϯ std. dev.) were as follows: Na ϩ