Hair Cell Membrane Potential Research Articles

Measurement of low-frequency receptor potentials in inner hair cells: a theoretical analysis

The measurement of the low-frequency responses of inner hair cells is complicated by the fact that extracellular cochlear microphonic may be larger than the intracellular inner hair cell potential. A subtraction technique is proposed in which the extracellular and intracellular potentials are recorded sequentially and then subtracted to give an estimate of the true membrane potential. A theoretical analysis is presented which predicts that this calculated inner hair cell membrane potential is a more accurate measure of inner hair cell receptor current than the membrane potential measured with respect to a remote indifferent electrode.

Unusual discharge patterns of single fibers in the pigeon's auditory nerve.

Extracellular recording from single auditory nerve fibers in the pigeon, Columba livia, revealed some unusual discharge patterns of spontaneous and evoked activity. Time interval histograms (TIHs) of spontaneous activity showed a random interval distribution in 73% of the auditory fibers (Fig. 1a). The remaining 27% revealed periodicity in the TIHs (Fig. 1b-e), determined by the characteristic frequency (CF) of a given fiber. Normally, those fibers had a CF less than 2.2 kHz. In both cases spontaneous activity was irregular. The time pattern of quasiperiodic spontaneous firing in different auditory fibers is described by three main types of autocorrelation histograms (ACHs; decaying, nondecaying, and modulated), reflecting the spontaneous oscillations of the hair cell membrane potential (Fig. 1b-d). Single-tone suppression in auditory fibers with quasi-periodic spontaneous activity was found (Figs. 2, 10) and it could be observed if the eighth nerve was cut. There was no suppressive effect in fibres with random spontaneous firing. The frequency selectivity properties of auditory fibers were studied by means of an automatic method. Both 'simple' (Fig. 4) and 'complex' (Figs. 7, 8) response maps were found. Apart from the usual excitatory area, complex response maps were characterized by suppressive areas lying either above (Fig. 7), below (Fig. 8e), or on both sides of the CF (Fig. 8a-c). Generally, complex response maps were observed for fibers showing quasiperiodic spontaneous activity (Figs. 7, 8). Input-output functions at frequencies evoking single-tone suppression were nonmonotonic, while they were always monotonic at frequencies near the CF (Fig. 12). No difference in sharpness was observed between normal frequency threshold curves (FTCs) and excitatory areas of 'complex' response maps (Fig. 9). 'On-off' responses evoked by suppressive stimuli were found (Figs. 2, 3). They had a periodic pattern determined by the CF and did not depend on the stimulus frequency (Fig. 3). Low-CF fibers were observed which changed their time discharge structure to tone levels about 45 dB lower than their thresholds at the CF (Fig. 6). The observed features of the discharge patterns of the pigeon's auditory fibers reflect the distinctive nature of the fundamental mechanisms of auditory analysis in birds that are connected with electrical tuning of the hair cells and probably with the micromechanics of the bird's cochlea.

The mechanical properties of ciliary bundles of turtle cochlear hair cells.

The mechanical behaviour of the ciliary bundles of hair cells in the turtle cochlea was examined by deflecting them with flexible glass fibres of known compliance during simultaneous intracellular recording of the cell's membrane potential. Bundle motion was monitored through the attached fibre partially occluding a light beam incident on a photodiode array. The change in photocurrent was assumed to be proportional to bundle displacement. For deflexions of 1-100 nm towards the kinocilium, the stiffness of the ciliary bundles was estimated as about 6 X 10(-4) N/m, with the fibre attached to the top of the bundle. When the fibre was placed at different positions up the bundle, the stiffness decreased approximately as the inverse square of the distance from the ciliary base. This suggests that the bundles rotate about an axis close to the apical pole of the cell and have a rotational stiffness of about 2 X 10(-14) N. m/rad. Step displacements of the fixed end of the flexible fibre caused the hair cell's membrane potential to execute damped oscillations; the frequency of the oscillations in different cells ranged from 20 to 320 Hz. Displacements towards the kinocilium always produced membrane depolarization. The amplitude of the initial oscillation increased with displacements up to 100 nm and then saturated. For small displacements of a few nanometres, the hair cell's mechanoelectrical sensitivity was estimated as about 0.2 mV/nm. Force steps delivered by the flexible fibre caused the bundle position to undergo damped oscillations in synchrony with the receptor potential. The mechanical oscillations could be abolished with large depolarizing currents that attenuated the receptor potential. When placed against a bundle, a fibre's spontaneous motion increased and became quasi-sinusoidal with an amplitude several times that expected from the compliance of the system. It is suggested that the hair bundle drives the fibre. We conclude that turtle cochlear hair cells contain an active force generating mechanism.

Efferent modulation of hair cell tuning in the cochlea of the turtle.

Intracellular recordings were made from turtle cochlear hair cells in order to study the changes in their tuning properties resulting from electrical stimulation of the efferent axons. Efferent stimulation caused a reduction in the amplitude of the receptor potential at the hair cell's most sensitive or characteristic frequency, an increased amplitude at frequencies more than an octave below the characteristic frequency, and no change at very high frequencies. These differential effects resulted in a broadening of each cell's tuning curve, which, during maximal efferent stimulation degenerated from a sharply tuned resonance to a critically damped low-pass filter. Efferent alterations in tuning were also inferred from the oscillations in membrane potential produced by acoustic clicks or extrinsic currents. The quality factor (Q) of tuning, derived from the decay of the oscillations, was progressively reduced with synaptic hyperpolarizations up to about 5 mV in amplitude. A consequence of efferent action was that the wave forms of transient pressure changes were more faithfully encoded as changes in hair cell membrane potential. Hyperpolarization of a hair cell by steady current injection resulted in a lowering of its characteristic frequency and quality factor, and an increase in steady-state resistance. By comparison, for a given reduction in quality factor, efferent stimulation was associated with a smaller change in characteristic frequency. This difference is expected if the resonance is also damped by the shunting action of the synaptic conductance. Perfusion with perilymphs containing 0.5-15 mM of the potassium channel blocker, tetraethylammonium bromide (TEA) reduced the hair cell's frequency selectivity, whether assayed acoustically or with extrinsic currents. Lower TEA concentrations abolished the efferent inhibitory post-synaptic potential with only a minor change in tuning. TEA produced other effects different from efferent stimulation including (i) a lowering of the characteristic frequency, and (ii) a highly asymmetric receptor potential. These observations suggest that the efferents do not simply block membrane conductances associated with tuning. We conclude that the efferent modification of the shape of the tuning curve may be a composite result of the synaptic conductance and the hyperpolarization of the hair cell membrane.

Injection of ac current into scala media alters the sound pressure at the tympanic membrane: Variations with electrical stimulus parameters

The sound pressure near the tympanic membrane of gerbils was measured using a probe tube microphone during the injection of ac current into the basal turn of the cochlea. Electrical currents of up to 5 μA with frequencies ranging from 25 to 1500 Hz were used. The sound pressure signal was sampled, averaged, and stored by computer for subsequent Fourier analysis. The sound pressure is modulated as much as 1% by the electrical current. The modulation is proportional to current level and is a low‐pass function of electrical frequency with a time constant of approximately 1 ms. The modulation disappears in dead animals and is not observed if the current injection electrode is not in scala media. In addition, the modulation can vary with time and often deteriates in long experiments. Our data may be the result of changes in stiffness of the haircell stereocilia, mediated by changes in haircell membrane potential.