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. 2002 Oct 11;545(Pt 1):279–288. doi: 10.1113/jphysiol.2002.025205

Passive basilar membrane vibrations in gerbil neonates: mechanical bases of cochlear maturation

Edward H Overstreet III *, Andrei N Temchin *, Mario A Ruggero *
PMCID: PMC2290655  PMID: 12433967

Abstract

Using a laser velocimeter, basilar membrane (BM) responses to tones were measured in neonatal gerbils at a site near the round window of the cochlea. In adult gerbils, ‘active’ BM responses at this site are most sensitive at 34–37 kHz and exhibit a compressive non-linearity. Postmortem, BM responses in adults become ‘passive’, i.e. linear and insensitive, and the best frequency (BF) shifts downwards by about 0.5 octaves. At 14 and 16 days after birth (DAB), BM responses in neonatal gerbils were passive but otherwise very different from postmortem responses in adult gerbils: BF was more than an octave lower, the steep slopes of the phase vs. frequency curves were shifted downwards in frequency by nearly 1 octave, and the maximum phase lags amounted to only 180 deg relative to stapes. BFs and phase lags increased systematically between 14 and 20 DAB, implying drastic alterations of the passive material properties of cochlear tissues and accounting for a large part of the shift in BF that characterizes maturation of auditory nerve responses during the same period.

During the third week after birth, the basal (or high-frequency) region of the cochlea of gerbils and other altricial (late developing) mammals undergoes drastic functional changes: the best frequency (BF) of gross cochlear potentials shifts towards higher values (Harris & Dallos, 1984; Yancey & Dallos, 1985; Arjmand et al. 1988) and the responses of auditory nerve fibres exhibit large increases of sensitivity, sharpness of frequency tuning and BF (Echteler et al. 1989; Müller, 1991, 1996). (We use ‘best frequency’, or BF, to refer to the stimulus frequency that elicits the most sensitive auditory response, regardless of age or cochlear health; we reserve ‘characteristic frequency’, or CF, to refer to the stimulus frequency that elicits the most sensitive responses to low-level stimulation in normal adult cochleae.) Because frequency tuning in adult mammalian cochleae originates in the vibrations of the basilar membrane (BM; von Békésy, 1960; Narayan et al. 1998; reviewed in Robles & Ruggero, 2001), it is widely thought that the developmental changes in neural frequency tuning also reflect underlying shifts in BM vibrations (Lippe & Rubel, 1983; Rubel & Ryals, 1983; reviewed in Manley, 1996; Romand, 1997). However, the evidence supporting this idea, although substantial, is entirely indirect and not definitive.

We tested the hypothesis that the BF of BM vibrations at the base of the gerbil cochlea increases during the third postnatal week. BM responses to tones were measured at a site of the cochlea located 1.2 mm from the round window, where responses in adult gerbils have a CF of 34–37 kHz and exhibit a compressive non-linearity, indicative of an active process, which is abolished by death (Overstreet et al. 2002b). By referencing BM vibrations to stapes vibrations measured in the same ears, the intrinsic maturation of BM vibrations was studied in isolation from the possibly confounding effects of middle ear development (Overstreet & Ruggero, 2002). The hypothesis of a developmental BF shift of BM vibrations was confirmed and, most importantly, it was shown to largely reflect the maturation of passive processes.

The substance of this paper, part of a doctoral dissertation submitted by E.H.O. to the University of Minnesota, Twin Cities, in 2000, has been published as a conference abstract (Overstreet & Ruggero, 1999).

Methods

Animal preparation

All experiments were conducted in accordance with guidelines of Northwestern University's Animal Care and Use Committee. Subjects were anaesthetized neonatal Mongolian gerbils (Meriones unguiculatus) aged 14–20 days after birth (DAB; determined with a precision of ± 8 h), day zero being the day of birth. To help ensure equal maturation rates, all litters were culled to six pups and only neonates weighing within one standard deviation of the mean for a given age (Woolf & Ryan, 1985) were used.

Gerbils were sedated with a 0.01 ml subcutaneous injection of ketamine HCl (100 mg ml−1) and initially anaesthetized with an intraperitoneal injection of sodium pentobarbital (48 mg kg−1). Additional doses of sodium pentobarbital were administered as needed to maintain deep anaesthesia, as indicated by the absence of limb withdrawal reflex (tested every 30 min). The animals were kept hydrated with 0.10 ml injections of Pedialyte i.p. (Abbott, USA; each litre of this solution contains 45 mequiv Na+, 20 mequiv K+, 35 mequiv Cl−, 30 mequiv citrate and 25 g dextrose), administered at ≈90 min intervals. The gerbils were placed upon a vibration-isolation table within a sound-insulated chamber. Rectal temperature was monitored and maintained at 39 °C by means of a servo-controlled and battery-powered electrical heating pad wrapped around the animal. All gerbils were tracheotomized and the surgical opening to the trachea was lined with cotton wicks to maintain a patent airway.

The gerbil's head was firmly affixed to a holder which was heated to maintain normal cochlear temperature during physiological recordings (to compensate for the effects of deep anaesthesia and the widely opened bulla; Brown et al. 1983; Shore & Nuttall, 1985). The left pinna was removed to allow the insertion into the ear canal of a speculum containing the probe tube of a miniature microphone and the sound source. After opening the ventral aspect of the bulla, a silver-wire ball electrode was placed on the bone near the round window to record compound action potentials evoked by tone pips. At the end of the experiments, gerbils were decapitated while still under deep anaesthesia.

Sound system and its calibration

Acoustic stimuli were produced by a modified tweeter (Chan et al. 1993) coupled to the speculum. The tweeter was driven with electrical tones (1-40 kHz) digitally synthesized under computer control. Stimulus sound pressures as a function of frequency were estimated using a hybrid calibration combining in situ probe-microphone measurements with an average calibration using a small artificial cavity simulating the external ear canal with a 1/8 in condenser microphone replacing the tympanic membrane (Overstreet & Ruggero, 2002).

Compound action potential thresholds

Compound action potential thresholds were measured using an automated procedure which, for each stimulus frequency, determined the sound pressure level (SPL; expressed in decibels referenced to 20 μPa) required to achieve a predetermined action potential magnitude (typically 7 μV). Stimuli were tone pips with onset phases randomized to cancel out cochlear microphonics and ramped with a 0.75 ms rise/fall time to efficiently synchronize the neural spikes without losing frequency resolution due to spectral spread.

Laser velocimetry data collection and analysis

Stimuli for the measurement of stapes and BM vibrations were 5 ms tones presented every 50 ms with rise/fall times of 0.75 ms. BM and stapes velocity responses to 256–4098 stimulus repetitions were time averaged for each stimulus condition, depending upon the signal-to-noise ratio. Only averaged responses greater than 6 dB above the average noise floor were used. Stimulus frequencies were 1–40 kHz, presented at intervals of 500 Hz.

Velocity responses were recorded with a laser vibrometer (Ruggero & Rich, 1991a) coupled to a compound microscope focused on one or more microbeads placed on the head of the stapes or on the BM. The velocimeter produced a voltage signal (proportional to target velocity) which was digitized with 16-bit resolution at a sampling rate of 166 kHz and stored in a PC computer for further processing.

The amplitudes and phases of averaged velocity responses were computed by Fourier transformation. The amplitude and phase curves of Fig. 1 and Fig. 3 indicate median values computed over 1 kHz bands centred at 500 Hz intervals.

Figure 1. Gains of BM responses to tones relative to stapes vibrations in neonatal gerbils.

Figure 1

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Each panel presents individual gains for neonatal gerbils within one age group, as well as a median curve (thick line). The arrows indicate the BFs, defined here as the ‘centre of gravity’ of the tuning curves (rather than their peak frequency). The median curves are truncated at the highest frequencies for which data are available from at least 80 % of subjects in each age group.

Figure 3. Phases of BM responses to tones relative to stapes vibrations in neonatal gerbils.

Figure 3

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Each panel presents individual phase vs. frequency curves of BM velocity towards scala tympani relative to inward stapes velocity as well as a median curve (thick line). The median curves are truncated at the highest frequencies for which data are available from at least 80 % of subjects in each age group. BFs (from Fig. 1) are indicated by arrows.

Results

Useful BM recordings were obtained in the cochleae of 33 Mongolian gerbils aged 14, 16, 18 or 20 days at sites located about 1.2 mm from their extreme basal end, where responses in adults have CFs of 34–37 kHz (Overstreet et al. 2002b).

Compound action potential thresholds in neonatal gerbils

As a control of the functional state of the experimental cochleae, compound action potential thresholds were measured immediately after opening the bulla and before the bony otic capsule or the round window were disturbed. Thresholds were normalized to the magnitudes of stapes vibration to estimate intrinsic cochlear sensitivity, i.e. in isolation from developmental changes in middle ear responses. Thresholds at 38.3 kHz (i.e. near the CF of the BM recording site in adult gerbils) typically could not be measured in 14- or 16-DAB neonatal gerbils because they exceeded the highest attainable stimulus levels (about 90 dB SPL). In 18- and 20-DAB neonates, the 38.3 kHz thresholds of intact cochleae (not shown here) were higher by 48 and 40 dB, respectively, than in adults (Overstreet et al. 2002a). Thresholds were often further elevated by subsequent experimental procedures, including excision of the round window membrane and placement of reflective beads on the BM. These acute threshold elevations were usually small, averaging 3 dB in 18-DAB gerbils and 9 dB in 20-DAB gerbils.

The magnitudes of BM vibrations in neonatal gerbils

In healthy cochleae of adult gerbils, BM responses to near-CF tones at the 1.2 mm site grow at (non-linear) compressive rates: vibration sensitivity is largest at low stimulus levels and diminishes at higher levels (Overstreet et al. 2002b). The non-linearity is vulnerable, being easily abolished by cochlear injury or death. Minimally compressive input-output functions could be measured in only two neonatal gerbils, aged 18 and 20 days. In these two animals, the loss of compressive non-linearity was accompanied by elevations of the compound action potential thresholds. BM responses in all other neonates grew linearly with stimulus intensity regardless of stimulus frequency, their sensitivity was remarkably stable over time and exhibited no change in vibration patterns immediately postmortem.

Figure 1A-D present individual data for the magnitudes of BM vibrations relative to stapes vibration in neonatal gerbils, plotted against stimulus frequency and grouped according to age. In 14-DAB neonates (Fig. 1A), BM responses were severalfold larger than stapes vibrations, very variable and poorly frequency-tuned. In 16-DAB gerbils (Fig. 1B), frequency tuning remained ill-defined with a wide plateau of nearly uniform magnitude, about nine times larger than stapes vibration, extending between 12 and 34 kHz. In 18- and 20-DAB gerbils (Fig. 1C and D), the peak plateaus grew only slightly in magnitude but their high-frequency cutoffs shifted systematically towards higher frequencies. Because the peak magnitudes of BM responses in neonates were poorly defined, BF was determined by computing the ‘centre of gravity’ of the magnitudes (i.e. the frequency dividing the area under the magnitude vs. frequency curve between the −6 dB cutoffs into equal halves). The BFs are indicated by arrows in Fig. 1 and Fig. 3.

Figure 2 allows side-by-side comparison of the median magnitudes of stapes-normalized BM responses to tones in neonatal gerbils aged 14, 16, 18 and 20 DAB. Figure 2 shows that although the gains of the tuning curves remained relatively stable at frequencies well below BF after 16 DAB, their upper-frequency arms migrated towards higher frequencies. Response sensitivity at the BF also increased but only by small amounts.

Figure 2. The magnitude of BM responses in neonatal gerbils and in postmortem and in vivo adults.

Figure 2

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The median gain curves of Fig. 1 are compared with peak response gains for in vivo vibrations of the cochleae of two adult gerbils, as well as a median curve computed from postmortem adult data (Overstreet et al. 2002b).

For reference, Fig. 2 also shows responses in adult gerbils, both in vivo and postmortem. In the case of neonates and postmortem adults, in which responses were linear, stapes-normalized gains can be taken as the magnitudes of transfer functions that fully describe the vibrations of the BM for stimuli of arbitrary level. In the case of in vivo adults, however, in which responses grow with stimulus level at compressive rates (i.e. less than 1 dB response growth per 1 dB of stimulus increase), gain vs. frequency curves vary as a function of stimulus level, becoming broader and less sensitive as stimulus levels are raised. The in vivo curves of Fig. 2 represent peak (maximal) response gain at each frequency in the relatively healthy cochleae of two gerbils (from Fig. 6 of Overstreet et al. 2002b). Peak gains and BFs in 20-DAB neonates were roughly similar to those of postmortem adults. The BFs of BM responses in the cochleae of 20-DAB neonates were lower (by about 0.5 octaves) than those of in vivo adults and their frequency tuning was much broader. The gains at BF of 20-DAB neonates were lower by 26 dB than the gains of CF responses in adults. At 34–37 kHz, BM responses in normal adult cochleae exceeded responses in 20- and 18-DAB neonates by more than 40 and 50 dB, respectively.

The phases of BM vibration in neonatal gerbils

In gerbils aged 14 days (Fig. 3A), BM velocity responses towards scala tympani exhibited a phase lead (relative to inward velocity of the stapes) amounting to ≈0.25 periods at low frequencies and increasingly lagged at higher frequencies. Response phases changed substantially as a function of increasing age, with the steep segment of the phase vs. frequency curve migrating progressively towards higher frequencies. At 16, 18 and 20 DAB (Fig. 3B-C), the phase vs. frequency curves all reached plateaus at a lag of approximately 0.5 periods.

Figure 4 allows comparison of the phase curves for responses of neonatal gerbils with those recorded in vivo and postmortem in adults. The response phases of 20-DAB gerbils attained a maximum lag at high frequencies of less than 0.5 periods relative to stapes vibration, contrasting with either postmortem or in vivo responses in adults. In adults in vivo, the phase curves reached a lag of 1.5 periods at CF and continued their roll-off well past that frequency. In adults, the median postmortem curve appeared to approach a plateau at 1.5 periods.

Figure 4. Phases of BM responses in neonatal gerbils and in postmortem and in vivo adults.

Figure 4

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The median gain curves of Fig. 3 are compared with phase curves for BM responses recorded in vivo in two adults gerbils, as well as a median curve computed from postmortem adult data (Overstreet et al. 2002b).

The phase vs. frequency curves of neonates had shorter steep segments and correspondingly longer plateaus than those of adults, either in vivo or postmortem. This indicates that the 1.2 mm BM site of the gerbil cochlea, which in adults supports a travelling wave that peaks at 34–37 kHz, cannot support travelling waves at frequencies higher than about 15 kHz in 14-DAB neonates or 30 kHz in 20-DAB neonates.

Summary of passive developmental changes in BM vibration during the third postnatal week

Changes in the frequency tuning of BM vibrations as a function of age are summarized in Fig. 5A. BF increased systematically, from 9.3 kHz at 14 DAB to 21.9 kHz at 20 DAB. At 14 DAB, the BF was about 1.7 octaves lower than the (adult) in vivo CF and 1.1 octaves lower than the postmortem adult BF. Thus, the maturation of passive processes accounts directly for most (1.1 octaves) of the overall developmental growth in BF, the remainder (0.5-0.6 octaves) representing the BF shift due to the operation of the active process. Figure 5A also shows that, consistent with the developmental shift in BF, both the low- and high-frequency arms of the tuning curves (‘low 6 dB’ and ‘high 6 dB’, respectively), as well as the steep segments of the phase vs. frequency curves (measured at zero or −90 deg), migrated towards higher frequencies during the third week after birth.

Figure 5. Summary of the development of BM responses in neonatal gerbils.

Figure 5

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The abscissae indicate age and, for adults, whether responses were measured postmortem (> 90PM) or in vivo (> 90). A, BFs, frequencies higher (high 6 dB) and lower (low 6 dB) than BF at which stapes-normalized BM response gains are 6 dB lower than at BF, and frequencies at which BM phases lag stapes responses by 0 and 90 deg. B, peak sensitivity, expressed as a magnitude gain relative to stapes vibration. C, sharpness of frequency tuning, expressed as Q6dB and Q10dB (i.e. BF divided by bandwidth at −6 or −10 dB relative to the response magnitude at the BF).

Figure 5B shows that the peak magnitude gain of responses grew moderately between 14 and 20 DAB, when it approximately equalled the adult postmortem values. This change, however, was small compared with the difference between in vivo and postmortem values in adult gerbils.

Figure 5C shows the variation of two measures of relative sharpness of tuning (Q10dB and Q6dB: BF divided by the bandwidth of the magnitude vs. frequency curve at −10 or −6 dB relative to the BF response magnitude) as a function of age. Relative sharpness of tuning remained almost constant between 14 and 20 DAB and did not differ from the sharpness of tuning of postmortem responses in adults.

Discussion

The characteristics of BM vibrations in adult mammalian cochleae derive from passive and active processes (reviewed in Robles & Ruggero, 2001). The passive processes reflect material mechanical properties (i.e. mass, stiffness and damping) of cochlear tissues and fluids and consist of coupled pressure and displacement travelling waves that move from base to apex, respectively, in the cochlear fluids and on the BM (von Békésy, 1960; Lighthill, 1981; Ruggero, 1994; Olson, 1999). In contrast to the passive processes, which in adults produce linear, insensitive and broadly frequency-tuned BM responses that remain unaltered postmortem, the active process (Gold, 1948; Davis, 1983; Ruggero & Rich, 1991b; reviewed in Dallos, 1992; Robles & Ruggero, 2001) is characterized by a triad of highly vulnerable properties of BM vibration - high sensitivity, sharp frequency tuning and compressive non-linearity - which are abolished by death (Rhode, 1971, 1973; Sellick et al. 1982; Robles et al. 1986) and various noxious agents (Ruggero et al. 1996a; Ruggero & Rich, 1991b; Murugasu & Russell, 1995; reviewed in Ruggero et al. 1996b; Robles & Ruggero, 2001).

The rudimentary travelling wave of passive BM vibrations in neonatal gerbils

The present results demonstrate that at 14 and 16 DAB, passive BM responses at the 1.2 mm site of the gerbil cochlea are linear but otherwise very different from postmortem responses in adult gerbils: BF is 1.1 octaves lower and phase lags are small. Phase lag accumulation ceases at frequencies lower than 25 kHz, indicating that in neonates the 1.2 mm BM site cannot sustain travelling waves at the frequency (the CF) at which it responds most sensitively in adult cochleae. Analyses of travelling wave behaviour in the BMs of mammalian cochleae generally highlight two defining characteristics: asymmetry of frequency tuning, with a steeper slope on the high-frequency side (or, equivalently, on the leading, apical side of the travelling wave) and, most importantly, a large accumulation of phase lag as a function of increasing frequency (or, equivalently, as a function of increasing distance from the cochlear base) (Lighthill, 1981; Wilson, 1992). On both counts, BM vibrations at the 1.2 mm site of the cochlea of 14- and 16-DAB gerbils lie at one extreme of the spectrum of travelling wave behaviour at the base of the cochlea of mammals. This is especially evident in the small cumulative phase lag, which amounts to only three-quarters of one period above 25 kHz. One may question whether the BMs of 14- or 16-DAB gerbils actually sustain travelling waves as traditionally defined.

The maturation of passive BM vibrations accounts in great part for the concurrent shift in the BFs of auditory nerve fibres at the base of the cochlea

During the third week after birth, the frequency tuning of auditory nerve fibres innervating the base of the gerbil cochlea undergo profound changes, most notably large increases of BF (Echteler et al. 1989; Müller, 1996). Therefore, given that the frequency-tuning characteristics of auditory nerve fibres in adult mammals derive from, and closely resemble, the corresponding characteristics of BM vibrations (Sellick et al. 1982; Narayan et al. 1998; Ruggero et al. 2000), it is reasonable to seek the bases of these developmental changes in neural tuning in the vibrations of the BM. The present results demonstrate that most of the increase in BF at basal cochlear sites, about 1.1 octaves out of a total 1.7 octaves, results from developmental changes in passive mechanics (Fig. 2 and Fig. 5). A further 0.6 octave increase, identical in magnitude to the difference between the CF and the postmortem BF in adult gerbils (Overstreet et al. 2002b), presumably can be ascribed to the maturation of active BM mechanical processes (see later).

Structural origins of the development of passive BM mechanics at the base of the gerbil cochlea

Developmental changes of BM vibrations reminiscent of those that we measured in vivo were observed in an in vitro preparation of the gerbil cochlea, the ‘hemicochlea’ (Richter et al. 1998): the BF of BM responses to nearby fluid disturbances ‘shifted by 1.5 octaves towards higher frequencies … between 12 and 18′ DAB (Richter & Dallos, 2000). Other things (e.g. cochlear input impedance and stapes vibrations) being equal, increases of BF could result from developmental increases in the stiffness of the cochlear partition (Emadi et al. 2001) or decreases in the masses of the BM and/or the organ of Corti.

The present magnitude data (Fig. 2) are consistent with an increase of stiffness only for frequencies < 10 kHz in the period 14–16 DAB. Increases in the stiffness of the cochlear partition could derive from structural changes in the BM, such as increases in the thickness of the radial fibre bands (Echteler, 1995; Schweitzer et al. 1996) or increased density of proteoglycans (Munyer & Schulte, 1995), as well as the proliferation of tension fibroblasts in the spiral ligament (Echteler, 1995; Kuhn & Vater, 1997) or the maturation of the pillar cells of the organ of Corti (Souter et al. 1997), which probably contribute to the overall stiffness of the cochlear partition (Naidu & Mountain, 2001).

For ages 16 DAB and greater, BM vibration data (Fig. 2) are more consistent with a progressive decrease in the effective mass of the cochlear partition. Several studies (Echteler, 1995; Schweitzer et al. 1996; Souter et al. 1997) have presented evidence favouring such a decrease during the third postnatal week in gerbils but contrary evidence also exists (Richter et al. 2000).

Evidence from compound action potential thresholds on the maturation of the active process in the gerbil cochlea

In addition to changes in BF, the frequency-threshold tuning curves of auditory nerve fibres innervating the base of the gerbil cochlea undergo other drastic alterations during the third week of postnatal development: the thresholds at BF decrease, and the magnitude of the tip-to-tail ratio increases, by at least 40 dB (Echteler et al. 1989; Müller, 1996). These developmental changes, which resemble (in reverse chronology) the changes that initially sensitive BM responses in normal adult cochleae undergo postmortem (Rhode, 1971, 1973; Sellick et al. 1982; Robles et al. 1986) or after noxious manipulations (Ruggero & Rich, 1991b; Murugasu & Russell, 1995; Ruggero et al. 1996a) must principally reflect active properties of BM vibrations (Gold, 1948; Davis, 1983).

Compound action potential thresholds were measured in all neonatal gerbils immediately after opening the bulla, when the cochlea was morphologically (and, presumably, also functionally) intact. After normalization to stapes vibrations, these thresholds serve to quantify the overall maturation of cochlear processes, including BM mechanics and signal transmission through inner hair cells and their synapses. Thresholds at 38.3 kHz were at least 80 dB higher in 14- and 16-DAB neonates than in adult gerbils and were still 40 dB higher than in adults by 20 DAB (Overstreet et al. 2002a). Taking into account that the peak in vivo sensitivity of BM responses in adult gerbils exceeds the peak magnitude of postmortem responses by only about 30 dB (see Fig. 2 and also Cooper, 2000; Ren & Nuttall, 2001; Overstreet et al. 2002b), it is likely that the high neural thresholds in the intact cochleae of 14- and 16-DAB neonates partly reflect the complete absence of active processes. Similarly, the 40 dB (or greater) reduction in thresholds during the course of the third week after birth probably reflects a rapid growth of cochlear amplification during the third postnatal week, which nevertheless remains incomplete by 20 DAB. Excision of the round window caused small threshold elevations in most 18- and 20-DAB gerbils (see first section of Results). Additionally, in two cases, threshold elevations occurred during BM recordings, accompanied by loss of pre-existing weak compressive non-linearities. In conclusion, compound action potential thresholds and their vulnerability (or lack thereof) suggest that the 1.2 mm site of the gerbil cochlea first exhibits weak mechanical amplification between 16 and 18 DAB.

Developmental bases of cochlear amplification in neonatal gerbils

In the adult mammalian cochlea, the active process apparently consists of a feedback loop linking BM mechanics with receptor potentials (Ruggero & Rich, 1991b) and voltage-driven somatic electromotility (Brownell et al. 1985; Ashmore, 1987) in the outer hair cells (for reviews, see Dallos, 1992; Robles & Ruggero, 2001). Therefore, deficiencies due to immaturity of passive BM mechanics, mechanoelectrical transduction or somatic electromotility should render the active process ineffective. An intrinsic deficiency in somatic electromotility during the third week after birth appears to be ruled out by the demonstration that motile responses of gerbil outer hair cells in vitro achieve adult magnitudes by 14 days after birth (He et al. 1994). We believe that the development of the active process, and hence of sensitive and non-linear BM responses, during the third postnatal week result from the concurrent maturation of both the endocochlear potential and passive BM mechanics.

A crucial factor in bringing about full expression of the active process at the base of the gerbil cochlea during the third postnatal week is the development of the endocochlear potential, which in adults controls to a great extent the magnitude of receptor currents and potentials (Davis, 1965; Russell, 1983). The endocochlear potential is only 2–3 mV at 10 DAB, grows rapidly between 10 and 20 DAB (Woolf et al. 1986) and more slowly thereafter, reaching adult values between 30 and 90 DAB (McGuirt et al. 1995). A low endocochlear potential should decrease mechanoelectrical transduction in outer hair cells and, consequently, also diminish the boost that the latter provides to BM vibrations in normal adult cochleae (the active process). This is probably the mechanism underlying the reductions in BM compressive non-linearity, sensitivity and sharpness of tuning caused by systemic furosemide injection (Ruggero & Rich, 1991b).

The other indispensable factor that permits expression of the active process at the base of the gerbil cochlea is the development, during the third postnatal week, of passive BM mechanics. Our premise is that the output of cochlear amplification (i.e. active BM vibrations) depends on the magnitude of the substrate (i.e. passive BM vibrations). Thus, for example, even if the intrinsic gain of cochlear amplification at the 1.2 mm site were identical in 18-DAB neonates and adult gerbils (an unlikely possibility since the endocochlear potential is not fully developed at that age; McGuirt et al. 1995), BM vibrations at CF (34-37 kHz) would presumably be 16 dB lower in magnitude than in adults (since at those frequencies passive BM vibrations in 18-DAB neonates are 16 dB lower than BM vibrations in postmortem adults). Even more insensitive BM vibrations would be expected at 14 and 16 DAB, when the endocochlear potential (and presumably also passive BM vibrations) is substantially smaller than that at 18 DAB.

We agree with Mills and Rubel that ‘active cochlear mechanics … are intrinsically tied to passive mechanics’. Furthermore, our results confirm their conjecture that the (passive) ‘base cutoff frequency increases quickly … following the onset of hearing’ (Mills & Rubel, 1998). However, we strongly disagree with their conclusion that ‘the cochlear amplifier appears to be mature, or nearly so, in the base … at or soon after the onset of hearing’, i.e. at 14 or 15 DAB (Mills & Rubel, 1998). The latter conclusion was based on the unproven and probably erroneous premise that the gain of the active process at the base of the cochlea can be correctly estimated by measuring the effects of furosemide on distortion-product otoacoustic emissions.

The role of passive BM vibrations may be even more decisive if, as suggested by theoretical considerations as well as empirical evidence, there is a filter additional to that of passive BM mechanics at each cochlear site. Indeed, computational models of BM vibrations in active mammalian cochleae generally include band-pass (or, exceptionally, low-pass; Hubbard, 1993) filters in the feedback loop that links passive BM vibrations with the active process and with receptor potentials (see review by Robles & Ruggero, 2001). Consistent with this idea, BM responses to clicks at the base of the cochlea consist of an initial passive frequency glide, which is linear and remains postmortem, and a later active component, which is non-linear and highly labile (Recio et al. 1998). The fact that the onset of the active component nearly coincides with the instant when the glide reaches a frequency near CF (Recio et al. 1998) suggests that either the active process is itself frequency specific or that a band-pass filter is interposed in the feedback loop. In other words, the active process is fully expressed only if the frequency of passive BM vibration coincides with the BF of an additional filter linked to somatic electromotility (which is not intrinsically frequency tuned; Dallos, 1992). The ears of lower vertebrates do contain ‘second filters’, such as the electrical ones in the hair cells of the turtle basilar papilla (Crawford & Fettiplace, 1981), or ‘tuned amplifiers’, such as those in the stereocilia of amphibian saccular hair cells (Martin & Hudspeth, 1999). However, the existence of ‘second filters’ in mammalian cochleae (e.g. Zwislocki & Kletsky, 1979; Brundin et al. 1989) remains unproven.

Significance of the findings in neonatal gerbils for understanding cochlear development in other mammals

The gerbil is the only species in which morphological and neurophysiological studies of cochlear maturation have been complemented by measurements of either BM (the present work) or stapes vibrations (Overstreet & Ruggero, 2002). Therefore, until these measurements are extended to other species, the measurements in gerbils provide a unique window to view the onset of hearing in other mammals including those, like humans (Pujol et al. 1991; Pujol & Lavigne-Rebillard, 1992), in which cochlear development is essentially fully mature at birth. It may not be coincidental that two distinct subsystems of the gerbil cochlea, namely the passive mechanics of the BM/organ of Corti complex and the energy source of the active process (i.e. the endocochlear potential), develop rapidly and simultaneously during the third week after birth. Comparable periods of rapid cochlear maturation exist in other mammalian species. For example, maturation of compound action potential thresholds takes place during the 12 days preceding birth in the guinea-pig, and between birth and 12–15 DAB in the cat (Pujol & Hilding, 1973). It will be interesting to ascertain whether the maturation of compound action potential thresholds in these and other mammalian species are also synchronized with the development of passive basilar membrane mechanics and of the endocochlear potential.

Conclusions

The present results support the hypothesis that the changes in the frequency tuning characteristics of auditory nerve fibres arise from the maturation of BM vibrations. Further, because the present vibration data all came from cochleae in which the active process was not operating, these results indicate that the structural characteristics of the gerbil cochlear partition, which determine passive response properties, undergo significant developmental changes during the third week after birth. Finally, compound action potential thresholds suggest that active processes near the round window of the cochlea develop during the third postnatal week but remain immature at 20 DAB, probably due to residual immaturity of both passive BM mechanics and endocochlear potential.

Acknowledgments

We thank Jon Siegel for his help in the modification of the Radio Shack tweeter. We are especially grateful to David Mountain, whose many thoughtful comments on previous versions of this paper contributed greatly to improving our interpretation of the data. We were supported by grant R01-DC-00419 from the National Institute on Deafness and Other Communication Disorders.

References

  1. Arjmand E, Harris D, Dallos P. Developmental changes in frequency mapping of the gerbil cochlea: comparison of two cochlear locations. Hearing Research. 1988;32:93–96. doi: 10.1016/0378-5955(88)90149-9. [DOI] [PubMed] [Google Scholar]
  2. Ashmore JF. A fast motile response in guinea-pig outer hair cells: the cellular basis of the cochlear amplifier. Journal of Physiology. 1987;388:323–347. doi: 10.1113/jphysiol.1987.sp016617. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Brown MC, Smith DI, Nuttall AL. Anesthesia and surgical trauma: their influence on the guinea pig compound action potential. Hearing Research. 1983;10:345–358. doi: 10.1016/0378-5955(83)90097-7. [DOI] [PubMed] [Google Scholar]
  4. Brownell WE, Bader CR, Bertrand D, de Ribaupierre Y. Evoked mechanical responses of isolated cochlear outer hair cells. Science. 1985;227:194–196. doi: 10.1126/science.3966153. [DOI] [PubMed] [Google Scholar]
  5. Brundin L, Flock A, Canlon B. Sound-induced motility of isolated cochlear outer hair cells is frequency-specific. Nature. 1989;342:814–816. doi: 10.1038/342814a0. [DOI] [PubMed] [Google Scholar]
  6. Chan JC, Musicant AD, Hind JE. An insert earphone system for delivery of spectrally shaped signals for physiological studies. Journal of the Acoustical Society of America. 1993;93:1496–1501. doi: 10.1121/1.406807. [DOI] [PubMed] [Google Scholar]
  7. Cooper NP. Basilar membrane vibrations in the basal turn of the gerbil cochlea. Association for Research in Otolaryngology Mid-Winter Meeting Abstracts. 2000;23:205. [Google Scholar]
  8. Crawford AC, Fettiplace R. An electrical tuning mechanism in turtle cochlear hair cells. Journal of Physiology. 1981;312:377–412. doi: 10.1113/jphysiol.1981.sp013634. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Dallos P. The active cochlea. Journal of Neuroscience. 1992;12:4575–4585. doi: 10.1523/JNEUROSCI.12-12-04575.1992. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Davis H. A model for transducer action in the cochlea. Cold Spring Harbor Symposia on Quantitative Biology. 1965;30:181–190. doi: 10.1101/sqb.1965.030.01.020. [DOI] [PubMed] [Google Scholar]
  11. Davis H. An active process in cochlear mechanics. Hearing Research. 1983;9:79–90. doi: 10.1016/0378-5955(83)90136-3. [DOI] [PubMed] [Google Scholar]
  12. Echteler SM. Structural correlates of frequency-place map development. Association for Research in Otolaryngology Mid-Winter Meeting Abstracts. 1995;18:111. [Google Scholar]
  13. Echteler SM, Arjmand E, Dallos P. Developmental alterations in the frequency map of the mammalian cochlea. Nature. 1989;341:147–149. doi: 10.1038/341147a0. [DOI] [PubMed] [Google Scholar]
  14. Emadi GA, Richter CP, Dallos P. Changes of stiffness in the gerbil cochlea after the onset of hearing. Association for Research in Otolaryngology Mid-Winter Meeting Abstracts. 2001;24:27. [Google Scholar]
  15. Gold T. Hearing. II. The physical basis of the action of the cochlea. Proceedings of the Royal Society B. 1948;135:492–498. [Google Scholar]
  16. Harris DM, Dallos P. Ontogenetic changes in frequency mapping of a mammalian ear. Science. 1984;225:741–743. doi: 10.1126/science.6463651. [DOI] [PubMed] [Google Scholar]
  17. He DZ, Evans BN, Dallos P. First appearance and development of electromotility in neonatal gerbil outer hair cells. Hearing Research. 1994;78:77–90. doi: 10.1016/0378-5955(94)90046-9. [DOI] [PubMed] [Google Scholar]
  18. Hubbard AE. A traveling-wave amplifier model of the cochlea. Science. 1993;259:68–71. doi: 10.1126/science.8418496. [DOI] [PubMed] [Google Scholar]
  19. Kuhn B, Vater M. The postnatal development of F-actin in tension fibroblasts of the spiral ligament of the gerbil cochlea. Hearing Research. 1997;108:180–190. doi: 10.1016/s0378-5955(97)00051-8. [DOI] [PubMed] [Google Scholar]
  20. Lighthill J. Energy flow in the cochlea. Journal of Fluid Mechanics. 1981;106:149–213. [Google Scholar]
  21. Lippe W, Rubel EW. Development of the place principle: tonotopic organization. Science. 1983;219:514–516. doi: 10.1126/science.6823550. [DOI] [PubMed] [Google Scholar]
  22. McGuirt JP, Schmiedt RA, Schulte BA. Development of cochlear potentials in the neonatal gerbil. Hearing Research. 1995;84:52–60. doi: 10.1016/0378-5955(95)00015-v. [DOI] [PubMed] [Google Scholar]
  23. Manley GA. Ontogeny of frequency mapping in the peripheral auditory system of birds and mammals: A critical review. Auditory Neuroscience. 1996;3:199–214. [Google Scholar]
  24. Martin P, Hudspeth AJ. Active hair-bundle movements can amplify a hair cell's response to oscillatory mechanical stimuli. Proceedings of the National Academy of Sciences of the USA. 1999;96:14306–14311. doi: 10.1073/pnas.96.25.14306. [DOI] [PMC free article] [PubMed] [Google Scholar]
  25. Mills DM, Rubel EW. Development of the base of the cochlea: place code shift in the gerbil. Hearing Research. 1998;122:82–96. doi: 10.1016/s0378-5955(98)00079-3. [DOI] [PubMed] [Google Scholar]
  26. Müller M. Developmental changes of frequency representation in the rat cochlea. Hearing Research. 1991;56:1–7. doi: 10.1016/0378-5955(91)90147-2. [DOI] [PubMed] [Google Scholar]
  27. Müller M. The cochlear place-frequency map of the adult and developing Mongolian gerbil. Hearing Research. 1996;94:148–156. doi: 10.1016/0378-5955(95)00230-8. [DOI] [PubMed] [Google Scholar]
  28. Munyer PD, Schulte BA. Developmental expression of proteoglycans in the tectorial and basilar membrane of the gerbil cochlea. Hearing Research. 1995;85:85–94. doi: 10.1016/0378-5955(95)00032-y. [DOI] [PubMed] [Google Scholar]
  29. Murugasu E, Russell IJ. Salicylate ototoxicity: the effects on basilar membrane displacement, cochlear microphonics, and neural responses in the basal turn of the guinea pig cochlea. Auditory Neuroscience. 1995;1:139–150. doi: 10.1523/JNEUROSCI.16-01-00325.1996. [DOI] [PMC free article] [PubMed] [Google Scholar]
  30. Naidu RC, Mountain DC. Longitudinal coupling in the basilar membrane. Journal of the Association for Research in Otolaryngology. 2001;2:257–267. doi: 10.1007/s101620010013. [DOI] [PMC free article] [PubMed] [Google Scholar]
  31. Narayan SS, Temchin AN, Recio A, Ruggero MA. Frequency tuning of basilar membrane and auditory nerve fibers in the same cochleae. Science. 1998;282:1882–1884. doi: 10.1126/science.282.5395.1882. [DOI] [PMC free article] [PubMed] [Google Scholar]
  32. Olson ES. Direct measurement of intra-cochlear pressure waves. Nature. 1999;402:526–529. doi: 10.1038/990092. [DOI] [PubMed] [Google Scholar]
  33. Overstreet EH, Richter CP, Temchin AN, Cheatham MA, Ruggero MA. High-frequency sensitivity of the mature gerbil cochlea and its development. Audiology and Neuro-otology. 2002a doi: 10.1159/000067892. in the Press. [DOI] [PMC free article] [PubMed] [Google Scholar]
  34. Overstreet EH, Ruggero MA. The development of basilar membrane mechanics at the hook region of the Mongolian gerbil cochlea. Association for Research in Otolaryngology Mid-Winter Meeting Abstracts. 1999;22:135–136. [Google Scholar]
  35. Overstreet EH, Ruggero MA. Development of wide-band middle ear transmission in the Mongolian gerbil. Journal of the Acoustical Society of America. 2002;111:261–270. doi: 10.1121/1.1420382. [DOI] [PMC free article] [PubMed] [Google Scholar]
  36. Overstreet EH, Temchin AN, Ruggero MA. Basilar membrane vibrations near the round window of the gerbil cochlea. Journal of the Association for Research in Otolaryngology. 2002b doi: 10.1007/s101620020023. (in the Press). Published online 27 February 2002. 10.1007/s101620020023. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Pujol R, Hilding D. Anatomy and physiology of the onset of auditory function. Acta Otolaryngologica. 1973;76:1–10. doi: 10.3109/00016487309121476. [DOI] [PubMed] [Google Scholar]
  38. Pujol R, Lavigne-Rebillard M. Development of neurosensory structures in the human cochlea. Acta Otolaryngologica. 1992;112:259–264. doi: 10.1080/00016489.1992.11665415. [DOI] [PubMed] [Google Scholar]
  39. Pujol R, Lavigne-Rebillard M, Uziel A. Development of the human cochlea. Acta Otolaryngologica Supplement. 1991;482:7–12. [PubMed] [Google Scholar]
  40. Recio A, Rich NC, Narayan SS, Ruggero MA. Basilar-membrane responses to clicks at the base of the chinchilla cochlea. Journal of the Acoustical Society of America. 1998;103:1972–1989. doi: 10.1121/1.421377. [DOI] [PMC free article] [PubMed] [Google Scholar]
  41. Ren T, Nuttall AL. Basilar membrane vibration in the basal turn of the sensitive gerbil cochlea. Hearing Research. 2001;151:48–60. doi: 10.1016/s0378-5955(00)00211-2. [DOI] [PubMed] [Google Scholar]
  42. Rhode WS. Observations of the vibration of the basilar membrane in squirrel monkeys using the Mössbauer technique. Journal of the Acoustical Society of America. 1971;49:1218–1231. doi: 10.1121/1.1912485. [DOI] [PubMed] [Google Scholar]
  43. Rhode WS. An investigation of postmortem cochlear mechanics. In: Møller AR, editor. Basic Mechanisms in Hearing. New York: Academic Press; 1973. pp. 49–63. [Google Scholar]
  44. Richter CP, Dallos P. Micromechanics contribute to the shift in the frequency place code in developing gerbils. Association for Research in Otolaryngology Mid-Winter Meeting Abstracts. 2000;23:250–251. [Google Scholar]
  45. Richter CP, Edge RM, He DZZ, Dallos P. Development of the gerbil inner ear observed in the hemicochlea. Journal of the Association for Research in Otolaryngology. 2000;1:195–210. doi: 10.1007/s101620010019. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Richter CP, Evans BN, Edge R, Dallos P. Basilar membrane vibration in the gerbil hemicochlea. Journal of Neurophysiology. 1998;79:2255–2264. doi: 10.1152/jn.1998.79.5.2255. [DOI] [PubMed] [Google Scholar]
  47. Robles L, Ruggero MA. Mechanics of the mammalian cochlea. Physiological Reviews. 2001;81:1305–1352. doi: 10.1152/physrev.2001.81.3.1305. [DOI] [PMC free article] [PubMed] [Google Scholar]
  48. Robles L, Ruggero MA, Rich NC. Basilar membrane mechanics at the base of the chinchilla cochlea. I. Input-output functions, tuning curves, and response phases. Journal of the Acoustical Society of America. 1986;80:1364–1374. doi: 10.1121/1.394389. [DOI] [PubMed] [Google Scholar]
  49. Romand R. Modification of tonotopic representation in the auditory system during development. Progress in Neurobiology. 1997;51:1–17. doi: 10.1016/s0301-0082(96)00043-3. [DOI] [PubMed] [Google Scholar]
  50. Rubel EW, Ryals BM. Development of the place principle: acoustic trauma. Science. 1983;219:512–514. doi: 10.1126/science.6823549. [DOI] [PubMed] [Google Scholar]
  51. Ruggero MA. Cochlear delays and traveling waves: comments on ‘Experimental look at cochlear mechanics. Audiology. 1994;33:131–142. doi: 10.3109/00206099409071874. [DOI] [PMC free article] [PubMed] [Google Scholar]
  52. Ruggero MA, Narayan SS, Temchin AN, Recio A. Mechanical bases of frequency tuning and neural excitation at the base of the cochlea: comparison of basilar-membrane vibrations and auditory-nerve-fiber responses in chinchilla. Proceedings of the National Academy of Sciences of the USA. 2000;97:11744–11750. doi: 10.1073/pnas.97.22.11744. [DOI] [PMC free article] [PubMed] [Google Scholar]
  53. Ruggero MA, Rich NC. Application of a commercially-manufactured Doppler-shift laser velocimeter to the measurement of basilar-membrane vibration. Hearing Research. 1991a;51:215–230. doi: 10.1016/0378-5955(91)90038-b. [DOI] [PMC free article] [PubMed] [Google Scholar]
  54. Ruggero MA, Rich NC. Furosemide alters organ of Corti mechanics: evidence for feedback of outer hair cells upon the basilar membrane. Journal of Neuroscience. 1991b;11:1057–1067. doi: 10.1523/JNEUROSCI.11-04-01057.1991. [DOI] [PMC free article] [PubMed] [Google Scholar]
  55. Ruggero MA, Rich NC, Recio A. The effect of intense acoustic stimulation on basilar-membrane vibrations. Auditory Neuroscience. 1996a;2:329–345. [Google Scholar]
  56. Ruggero MA, Rich NC, Robles L, Recio A. The effects of acoustic trauma, other cochlear injury and death on basilar-membrane responses to sound. In: Axelsson A, Borchgrevink P-A, Hëllstrom D, Henderson RP, Hamernik RP, Salvi RJ, editors. Scientific Basis of Noise-Induced Hearing Loss. New York: Thieme Medical Publishers; 1996b. pp. 23–35. [Google Scholar]
  57. Russell IJ. Origin of the receptor potential in inner hair cells of the mammalian cochlea - evidence for Davis' theory. Nature. 1983;301:334–336. doi: 10.1038/301334a0. [DOI] [PubMed] [Google Scholar]
  58. Schweitzer L, Lutz C, Hobbs M, Weaver SP. Anatomical correlates of the passive properties underlying the developmental shift in the frequency map of the mammalian cochlea. Hearing Research. 1996;97:84–94. [PubMed] [Google Scholar]
  59. Sellick PM, Patuzzi R, Johnstone BM. Measurement of basilar membrane motion in the guinea pig using the Mössbauer technique. Journal of the Acoustical Society of America. 1982;72:131–141. doi: 10.1121/1.387996. [DOI] [PubMed] [Google Scholar]
  60. Shore SE, Nuttall AL. The effects of cochlear hypothermia on compound action potential tuning. Journal of the Acoustical Society of America. 1985;77:590–598. doi: 10.1121/1.391877. [DOI] [PubMed] [Google Scholar]
  61. Souter M, Nevill G, Forge A. Postnatal maturation of the organ of Corti in gerbils: morphology and physiological responses. Journal of Comparative Neurology. 1997;386:635–651. [PubMed] [Google Scholar]
  62. von BékÉsy G. Experiments in Hearing. New York: McGraw-Hill; 1960. [Google Scholar]
  63. Wilson JP. Cochlear mechanics. In: Cazals Y, Demany L, Horner K, editors. Auditory Physiology and Perception. Oxford: Pergamon Press; 1992. pp. 71–84. [Google Scholar]
  64. Woolf NK, Ryan AF. Ontogeny of neural discharge patterns in the ventral cochlear nucleus of the mongolian gerbil. Brain Research. 1985;349:131–147. doi: 10.1016/0165-3806(85)90138-5. [DOI] [PubMed] [Google Scholar]
  65. Woolf NK, Ryan AF, Harris JP. Development of mammalian endocochlear potential: normal ontogeny and effects of anoxia. American Journal of Physiology. 1986;250:R493–498. doi: 10.1152/ajpregu.1986.250.3.R493. [DOI] [PubMed] [Google Scholar]
  66. Yancey C, Dallos P. Ontogenic changes in cochlear characteristic frequency at a basal turn location as reflected in the summating potential. Hearing Research. 1985;18:189–195. doi: 10.1016/0378-5955(85)90011-5. [DOI] [PubMed] [Google Scholar]
  67. Zwislocki JJ, Kletsky EJ. Tectorial membrane: a possible effect on frequency analysis in the cochlea. Science. 1979;204:639–641. doi: 10.1126/science.432671. [DOI] [PubMed] [Google Scholar]

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