| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
CLASSICAL PERSPECTIVES |
1 School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK
2 Institute for Science and Technology in Medicine and School of Life Sciences, Keele University, Keele ST5 5BG, UK
Email: c.j.kros{at}sussex.ac.uk and m.g.evans{at}keele.ac.uk
A major quest in hearing research over the last 35 years or so has been, and continues to be, how mammalian hearing achieves its remarkable frequency selectivity and sensitivity over a large frequency range, down from 20 Hz up to over 100 kHz in some species. Readers of The Journal of Physiology have had a ringside seat over the years enabling them to follow how this quest has led to the current state of the debate, which is focused on the precise role of the outer hair cells in cochlear tuning and amplification (Dallos et al. 2006; Fettiplace, 2006). How did it all begin? Evans (1972) found that auditory nerve fibres in the guinea-pig were much more sharply tuned than the basilar membrane vibrations that had been reported in the same species. He noted that if the physiological condition of the cochlea was in any way compromised the nerve fibres became less sensitive and more broadly tuned, just like the basilar membrane vibrations. To explain his findings Evans proposed a physiologically vulnerable ‘second filter’, situated somewhere between the broad ‘first filter’ of the basilar membrane and the auditory nerve fibres. The search was on to find the whereabouts of this sharply tuned band-pass filter.
One obvious place to look was the mechanosensory hair cells inside the cochlea, but attempts to use microelectrodes to record receptor potentials from mammalian cochlear hair cells had proved unsuccessful for many years, due to the difficulty of gaining access to these fragile cells buried deep within the temporal bone. Two very different approaches were used to try and overcome this problem. The first was to stubbornly persist in trying to record from mammalian cochlear hair cells hoping that success would come at some point. The other was to turn to more robust auditory organs of lower vertebrates with a simpler architecture, in the hope that similar principles might nevertheless apply. These preparations also had the advantage that the hair cells survived much better in vitro than their fragile mammalian counterparts, allowing for more stable recording conditions and more elaborate experimental manipulations than could be achieved in vivo. The two papers that are the subject of this Classical Perspective (Russell & Sellick, 1978; Crawford & Fettiplace, 1981) are some of the earliest successful examples of both approaches.
Russell & Sellick (1978) were the first to succeed in recording receptor potentials from inner hair cells (from which most of the afferent fibres of the auditory nerve derive) situated in the guinea-pig cochlea in vivo. One of the novel findings from these heroic experiments was that the receptor potentials consisted of two components: one was at the frequency of the stimulus tone (‘AC response’), the other was a steady depolarization (‘DC response’). The AC response was progressively attenuated as the sound frequency was increased and the authors concluded that this must be due to filtering by the electrical (‘RC’) time constant of the inner hair cell membrane, leaving only the smooth depolarizing DC response for high-frequency tones above a few kilohertz. The mammalian cochlea is tonotopically organized, its apical end responding best to low-frequency sounds and its basal end to high frequencies. At the recording location used, inner hair cells responded best to frequencies between 14 and 32 kHz at which the AC responses were strongly attenuated. This prompted the conclusion that in the basal, high-frequency end of the cochlea the DC component is the one solely responsible for neural excitation. At this point it was not known why the receptor potentials were asymmetrical (later in vitro experiments showed this to be a property of the mechano-electrical transducer currents), but it was clearly necessary for signalling the reception of high-frequency sounds. In a follow-up paper (Russell & Sellick, 1983) it was noted that the progressive attenuation of the AC component of the receptor potential was correlated with the decrease in phase locking (the ability to preserve the periodicity of an auditory stimulus) of auditory nerve fibre responses at frequencies above a kilohertz or so.
This work also got a step closer to answering the vexed question of the ‘second filter’: the inner hair cells turned out to be as sharply tuned as the auditory nerve fibres, placing the filter somewhere between the broadly tuned basilar membrane and the inner hair cells. Could it be in the inner hair cells themselves? The lateral approach taken in the other classical paper, by Crawford & Fettiplace (1981), gave some intriguing clues by analogy. They presented microelectrode recordings from hair cells situated in the basilar papilla, the auditory organ of the turtle. The recordings were done in vitro in an isolated half-head preparation, so sound stimulation could be applied through the ear canal. In reptiles too, the auditory nerve fibres were found to be much more sharply tuned than the basilar membrane. A previous paper (Crawford & Fettiplace, 1980) had already established that auditory nerve fibres and hair cells of the turtle were equally sharply tuned, and that the hair cells were tonotopically arranged on the basilar membrane (just as in mammals, but only over a restricted frequency range of a few tens of hertz to 700 Hz). The remarkable observation elaborated in Crawford & Fettiplace (1981) was that, if a small electrical current was injected through the recording microelectrode inserted in a hair cell, the membrane potential oscillated at the same frequency to which the cell was most sensitive acoustically. Moreover, the rate at which these oscillations died out indicated that the sharpness of the ‘electrical tuning’ of these cells was strongly correlated with that of their tuning to sound stimulation. This applied to hair cells across the frequency range of turtle hearing, leading to the conclusion that, in the turtle, sharp frequency tuning must originate in the electrical properties of the hair cells. Clearly either the referees or the authors were not quite ready to believe this finding at a time when models of the cochlea were built by physicists and engineers fond of pendulums, springs and dashpots, for a series of experiments is addressed to the question: ‘does current injection move the intracellular electrode?’.
How could this ‘electrical resonance’ be explained in terms of the physiology of the hair cells? The authors briefly mention that the resonance could be electromechanical, but in the absence of evidence for the involvement of a mechanical element this was not pursued at the time. Instead, it was postulated that the mechanism was purely electrical, mainly due to a voltage-dependent K+ conductance in the basolateral membrane rapidly opposing any membrane depolarization, whether imposed by the transducer current or artificially by current injection, and thus setting up a voltage oscillation. One of the key pieces of evidence for this hypothesis was that the resonance disappeared when current injection hyperpolarized the hair cell beyond the K+ equilibrium potential of about –80 mV. The paper mentions ‘a major difficulty’ with this hypothesis, namely how could the frequency of the resonance change systematically in a tonotopic fashion? It was suggested that this could be achieved by changing the density of the – at this stage presumed – K+ channels from cell to cell. To study whether such ion channels were indeed present in hair cells required the ability to voltage clamp the cells, something not possible with a microelectrode due to its high resistance. The advent of the patch-clamp technique brought the required breakthrough. This enabled Art & Fettiplace (1987) to establish that individual turtle hair cells retained their electrical resonance when isolated from the basilar papilla, confirming that sharp tuning in the turtle auditory system is indeed exclusively a property of the hair cells, and to demonstrate that the tonotopic frequency gradient is brought about by a gradual variation of the kinetics and density of Ca2+-activated K+ channels. Using an isolated basilar papilla preparation, the electrical resonance was even found to be coupled to a force-generating mechanism in the hair bundles, which resonated at the same frequency, confirming the participation of a mechanical element after all (Crawford & Fettiplace, 1985).
Here we had an intellectually satisfying mechanism to ensure sharp tuning over the complete frequency range in an auditory organ, but how general was it? Electrical resonance supported by Ca2+-activated K+ currents has been studied in hair cells isolated from the bullfrog sacculus (Hudspeth & Lewis, 1988) and from the chick cochlea (Fuchs & Evans, 1990). The first patch clamp recordings of isolated mammalian inner hair cells showed that, despite having not dissimilar K+ conductances, they did not have any significant electrical resonance (Kros & Crawford, 1990) and neither did outer hair cells (Housley & Ashmore, 1992). While it is good to retain some scepticism about recordings from isolated mammalian hair cells, as they are much more easily damaged than their lower-vertebrate counterparts, a reasonable explanation for their lack of electrical resonance is that the much extended frequency range of mammals would require unfeasibly high channel densities and unrealistically fast kinetics. Instead, we need to consider progress made with the other strand of investigation mentioned at the beginning, that of recording from mammalian cochlear hair cells in vivo. A key observation was made by Brown & Nuttall (1984), who found that stimulation of efferent nerve fibres, which contact outer, but not inner, hair cells, reduced the sensitivity of inner hair cells in the guinea-pig cochlea (Fig. 1). This could only be explained if the outer hair cells could somehow change the micromechanics of the cochlea. Soon after Brownell et al. (1985) reported a fascinating phenomenon: isolated outer hair cells could rapidly change their length in response to injection of current steps through a microelectrode. Patch-clamp studies showed that cell length was controlled by membrane voltage and that cell motility was not dependent on actin–myosin interactions (Ashmore, 1987). These observations provided the basis for the current hypothesis that there is a firm division of labour between sensory inner hair cells and mechanically active outer hair cells in the mammalian cochlea. The outer hair cells, acting as the cochlea's force generators, are clearly crucial for sharp tuning and high sensitivity. The forces could be due either to prestin-mediated electromotility or associated with hair bundle movements analogous to those first described in turtle hair cells (Crawford & Fettiplace, 1985), or both (see Dallos et al. 2006; Fettiplace, 2006). Meanwhile, techniques for recording basilar membrane mechanics had also steadily improved since the early efforts by Von Békésy (1960), and it was found that the basilar membrane was after all as sharply tuned as the nerve fibres provided the cochlea, and in particular the outer hair cells, were in tip-top condition (e.g. Sellick et al. 1982). It is a fortunate fluke of history that the basilar membrane did not initially seem to be sharply tuned, as otherwise the search for the second filter, which has taught us so much about the physiology of hair cells in mammals and lower vertebrates, might never have started.
|
References
Art
JJ
&
Fettiplace
R (1987). Variation of membrane properties in hair cells isolated from the turtle cochlea. J Physiol
385, 207–242.
Ashmore
JF (1987). A fast motile response in guinea-pig outer hair cells: the cellular basis of the cochlear amplifier. J Physiol
388, 323–347.
Brown
MC
&
Nuttall
AL (1984). Efferent control of cochlear inner hair cell responses in the guinea-pig. J Physiol
354, 625–646.
Brownell WE, Bader CR, Bertrand D, de Ribaupierre Y (1985). Evoked Mechanical responses of isolated cochlear outer hair cells. Science 227, 191–196.
Crawford
AC
&
Fettiplace
R (1980). The frequency selectivity of auditory nerve fibres and hair cells in the cochlea of the turtle. J Physiol
306, 79–125.
Crawford
AC
&
Fettiplace
R (1981). An electrical tuning mechanism in turtle cochlear hair cells. J Physiol
312, 377–412.
Crawford
AC
&
Fettiplace
R (1985). The mechanical properties of ciliary bundles of turtle cochlear hair cells. J Physiol
364, 359–379.
Dallos
P, Zheng
J
&
Cheatham
MA (2006). Prestin and the cochlear amplifier. J Physiol
576, 37–42.
Evans
EF (1972). The frequency response and other properties of single fibres in the guinea-pig cochlear nerve. J Physiol
226, 263–287.
Fettiplace
R (2006). Active hair bundle movements in auditory hair cells. J Physiol
576, 29–36.
Fuchs
PA
&
Evans
MG (1990). Potassium currents in hair cells isolated from the cochlea of the chick. J Physiol
429, 529–551.
Housley
GD
&
Ashmore
YF (1992). Ionic currents of outer hair cells isolated from the guinea-pig cochlea. J Physiol
448, 73–98.
Hudspeth
AJ
&
Lewis
RS (1988). Kinetic analysis of voltage- and ion-dependent conductances in saccular hair cells of the bull-frog, Rana catesbeiana. J Physiol
400, 237–274.
Kros
CJ
&
Crawford
AC (1990). Potassium currents in inner hair cells isolated from the guinea-pig cochlea. J Physiol
421, 263–291.
Russell
IJ
&
Sellick
PM (1978). Intracellular studies of hair cells in the mammalian cochlea. J Physiol
284, 261–290.
Russell
IJ
&
Sellick
PM (1983). Low-frequency characteristics of intracellularly recorded receptor potentials in guinea-pig cochlear hair cells. J Physiol
338, 179–206.
Sellick PM, Patuzzi R, Johnstone BM (1982). Measurement of basilar membrane motion in the guinea pig using the Mössbauer technique. J Acoust Soc Am 72, 131–141.[CrossRef][Medline]
von Békésy G (1960). Experiments in Hearing. McGraw-Hill, New York.
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
