Several distinct mechanisms have evolved in the auditory periphery to extract frequency information from a sound. In the mammalian cochlea, a travelling wave on the basilar membrane enhanced by a physiologically vulnerable neuromechanical interaction performs the primary frequency separation. In lizards, tuning is likely to depend on structures in the papilla other than the basilar membrane, and tuning in the auditory nerve is correlated with the length of the stereocilia. In turtles and possibly some bird species, an electrical resonance in the receptor cells is responsible for frequency selectivity. In addition to those organs obviously specialized to detect acoustic stimuli, afferents of the vestibular system can exhibit tuning to low-frequency airborne sounds, despite the absence of mechanical frequency separation by accessory structures. I report here that in the frog saccule, a vestibular organ apparently constructed for the detection of vibratory accelerations, frequency tuning may arise from an electrical resonance intrinsic to the hair cells. The mechanism is similar to that found in turtle and ensures that a stimulus with frequency corresponding to the membrane resonant frequency will produce the largest signal in the cell. This type of tuning may thus be quite widespread. Oscillatory mechanisms have been reported in sensory cells of other modalities in several lower vertebrates, and may even contribute to their sensitivity, although such mechanisms do imply that the signal-to-noise ratio is degraded near threshold.

Action Potentials↗

PubMedDate not supplied

Ionic basis of membrane potential in outer hair cells of guinea pig cochlea.

Mammalian hearing involves features not found in other species, for example, the separation of sound frequencies depends on an active control of the cochlear mechanics. The force-generating component in the cochlea is likely to be the outer hair cell (OHC), one of the two types of sensory cell through which current is gated by mechano-electrical transducer channels sited on the apical surface. Outer hair cells isolated in vitro have been shown to be motile and capable of generating forces at acoustic frequencies. The OHC membrane is not, however, electrically tuned, as found in lower vertebrates. Here we describe how the OHC resting potential is determined by a Ca2+-activated K+ conductance at the base of the cell. Two channel types with unitary sizes of 240 and 45 pS underlie this Ca2+-activated K+ conductance and we suggest that their activity is determined by a Ca2+ influx through the apical transducer channel, as demonstrated in other hair cells. This coupled system simultaneously explains the large OHC resting potentials observed in vivo and indicates how the current gated by the transducer may be maximized to generate the forces required in cochlear micromechanics.

Animals↗