Hair Cells Of Cochlea Research Articles

Intracellular studies of hair cells in the mammalian cochlea.

1. Intracellular recordings were made from inner hair cells in the first turn of the guinea-pig cochlea, the recording sites being confirmed by the injection of Procion yellow dye and subsequent histology. 2. The receptor potential, in response to a pure tone burst, consisted of an AC response which followed the wave form of the stimulus and was analogous to the extracellularly recorded cochlear microphonic and a depolarizating DC response which followed the envelope of the tone burst and was analogous to the extracellularly recorded summating potential. 3. The DC response was broadly tuned at high sound pressure having a maximal amplitude of 27 mV at a sound pressure level of ca. 100 db; however the bandwidth of the response was reduced at lower sound pressure level. Isoamplitude curves for the DC response were indistinguishable from the threshold curves for auditory nerve fibres. 4. The AC response was tuned in a similar fashion to the DC response except that it was attenuated at 6-9 db/octave with respect to the DC response. It is suggested that this difference was due to the effect of membrane capacitance and resistance on the AC response. In contrast the extracellularly recorded AC component was not subject to this attenuation. 5. The total resistance and capacitance in three cells were found to be 46-61 Momega and 7.8-15.8 muF respectively. 6. Intracellular resistance changes were measured during sound stimulation, the resistance change being proportional to the DC receptor potential, indicating constant current flow through the hair cell. The current varied between 0.37 and 0.81 nA between cells. The time constant for seven cells was found to lie between 0.31 and 0.76 msec. 7. A map of the basilar membrane showing position of hair cells against characteristic frequency corresponded to the cut-off frequencies of the basilar membrane mechanical measurements and the innervation sites of spiral ganglion cells.

Extra internal hair cells. A scanning electron microscopic study.

The extra internal hair cell (EIHC) of the human cochlea was observed by means of a scanning electron microscope. The EIHC was found not infrequently in all turns of the human cochlea. It was located medial to the IHC row. The inner pillar cells showed an abnormal structure. The anatomical relationships between the displaced IHC and EIHC, and the inner pillar cell were classified into five types. The origin of these anomalies is discussed form an embryological viewpoint.

Tuning properties of cochlear hair cells

THE mammalian cochlea is generally considered to be a frequency analyser. When the ear is stimulated by a pure tone the basilar membrane vibrates. The amplitude of the vibrations varies along the length of the membrane with the position of the maximum depending in a graded way upon the frequency of the tone1,2. The basal turn responds maximally to high frequencies, the apex to low frequencies. Consequently hair cells situated along the membrane are frequency selective. The nerve fibres innervating the basilar membrane have low thresholds over a narrow range of frequencies corresponding to the displacement maxima of the section of the basilar membrane that they innervate3. But the neural tuning curves (the relationship between threshold and frequency for individual nerve fibres) are much sharper than the mechanical tuning curves of the basilar membrane (the relationship between amplitude of displacement and frequency at a single point on the membrane) and this discrepancy has led to the proposal of a “second filter”4. In this paper we describe the tuning of intracellularly recorded receptor potentials from histologically identified hair cells (IHC) of the guinea pig cochlea in an effort to investigate the nature of the hypothetical sharpening process and we conclude that IHCs are tuned as sharply as auditory nerve fibres. To our knowledge this is the first time intracellular recordings have been made from inner hair cells in the mammalian cochlea.

Atoxyl-Induced damage to the sensory cells in the organ of corti in the guinea pig cochlea

Atoxyl-induced damage to the inner and the outer hair cells in the guinea pig cochlea starts in the apical coil and spreads successively downward toward the round window. Vesicular degeneration of both the inner and the outer hair cells and the cell organelles occurs. The degeneration is probably caused by an alteration in ionic composition of the endolymph resulting from primary changes of the stria vascularis (vesiculation and rejection of degenerated marginal and intermediate cells) as reported by Anniko and Wersäll (1975). However, a direct action of atoxyl on the sensory cells of the organ of Corti cannot be excluded.

Cochlear innervation in the guinea pig. I. The inner spiral bundle region.

The organization of nerve fibers in the vicinity of the inner hair cells of the guinea pig cochlea was studied in silver-stained whole-mount specimens by light microscopy. In all turns of the organ of Corti the inner spiral bundle (ISB) was found to contain both short and long spiral fascicles coursing toward either the base or the apex. The ISB is largest and its organization is most complex in the upper basal and lower second turns. It becomes markedly reduced in size in the apical and lower basal turns. Many ISB fibers cross the tunnel of Corti to the outer hair cells. Surgical interruption of the efferent nerve supply in the brain stem resulted in degeneration of all ISB fibers. After elimination of the efferents it could be seen that the majority of afferent fibers end on the inner hair cells.

Translation of auditory stimuli into neural codes.

A recent theory has hypothesized that the brain works on the basis of encoding information as the intervals between the individual spikes in spike trains. The number of spikes in spike trains is thought to be a constant within a given species, and 22 spikes has been hypothesized for humans. Microscopic evidence that the hair cells of the human cochlea have 22 hairs each is thought to support the basic theory. This evidence, in conjunction with the assumptions of the basic theory, is used to provide an explanatory framework for the basis of neural coding of auditory frequency and amplitude.

Structure of the nerve endings on the external hair cells of the guinea pig cochlea as studied by serial sections

The nerve endings about the external hair cells of the organ of Corti in the guinea pig have been studied by means of serial sections, and plastic reconstructions. Three kinds of nerve endings were found most often. Type 1 is small ending, containing a few or a moderate number of vesicles. A dense osmiophilic rod with vesicles about it is often found in the hair cell cytoplasm, adjacent to this ending. Type 2 is a large ending, filled with vesicles and mitochondria. A pair of membranes is always found in the hair cell cytoplasm covering the synaptic membrane adjacent to the Type 2 ending. Type 2a is similar in size and cytoplasmic contents to Type 2, but it only touches the hair cell, whereas it has wide contact with other neural structures. Other endings, not conforming to the above, are less frequently found. Two types of hair cells, classed according to the distribution of these endings, are present. Type A shows Type 1 and Type 2 endings in approximately equal numbers, the total number being less in the basal turn. Type B shows a few Type 2 and Type 2a endings, and many Type 1 endings. These hair cells are found in an orderly arrangement along the basilar membrane. Only Type A cells were observed in the first turn of the cochlea. The second turn showed Type B cells in the third row. The third and fourth turns showed Type B cells in the second and third rows.

A synaptic structure in the hair cells of the guinea pig cochlea

A synaptic structure has been observed in the external and internal hair cells of the guinea pig cochlea. It is a dense osmiophilic rod-like structure, surrounded by vesicles. In the external hair cell it is present only adjacent to the small type 1 nerve endings.