Further scanning electron microscope studies of lizard auditory papillae

Somatic motility and hair bundle mechanics, are both necessary for cochlear amplification?

Hearing organs have evolved to detect sounds across several orders of magnitude of both intensity and frequency. Detection limits are at the atomic level despite the energy associated with sound being limited thermodynamically. Several mechanisms have evolved to account for the remarkable frequency selectivity, dynamic range, and sensitivity of these various hearing organs, together termed the active process or cochlear amplifier. Similarities between hearing organs of disparate species provides insight into the factors driving the development of the cochlear amplifier. These properties include: a tonotopic map, the emergence of a two hair cell system, the separation of efferent and afferent innervations, the role of the tectorial membrane, and the shift from intrinsic tuning and amplification to a more end organ driven process. Two major contributors to the active process are hair bundle mechanics and outer hair cell electromotility, the former present in all hair cell organs tested, the latter only present in mammalian cochlear outer hair cells. Both of these processes have advantages and disadvantages, and how these processes interact to generate the active process in the mammalian system is highly disputed. A hypothesis is put forth suggesting that hair bundle mechanics provides amplification and filtering in most hair cells, while in mammalian cochlea, outer hair cell motility provides the amplification on a cycle by cycle basis driven by the hair bundle that provides frequency selectivity (in concert with the tectorial membrane) and compressive nonlinearity. Separating components of the active process may provide additional sites for regulation of this process.

Spontaneous otoacoustic emissions in lizards: a comparison of the skink-like lizard families Cordylidae and Gerrhosauridae

Lizard families can be grouped into larger units comprising those families that are closely related and whose auditory papillae are morphologically very similar. Based on the few species studied at that time [Manley, G.A., 1997. Diversity in hearing-organ structure and the characteristics of spontaneous otoacoustic emissions in lizards. In: Lewis, E.R., Long, G.R., Lyon, R.F., Narins, P.M., Steele, C.R. (Eds.), Diversity in Auditory Mechanics. World Scientific Publishing Co., Singapore, pp. 32-38], it was suggested that SOAE spectral patterns are strongly influenced by papillar anatomy. However, in two family groups, only one single species has been studied and we have no data on the regularity of pattern within related lizard families. Within the group of skink-like lizards, whose papillae all have salletal tectorial structures, the only detailed SOAE studies so far were on the skink genus Tiliqua. To ascertain the similarity of SOAE in species from families related to the skinks, we have studied one species each from two families that are closely related to skinks, the Cordylidae (Girdle-tailed lizards) and the Gerrhosauridae (plated lizards). Gerrhosaurus and Cordylus have a similar number and amplitudes of SOAE to Tiliqua (Skinkidae). The maximal frequency shifts of SOAE under the influence of external tones is also similar to that of Tiliqua. However, the maximal suppression and maximal facilitation are smaller. In general, the patterns displayed by the SOAE of lizards of these two new families are recognizably similar to the skink Tiliqua, suggesting that the anatomy of the papilla and the tectorial structures do play an important role in determining how SOAE are manifested in papillae that possess tectorial sallets.

Innervation of a lizard auditory organ having gap junctions between most hair cells: a serial transmission electron microscopy study

Two apical unidirectional and 16 basal bidirectional papillar hair cells of the yucca night lizard, Xantusia vigilis, were serially sectioned for transmission electron microscopy (TEM) to determine the pattern of hair cell innervation. The 16 bidirectional hair cells (central group) were sectioned across the entire width of the papilla and consisted of four complete hair cells in each of the first three rows and the upper (or neural) half of the four hair cells in the fourth or last row. Both hair cell types were nonexclusively innervated, i.e., each afferent nerve fiber innervated two or more hair cells. The apical unidirectional hair cells were innervated by six or seven different afferent nerve fibers and five or six efferent fibers. The afferent nerve fibers made an average of 52.5 synapses/hair cell. In the central group of 16 bidirectional hair cells, 25 different afferent nerve fibers innervated an average of 4.5 hair cells. The average number of hair cells innervated by the eight afferent nerve fibers limited to the central group was 5.4. An unusual finding was the presence of gap junctions directly interconnecting more than half the hair cells in both papillar segments. In the bidirectional hair cell region, it was possible to count the number of gap junctions between 24 contiguous hair cells. The average number of gap junctions was four per hair cell, and all bidirectional hair cells were either directly or indirectly interconnected by gap junctions. The possible functions of a nonexclusive type of hair cell innervation and the presence of large numbers of gap junctions are discussed.

How the ear's works work

The senses of hearing and equilibrium depend on sensory receptors called hair cells which can detect motions of atomic dimensions and respond more than 100,000 times a second. Biophysical studies suggest that mechanical forces control the opening and closing of transduction channels by acting through elastic components in each hair cell's mechanoreceptive hair bundle. Other ion channels, as well as the mechanical and hydrodynamic properties of hair bundles, tune individual hair cells to particular frequencies of stimulation.

The organization of tip links and stereocilia on hair cells of bird and lizard basilar papillae

Auditory papillae from three species of bird (pigeon, starling, and chick), and two species of European lizard (Podarcis muralis and Podarcis sicula) were examined by scanning electron microscopy. Hair bundles from all papillae showed tip links oriented along the direction of gradation in heights of the stereocilia (i.e. parallel to the hair-cell axis of bilateral symmetry, and so parallel to the excitatory-inhibitory axis for mechanotransduction). This orientation was seen irrespective of the overall orientation of the hair bundle within the papilla. The stereocilia formed columns, joined by the tip links, which ran parallel to the hair-cell axis of bilateral symmetry. The stereocilia within the same column tended to stay together, while those in different columns tended to separate during preparation. In many columns all the stereocilia tended to be a little taller, or a little shorter, than the equivalent stereocilia in adjacent columns, suggesting that all the stereocilia within one column had been affected by a common height determinant during development. In addition, links running laterally between stereocilia were seen, in a band near the base of the stereocilia. The results are consistent with the hypothesis that tip links are a universal feature of mechano-transducing acousticolateral hair cells, and that they are involved in sensory transduction. The results also support suggestions that the tip links may play a role in determining the heights of the stereocilia during development.

The actin filament content of hair cells of the bird cochlea is nearly constant even though the length, width, and number of stereocilia vary depending on the hair cell location

By direct counts off scanning electron micrographs, we determined the number of stereocilia per hair cell of the chicken cochlea as a function of the position of the hair cell on the cochlea. Micrographs of thin cross sections of stereociliary bundles located at known positions on the cochlea were enlarged and the total number of actin filaments per stereocilium was counted and recorded. By comparing the counts of filament number with measurements of actin filament bundle width of the same stereocilium, we were able to relate actin filament bundle width to filament number with an error margin (r2) of 16%. Combining this data with data already published or in the process of publication from our laboratory on the length and width of stereocilia, we were able to calculate the total length of actin filaments present in stereociliary bundles of hair cells located at a variety of positions on the cochlea. We found that stereociliary bundles of hair cells contain 80,000-98,000 micron of actin filament, i.e., the concentration of actin is constant in all hair cells with a range of values that is less than our error in measurement and/or biological variation, the greatest variation being in relating the diameters of the stereocilia to filament number. We also calculated the membrane surface needed to cover the stereocilia of hair cells located throughout the cochlea. The values (172-192 micron 2) are also constant. The implications of our observation that the total amount of actin is constant even though the length, width, and number of stereocilia per hair cell vary are discussed.

Auditory receptor of the red-eared turtle: I. General ultrastructure

The auditory receptor of the red-eared turtle has been the subject of intensive electrophysiological study within the last decade, yet the details of its ultrastructure have remained uninvestigated. In the present report information is derived from an analysis of specimens prepared for light, scanning electron, and transmission electron microscopy. Attention is focused on the ultrastructure of hair cells, supporting cells, and nerve fibers within the sensory epithelium as well as the basilar membrane upon which it rests. A description of the receptor's relations to surrounding sensory epithelia, the limbus of the cochlear duct, and the basilar membrane is also included. Observations are discussed in the light of similar information from other reptilian auditory receptors and the mammalian organ of Corti.

Auditory hair cell innervational patterns in lizards

The pattern of afferent and efferent innervation of two to four unidirectional (UHC) and two to nine bidirectional (BHC) hair cells of five different types of lizard auditory papillae was determined by reconstruction of serial TEM sections. The species studies were Crotaphytus wislizeni (iguanid), Podarcis (Lacerta) sicula and P. muralis (lacertids), Ameiva ameiva (teiid), Coleonyx variegatus (gekkonid), and Mabuya multifasciata (scincid). The main object was to determine in which species and in which hair cell types the nerve fibers were innervating only one (exclusive innervation), or two or more hair cells (nonexclusive innervation); how many nerve fibers were supplying each hair cell; how many synapses were made by the innervating fibers; and the total number of synapses on each hair cell. In the species studies, efferent innervation was limited to the UHC, and except for the iguanid, C. wislizeni, it was nonexclusive, each fiber supplying two or more hair cells. Afferent innervation varied both with the species and the hair cell types. In Crotaphytus, both the UHC and the BHC were exclusively innervated. In Podarcis and Ameiva, the UHC were innervated exclusively by some fibers but nonexclusively by others (mixed pattern). In Coleonyx, the UHC were exclusively innervated but the BHC were nonexclusively innervated. In Mabuya, both the UHC and BHC were nonexclusively innervated. The number of afferent nerve fibers and the number of afferent synapses were always larger in the UHC than in the BHC. In Ameiva, Podarcis, and Mabuya, groups of bidirectionally oriented hair cells occur in regions of cytologically distinct UHC, and in Ameiva, unidirectionally oriented hair cells occur in cytologically distinct BHC regions.

The distribution of hair cell bundle lengths and orientations suggests an unexpected pattern of hair cell stimulation in the chick cochlea

A detailed analysis of the morphological polarity of the hair cell bundles on the chick cochlea was carried out. Although the pattern is identical from cochlea to cochlea, the morphological polarity of the bundles varies at different positions on the cochlea. More specifically, the hair cell bundles located immediately adjacent to the inferior and superior edges are oriented with their morphological polarity perpendicular to the margins. As we move across the cochlea (transect it), there is a gradual rotation in the polarity of the bundles so that in the center of the cochlea the hair cells are oriented at an angle to those at the edges. As we continue to the superior edge the polarity gradually rotates back again. The amount of rotation depends on the position of the transect such that at the extreme proximal end there is little rotation, while at the distal end the rotation is up to 90 degrees. The rotation is always in the same direction with the tallest rows of stereocilia nearest the distal end of the cochlea. Measurements of the length of the longest stereocilia in the hair cell bundles revealed that not only are the bundles systematically longer from the proximal to distal end of the cochlea, but also the hair cells on the superior edge are significantly longer than those on the inferior edge at the same distance from one end of the cochlea. If we draw on micrographs of the cochlea contour lines through hair cells whose stereocilia are the same height, these lines coincide with the morphological polarity of the hair cells included in these contours. Furthermore analysis of damage to the cochlea induced by pure tones of high intensity also roughly follows the same contour lines. We conclude that unlike what has been thought, the stimulation of hair cells by pure tones may not occur in a strictly transverse pattern, but instead may follow the oblique contours demonstrated here.

Neural tuning in the granite spiny lizard

Scanning electron microscopy was used to examine the basilar papilla of the granite spiny lizard. The papilla contains three distinct hair cell populations: an apical and a basal population with free-standing cilia, and a central population with a tectorial membrane. In the free-standing populations, stereocilium length decreases towards the ends of the papilla. Ciliary tuft morphology differs in the free-standing and the tectorial membrane populations, except that several of the free-standing hair cells with the shortest stereocilia have a tuft morphology like the hair cells in the tectorial membrane population. On the basis of single-fiber physiology, auditory nerve fibers can be divided into a low characteristic frequency (CF) and a high CF population. Mappings of the tonotopic organization of the nerve demonstrated two groups of high CF fibers that correspond to the two free-standing hair cell populations. The low CF fibers are associated with the tectorial membrane hair cell population. Fiber CF correlated with hair cell cilium length, not position on basilar membrane, for hair cells with free-standing cilia. Tonotopic organization of high CF fibers could be predicted reasonably well from the histogram of fiber CFs.

The cellular basis of hearing: the biophysics of hair cells

A crucial event in the hearing process is the transduction of mechanical stimuli into electrical signals by hair cells, the sensory receptors of the internal ear. Stimulation results in the rapid opening of ionic channels in the mechanically sensitive organelles of these cells, their hair bundles. These transduction channels, which are nonselectively permeable, are directly excited by hair-bundle displacement. Hair cells are selectively responsive to particular frequencies of stimulation, both due to the mechanical properties of their hair bundles and because of an ensemble of ionic channels that constitute an electrical resonator.

Quantitative studies of auditory hair cells and nerves in lizards

Because the lizard cochlear duct is anatomically accessible as well as relatively simple in structure it is an excellent model in which to study auditory hair cells, nerve fibers, and innervational patterns. The objectives of this study were to determine the intra- and interspecific variations of auditory hair cell and nerve fiber numbers, nerve fiber/hair cell ratios, and nerve fiber sizes in a varied of lizard species and to relate these to auditory function and phylogeny. Hair cell numbers were determined by SEM and serial frontal sections of the papilla basilaris and nerve fiber numbers and diameters by use of a Zeiss TGZ3 particle counter. The coefficient of variation of hair cell numbers varied from 3.2 to 16.6 (171 specimens, 15 species) and of nerve fiber numbers from 1.2 to 14.4 (381 specimens, 35 species). There was no correlation between hair cell or nerve fiber number and age or sex. The nerve fiber number/hair cell number ratio was 3.5-11.1/1 in small papillae basilares of the iguanid-agamid-anguid type, 2.4-3.2/1 in the teiid type, and 0.6-1.5/1 in the larger specialized papillae of the scincid and gekkonid types. Nerve fibers varied in diameter from 0.8 to 6.0 microns (largest percentage were 2-4 microns) and were unimodally distributed. Larger nerve fibers usually supplied the unidirectionally oriented hair cells of the papilla basilaris. Variations in hair cell and nerve fiber numbers in other vertebrate classes and the functional and phylogenetic aspects of lizard papilla basilaris structure and innervation are discussed.

Mechanoelectrical transducer has discrete conductances in the chick vestibular hair cell

Properties of mechanoelectrical transduction were studied at the single-cell level by applying a whole-cell recording variation of the patch-clamp technique to dissociated vestibular hair cells of chicks. The hair bundle was directly stimulated by a glass rod, and transduction currents were recorded from the cell body. After a triangular movement of the stimulating probe, the transduction current was generated stepwise between discrete levels of amplitude. The minimum step amplitude was -1.8 pA at -27 mV in Na-containing normal saline.

Scanning electron microscope studies of the auditory papillae of some iguanid lizards

The papillae basilares of 16 species (10 general) of iguanid lizards were studied by scanning electron microscopy. Variations in the surface structures of the auditory papillae showed the following major differences: 1)papillae with localization of the unidirectional hair cells at the apical end of the papilla (anolis carolinensis); 2)papillae with absence or loss of a portion of the apical bidirectional hair-cell segment (Basiliscus basiliscus); 3)papillae with a central, short ciliated, unidirectional hair-cell segment, and 3-6 irregular rows of long-ciliated bidirectional hair cells located in the apical and basal regions (Iguana iguana, Crotaphytus collaris, C. wislizeni, Dipsosaurus dorsalis, Sauromalus obesus); 4)papillae in which the apical and basal bidirectional hair-cell segments are reduced to two rows of hair cells (Sceloporus occidentalis, S. clarki, S. orcutti, S. jarrovi, S. undulatus, S. magister, Callisaurus draconoides, Uta stansburiana). The above differences in auditory papilla structure agree closely with other anatomical differences that delineate iguanid assemblages. Thus the species in the four groups above fall respectively into the following iguanid groups: 1)anolines, 2)basiliscines, 3)iguanines, and 4)sceloporines.

Evolution of inner-ear auditory apparatus in the frog

The morphology of typical anuran amphibian papillae is thoroughly distinct from that of urodeles. However, the morphological discontinuity lies not between the frogs and the salamanders, but between the most primitive living frog, Ascaphus truei, and the more derived anurans. Three features distinguishing the papillae of more derived anurans from that of Ascaphus apparently provide peripheral tonotopy in the former. The adaptive significance of a fourth feature, kinociliary bulbs, is not clear.

An electrical tuning mechanism in turtle cochlear hair cells

1. Intracellular recordings were made from single cochlear hair cells in the isolated half-head of the turtle. The electrical responses of the cells were recorded under two conditions: (a) when the ear was stimulated with low-intensity tones of different frequencies and (b) when current steps were injected through the intracellular electrode. The aim of the experiments was to evaluate the extent to which the cochlea's frequency selectivity could be accounted for by the electrical properties of the hair cells.2. At low levels of acoustic stimulation, the amplitude of the hair cell's receptor potential was proportional to sound pressure. The linear tuning curve, which is defined as the sensitivity of the cell as a function of frequency when the cell is operating in its linear range, was measured for a number of hair cells with characteristic frequencies from 86 Hz to 425 Hz.3. A rectangular current passed into a hair cell elicited a membrane potential change consisting of a damped oscillation superimposed on a step. Small currents produced symmetrical oscillations at the beginning and end of the pulse. Larger currents increased the initial ringing frequency if depolarizing and decreased it if hyperpolarizing.4. For small currents the frequency of the oscillations and the quality factor (Q) of the electrical resonance derived from the decay of the oscillations were close to the characteristic frequency and Q of the hair-cell linear tuning curve obtained from sound presentations.5. The hair cell's membrane potential change to small-current pulses or low-intensity tone bursts could be largely described by representing the hair cell as a simple electrical resonator consisting of an inductance, resistor and capacitor.6. When step displacements of 29-250 nm were applied to a micropipette, placed just outside a hair cell in the basilar papilla, an initial periodic firing of impulses could be recorded from single fibres in the auditory nerve. Currents of up to 1 nA, injected through the same micropipette, failed to produce any change in the auditory nerve discharge. The experiment demonstrates that current injection does not produce gross movements of the electrode tip.7. The contribution of the electrical resonance to hair-cell tuning was assessed by dividing the linear tuning curve by the cell's impedance as a function of frequency. The procedure assumes that the electrical resonance is independent of other filtering stages, and on this assumption the resonance can account for the tip of the acoustical tuning curve.8. The residual filter produced by the division was broad; it exhibited a high-frequency roll-off with a corner frequency at 500-600 Hz, similar in all cells, and a low-frequency roll-off, with a corner frequency from 30 to 350 Hz which varied from cell to cell but was uncorrelated with the characteristic frequency of the cell.9. The phase of the receptor potential relative to the sound pressure at the tympanum was measured in ten cells. For low intensities the phase characteristic was independent of the sound pressure. At low frequencies the receptor potential led the sound by 270-360 degrees , and in the region of the characteristic frequency there was an abrupt phase lag of 90-180 degrees ; the abruptness of the phase change depended upon the Q of the cell.10. The calculated phase shift of the electrical resonator as a function of frequency was subtracted from the phase characteristic of the receptor potential. The subtraction removed the sharp phase transition around the characteristic frequency, and in this frequency region the residual phase after subtraction was approximately constant at +180 degrees . This is consistent with the idea that the hair cells depolarize in response to displacements of the basilar membrane towards the scala vestibuli. The high-frequency region of the residual phase characteristic was similar in all cells.11. It is concluded that each hair cell contains its own electrical resonance mechanism which accounts for most of the frequency selectivity of the receptor potential. All cells also show evidence of a broad band-pass filter, the high frequency portion of which may be produced by the action of the middle ear.

The cochlear nuclei of snakes

The cochlear nuclei of three burrowing snakes (Xenopeltis unicolor, Cylindrophis rufus, and Eryx johni) and three non-burrowing snakes (Epicrates cenchris, Natrix sipedon, and Pituophis catenifer) were studied. The posterior branch of the statoacoustic nerve and its posterior ganglion were destroyed and the degenerated nerve fibers and terminals traced to primary cochlear nuclei in 13 specimens of Pituophis catenifer. All these snake species possess three primary and one secondary cochlear nuclei. The primary cochlear nuclei consist of a small nucleus angularis located at the cerebello-medullary junction and a fairly large nucleus magnocellularis forming a dorsal cap over the cephalic end of the alar eminence. Nucleus magnocellularis may be subdivided into a medially placed group of rounder cells, nucleus magnocellularis medialis, and a laterally placed group of more ovate and paler-staining cells, nucleus magnocellularis lateralis. A small but well-defined secondary nucleus which showed no degenerated nerve terminals after nerve root section, nucleus laminaris, underlies the cephalic part of both nucleus magnocellularis medialis and nucleus magnocellularis lateralis. Larger and better-developed cochlear nuclei were found in burrowing species than in non-burrowing species of snakes. Of the three burrowing species studied, Xenopeltis showed the greatest development of cochlear nuclei; Eryx cochlear nuclei were not quite as large but were better differentiated than in Xenopeltis; and Cylindrophis cochlear nuclei were fairly large but not as well developed nor as well differentiated as in either Xenopeltis or Eryx. The cochlear nuclei of the three non-burrowing snakes, Epicrates, Natrix, and Pituophis, were not as large nor as well developed as those of the burrowing snakes. There is some, but not complete, correlation between cochlear development and papilla basilaris length and number of hair cells. Thus, Xenopeltis and Eryx, with well-developed cochlear nuclei, have relatively long papillae basilares; but the boid, Epicrates, with less well-developed cochlear nuclei, has a fairly well-developed papilla basilaris. Cylindrophis, a burrowing species, shows only a moderate degree of cochear nuclei and papilla basilaris development. The non-burrowers, Natrix and Pituophis, have both small cochlear nuclei and relatively short papillae basilares.

The cochlear nuclei of some turtles

Histological sections of the brains of eight species of turtles representing six different families were studied in order to delineate the cochlear nuclei. In addition, the posterior eighth cranial nerve root and its ganglion were sectioned in 15 specimens of Kinosternon leucostomum, and the distribution of the degenerated nerve fibers and terminals was determined. Two primary and one probably secondary nuclei were demonstrated by the terminal degeneration pattern of the cochlear fibers. A spherical nucleus angularis and an elongated nuclus magnocellularis together form a column of primary cochlear nuclei in the dorsal alar lamina of the medulla. Heavy terminal degeneration is seen associated with these cells following transection of the posterior eighth nerve and ganglion. Nucleus magnocellularis is probably homologous with the nucleus magnocellularis medialis of lizards and crocodiles, and has been described in turtles as nucleus dorsalis magnocellularis by previous authors. A probably secondary cochlear nucleus, nucleus laminaris, lies just ventral to the nucleus magnocellularis. It is associated with the nucleus magnocellularis throughout its length but is shorter. Nucleus laminaris remains free of terminal degeneration after destruction of the posterior eighth nerve and ganglion. The cochlear nuclei of other turtle species were very similar to those of Kinosternon leucostomum.