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J Physiol. 2000 Apr 15;524 Pt 2:423-36. Links
Tonotopic variations of calcium signalling in turtle auditory hair cells.
• Ricci AJ, Gray-Keller M, Fettiplace R.
Department of Physiology, University of Wisconsin Medical School, Madison, WI 53706, USA.
fettiplace@physiology.wisc.edu
Turtle cochlear hair cells are electrically tuned by a voltage-dependent Ca2+ current and a Ca2+-dependent K+ current (IBK(Ca)). The effects of intracellular calcium buffering on electrical tuning were studied in hair cells at apical and basal cochlear locations tuned to 100 and 300 Hz, respectively. Increasing the intracellular BAPTA concentration changed the hair cell's resonant frequency little, but optimized tuning at more depolarized membrane potentials due to a positive shift in the half-activation voltage (V ) of the IBK(Ca). The shift in V depended similarly on BAPTA concentration in basal and apical hair cells despite a 2. 4-fold difference in the size of the Ca2+ current at the two positions. The Ca2+ current amplitude increased exponentially with distance along the cochlea. Comparison of V values and tuning properties using different BAPTA concentrations with values measured in perforated-patch recordings gave the endogenous calcium buffer as equivalent to 0.21 mM BAPTA in low-frequency cells, and 0.46 mM BAPTA in high-frequency cells. High conductance Ca2+-activated K+ (BKCa) channels recorded in inside-out membrane patches were 2-fold less Ca2+ sensitive in high-frequency than in low-frequency cells. Confocal Ca2+ imaging using the fluorescent indicator Calcium Green-1 revealed about twice as many hotspots of Ca2+ entry during depolarization in high-frequency compared to low-frequency hair cells. We suggest that each BKCa channel is gated by Ca2+ entry through a few nearby Ca2+ channels, and that Ca2+ and BKCa channels occupy, at constant channel density, a greater fraction of the membrane area in high-frequency cells than in low-frequency cells.
PMID: 10766923 [PubMed - indexed for MEDLINE]
1: J Physiol. 1985 Mar;360:397-421. Links
Efferent modulation of hair cell tuning in the cochlea of the turtle.
• Art JJ, Crawford AC, Fettiplace R, Fuchs PA.
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.
PMID: 3989721 [PubMed - indexed for MEDLINE]
Hear Res. 1983 Nov;12(2):199-208. Links
Auditory nerve responses to imposed displacements of the turtle basilar membrane.
• Crawford AC, Fettiplace R.
Impulse activity of single auditory nerve fibres was recorded in the isolated half-head of the turtle in response to displacements of a piezoelectric probe placed on the basilar membrane. The temporal pattern of firing in response to sinusoidal displacements of amplitude 0.1-1.0 nm r.m.s. at a fibre's characteristic frequency could be matched to that generated by low-level tonal stimuli delivered to the tympanum. Frequency-threshold curves for acoustic and mechanical stimuli had similar shapes and differed only at frequencies above 500 Hz where the middle ear should filter acoustic but not direct mechanical stimuli. Step displacements of the basilar membrane gave a transient periodic discharge which resembled the responses to acoustic clicks. Most fibres initially increased their firing rate for rarefaction clicks and displacements towards the scala tympani.
PMID: 6643291 [PubMed - indexed for MEDLINE]
J Aud Res. 1982 Jul;22(3):153-60. Links
Bone conduction hearing in turtles.
• Lenhardt ML.
Audiofrequencies were delivered to specimens of 3 turtle families by ac and bc in an attempt to elicit a behavioral response. Aerial signals with intensities up to 100 db SPL failed to elicit any consistent response. The same signals delivered directly to the carapace resulted in brisk head withdrawals. There was little energy loss measured along the shell suggesting it may serve in a minor role as an acoustic receptor. Turtles could use bc hearing in short distance detection even when withdrawn into the shell. Although the ear is likely mediating the head reflex, somatic mechanoreceptors cannot be completely excluded.
PMID: 7187665 [PubMed - indexed for MEDLINE]
Electroencephalogr Clin Neurophysiol. 1982 Dec;54(6):629-41. Links
The auditory brain stem response in five vertebrate classes.
• Corwin JT, Bullock TH, Schweitzer J.
In representative elasmobranchs, osteichthyans, amphibians, reptiles and birds, average evoked potentials in response to acoustic clicks and tone bursts were recorded intracranially, but outside the brain, or extracranially. Controls against artifacts and tests after transections show that these potentials conform to criteria for auditory brain stem responses (ABRs). Brief waves in a 10-15 msec sequence originate successively in the eighth nerve, medulla and midbrain; there is little contribution to the latter waves from the lower levels. This response pattern appears to be consistent within each species and is similar to that extensively studied in mammals. Some of its features are remarkably alike in all the vertebrate classes tested, implying a generality in the existence of a subset of auditory neurons at several brain levels that are highly synchronous in activity, even after several synapses, and geometrically oriented to add their macroscopic, open, dipole fields. The intensity, repetition rate and the power spectrum of the click stimuli have little effect on the ABR pattern, except when the peak energy is in the low frequency range. In the range below ca. 700 Hz frequency content has a considerable effect; lower frequencies broaden certain waves. Cooling has marked and differential effects on component processes. Reversing click phase, e.g. from initial compression to initial rarefaction, can show no effect or any of several effects, depending on the species. Tone bursts evoke onset ABRs and in some cases after a transitional period a sustained frequency following response. The ABR resembles a click evoked potential even when stimulus rise time is slow. Background tones of particular frequency are most efficient in masking click evoked ABRs; white noise is less efficient. The ABR should be useful in neuroethology since it can be studied without invading the brain. It can tell that the brain is sensitive to a sound. In an immobilized animal it can be recorded in a single sweep, or it can be averaged from an awake tethered animal. It shows good sensitivity and at least some correspondence with behavioral measures of hearing.
PMID: 6183096 [PubMed - indexed for MEDLINE]
...solo alcune delle 43 voci che mi sono uscite in bibliografia scientifica nel campo delle ricerche sulla capacità acustica nei cheloni.