Hair bundle profiles along the chick basilar papilla

Effectiveness of hair bundle motility as the cochlear amplifier

The effectiveness of hair bundle motility in mammalian and avian ears is studied by examining energy balance for a small sinusoidal displacement of the hair bundle. The condition that the energy generated by a hair bundle must be greater than energy loss due to the shear in the subtectorial gap per hair bundle leads to a limiting frequency that can be supported by hair-bundle motility. Limiting frequencies are obtained for two motile mechanisms for fast adaptation, the channel re-closure model and a model that assumes that fast adaptation is an interplay between gating of the channel and the myosin motor. The limiting frequency obtained for each of these models is an increasing function of a factor that is determined by the morphology of hair bundles and the cochlea. Primarily due to the higher density of hair cells in the avian inner ear, this factor is approximately 10-fold greater for the avian ear than the mammalian ear, which has much higher auditory frequency limit. This result is consistent with a much greater significance of hair bundle motility in the avian ear than that in the mammalian ear.

Closure of supporting cell scar formations requires dynamic actin mechanisms

In many vertebrate inner ear sensory epithelia, dying sensory hair cells are extruded, and the apices of surrounding supporting cells converge to re-seal the epithelial barrier between the electrochemically-distinct endolymph and perilymph. These cellular mechanisms remain poorly understood. Dynamic microtubular mechanisms have been proposed for hair cell extrusion; while contractile actomyosin-based mechanisms are required for cellular extrusion and closure in epithelial monolayers. The hypothesis that cytoskeletal mechanisms are required for hair cell extrusion and supporting cell scar formation was tested using bullfrog saccules incubated with gentamicin (6h), and allowed to recover (18h). Explants were then fixed, labeled for actin and cytokeratins, and viewed with confocal microscopy. To block dynamic cytoskeletal processes, disruption agents for microtubules (colchicine, paclitaxel) myosin (Y-27632, ML-9) or actin (cytochalasin D, latrunculin A) were added during treatment and recovery. Microtubule disruption agents had no effect on hair cell extrusion or supporting cell scar formation. Myosin disruption agents appeared to slow down scar formation but not hair cell extrusion. Actin disruption agents blocked scar formation, and largely prevented hair cell extrusion. These data suggest that actin-based cytoskeletal processes are required for hair cell extrusion and supporting cell scar formation in bullfrog saccules.

Spatial tuning curves along the chick basilar papilla in normal and sound-exposed ears

Intense sound exposure destroys chick short hair cells and damages the tectorial membrane. Within a few days postexposure, signs of repair appear resulting in nearly complete structural recovery of the inner ear. Tectorial membrane repair, however, is incomplete, leaving a permanent defect on the sensory surface. The consequences of this defect on cochlear function, and particularly frequency analysis, are unclear. The present study organizes the sound-induced discharge activity of cochlear nerve units to describe the distribution of neural activity along the tonotopic axis of the basilar papilla. The distribution of this activity is compared in 12-day postexposed and age-matched control groups. Spontaneous activity, tuning curves, and rate-intensity functions were measured in each unit. Discharge activity at 60 frequency and intensity combinations was identified in the tuning curves of hundreds of units. Activity at each of these criterion frequency/intensity combinations was plotted against the unit's characteristic frequency to construct spatial tuning curves (STCs). The STCs depict tone-driven cochlear nerve activity along the length of the papilla. Tuning sharpness, low- and high- frequency slopes, and the maximum response were quantified for each STC. The sharpness of tuning increased with increasing criterion frequency. However, within a frequency, increasing sound intensity yielded more broadly tuned STCs. Also, the high-frequency slope was consistently steeper than the low-frequency slope. The STCs of exposed ears exhibited slightly less frequency selectivity than control ears across all frequencies and larger maximum responses for STCs with criterion frequencies spanning the tectorial membrane defect. When rate-intensity types were segregated, differences were observed in the STCs between saturating and sloping-up units. We propose that STC shape may be determined by global mechanical events, as well as localized tuning and nonlinear processes associated with individual hair cells. The results indicated that 12 days after intense sound exposure, global and local contributions to spatially distributed neural activity are restored.

Coding of sound intensity in the chick cochlear nerve

Tuning curves, spontaneous activity, and rate-intensity (RI) functions were obtained from units in the chick cochlear nerve. The characteristic frequency (CF) was determined from each tuning curve. The shape of each RI function was subjectively evaluated and assigned to one of four RI types. The breakpoint, discharge rate at the highest SPLs, and slopes of the primary and secondary segments were quantified for each function. The CF and RI type were then related to these variables. A new RI function was observed in which the discharge activity in the secondary segment diminished as stimulus level increased above the breakpoint. This function was called a "sloping-down" type. In 959 units, saturating, sloping-up, sloping-down, and straight RI types were identified in 39.2, 35.5, 12.6, and 12.7% of the sample, respectively. The slope of the primary segment was nearly the same in each of the four types and averaged 5.48 S. s(-1). dB(-1) across all units. The slopes of the secondary segments formed four groupings when segregated by RI type based on the subjective assignments and averaged 0.03, 1.22, -0.90, and 3.95 S. s(-1). dB(-1) in the saturating, sloping-up, sloping-down, and straight types, respectively. The data describing the secondary segments of all units were fit with a multi-compartment polynomial and showed a continuous distribution that segregated, with some overlap, into the different RI categories. The proportion of RI types, as well as the secondary and primary slopes were approximately constant across CFs. In addition, it would appear that the other parameters that define the four types were, for the most part, homogeneously distributed across the frequency axis of the chick inner ear. Finally, a comparison of RI functions having a common CF suggested that the compressive nonlinearity that determines RI type may be a phenomenon localized to individual hair cells in the bird ear.