Cochlear Receptor Research Articles

Cytoskeletal and muscle-like elements in cochlear hair cells.

Monospecific antibodies to actin and to tubulin were used as immunofluorescent probes to evaluate the distribution of microtubules and actin filaments in the organ of Corti in mouse and guinea pig. The results indicate that in cochlear receptor cells actin and actin filaments as well as tubulin and microtubules are integral cytoskeletal elements. The presence of actin suggests a possible contractile mechanism within the sensory cilia whereas tubulin is thought to play an important role in the stability of sensory cells. Both proteins are discussed to form structural elements required for the mechano-chemical coupling in hearing.

Die mechanochemische Kopplung in der H�rzelle ben�tigt ein intaktes Zytoskelett

Monospecific antibodies to actin and to tubulin were used as immunofluorescent and ferritin-labelled probes to evaluate the distribution of microtubules and actin filaments in the hearing organs from mouse, guinea pig and fish. The results indicate that in cochlear receptor cells actin and actin filaments as well as tubulin and microtubules are integral cytoskeletal elements. The presence of actin suggests a possible contractile mechanism within the sensory cilia whereas tubulin is thought to play an important role in the stability of sensory cells. Both proteins are discussed to form structural elements required for the mechano-chemical coupling in hearing.

Acoustic responses after total destruction of the cochlear receptor: brainstem and auditory cortex.

Acoustically evoked neural activity has been recorded from the brainstem and auditory cortex of guinea pigs after complete destruction of the organ of Corti by the aminoglycosidic antibiotic amikacin. These responses to sound differ in important respects from the evoked potentials normally recorded from the auditory pathways. At the brainstem level they resemble the potentials reported by others after stimulation of the vestibular nerve.

Cochlear receptor development in the rat with emphasis on synaptogenesis

Maturation of the albino rat cochlea was studied using light and electron microscopy. Critical stages of receptor morphology were examined. At birth, cochlear structures are very immature, but even at this early stage synapses are recognizable. Under inner hair cells (IHCs) both afferent and efferent synapses are present. Under outer hair cells (OHCs) only afferent endings are seen. During the first postnatal week, synaptic development proceeds slowly. Between 6 to 12 days of age, substantial changes occur in the pattern of hair cell innervation. There are fewer efferent synapses at the IHC level and the first efferent junctions form on OHCs. In addition, a pattern of temporary innervation is seen under the OHC, with axo-dendritic synapses between efferent endings and afferent fibres. Between 12 and 16 days of age the main changes in hair cell innervation are at OHC level where afferent junctions regress and large efferent synapses form. By 16 days of age sensory-neural relationships seem adult-like. The results are discussed in relation to rat cochlear electrophysiological development and the period of supra-normal sensitivity to acoustic trauma.

Relations between cochlear fatigue and the asymmetrical nonlinearity of the cochlear microphonics.

It is generally assumed that noises have a detrimental effect when the cochlear receptor is overloaded and, more specifically, when the cochlear microphonic (CM) fails to increase linearly with intensity. In order to investigate further the relations between the nonlinearity of CM and damage to the cochlea, a series of experiments was carried out on guinea pigs to relate the short-term CM depression following the presentation of noises or tones with the nonlinearity and the assymmetry. The asymmetrical non-linearity of CM was measured in tracing the input-out functions and also the wave-forms. Two other important tests of the asymmetrical nonlinearity were used: the measure of interference and of summating potential (SP DIF). The results show that the fatigability is greater when there is a large negative asymmetry or a large negative SP. Variations in asymmetry and in SP were observed among individuals. Other changes of symmetry were provoked by asphyxia or by introducing solutions of KCl in the perilymph. These changes were well correlated with the fatigability. These results are interpreted in a model of the cochlear transducer derived from the model of Davis. The assymetry of a flux of potassium ions between endolymph and the hair cells is assumed to be responsible for the alterations associated with cochlear fatigue and trauma.

Enhancement of visual responses on the primary auditory cortex of the cat after an early destruction of cochlear receptors

Enhancement of visual responses on the primary auditory cortex of the cat after an early destruction of cochlear receptors

Re-examination of avian cochlear potentials.

THE aim of this study is to provide a basic description of the cochlear receptor potentials, the cochlear microphonic (CM) and the summating potential (SP) of the bird. Although several workers have examined these avian potentials in limited aspects, a comprehensive survey of their frequency and intensity behaviour is long overdue. Moreover, a re-examination of the avian potentials has seemed necessary because the potentials are reported to differ vastly from the mammalian CM and SP and one is left with the baffling implication that the middle and/or inner ears of these classes operate with a degree of dissimilarity exceeding what might reasonably be expected on the basis of structural difference. For example, the avian CM sensitivity function has been reported1 to be flatter than the middle ear transfer function in a comparable species2 thus implicating an unknown sort of cochlear element in the frequency response of the avian system. Second, the slopes of avian CM input-output functions have been reported as considerably less than unity and thus less steep than in the mammal1,3–7 leaving unanswered the question of whether the mechanics of the middle ear or basilar membrane, or peculiarities of the receptor cells per se, enforce the nonlinear intensity response in the bird. Third, if, as reported, the avian summating potential is appropriately described as a unitary negative response, manifest only at frequencies above 600 Hz (refs 4, 7–10), one can hardly reconcile this with the observation that the SP exists as both a positive and negative response in the mammal, and certainly occurs at even the lowest audible frequencies11,12 in that class. While each of the above differences implies a fundamental dichotomy between the avian and mammalian cochleae, such differences were not found in the present study. We did however use a new intracochlear electrode configuration in the elucidation of the parametric behaviour of the CM and SP in the pigeon. Fine tungsten electrodes, flame-sealed in capillary tubing, were placed in the basal scala vestibuli (SV) and the scala tympani (ST) proper. This electrode configuration made differential recordings possible and avoided electroanatomical effects, believed to be inherent in previous recessus tympani or round window recordings in the bird. The following differentially measured avian functions are discussed: the 1-µV r.m.s. CM isopotential curve, CM isovelocity curves (relative to a constant footplate velocity of 10−6 mm s−1), CM input–output functions, CM “degree of nonlinearity” functions and SP isovelocity functions. Our results show that these functions do not differ significantly from their mammalian counterparts.

Input–output functions of cochlear whole-nerve action potentials: Interpretation in terms of one population of neurons

It is commonly assumed that the two-segmented input–output functions of the whole-nerve action potential are a result of the activity of two sets of primary fibers: a low- and a high-sensitivity group. While anatomical studies show the existence of two populations of fibers, there are no clear-cut electrophysiological signs of two distinct populations responding with different thresholds. In light of this physiological result, it is desirable to construct a scheme for the growth of the whole-nerve action potential that involves only one group of fibers. This can be done in a qualitative fashion by considering the pattern of single-unit tuning curves and assessing the contribution of populations of units to the compound response. Thus the slowly growing low-level segment of the input–output function is identified with the sharp tip region of the tuning curves of the responding units, while the high-level, rapidly growing, segment is associated with the recruitment of higher-frequency units that respond on the tail segment of their tuning curves. Thus the low and high parts of the input–output functions are interpreted in terms of the excitation of narrow and wide segments of the cochlear receptor. Masking experiments that support the scheme are described. Subject Classification: [43]65.35, [43]65.42, [43]65.59, [43]65.40.

Outer hair cells do produce the preponderance of receptor potentials

Numerous experiments [e.g., Davis et al., Laryngoscope (1958); Dallos and Wang, Audiology (1974)] in which outer hair cell damage is accompanied by good preservation of inner hair cells, have indicated that, in such cochleas, the CM is suppressed by 30–40 dB. The conclusion drawn from such results was that outer hair cells generate a very large fraction of the recorded CM, and that in fact in a normal cochlea the CM is determined by the output of the outer hair cells. While these experiments are very suggestive, a direct confirmation of the dominant role of outer hair cells in CM production has been lacking. What is required for such confirmation is recording from cochlear regions that are denuded of inner hair cells. We are now reporting on such experiments. With very massive doses of kanamycin one can achieve a pattern of hair cell degeneration in the guinea pig's cochlea, whereby only outer hair cells remain in the fourth (apical) turn, and whatever inner hair cells are preserved are confined to more basal regions. Recordings of CM and SP from the scalar media of the fourth turn of such cochleas is compared to data from normal preparations. While many animals are seen to have severely depressed responses, several possess essentially normal receptor potentials. This finding tends to confirm the suggestion that outer hair cells are the primary source of the cochlear receptor potentials; both CM and SP. [Supported by grants from the NINCDS.]

Ribonucleic acid changes in the brain, spiral ganglion, and adrenal cortex of noise-exposed rats

Microspectrophotometric determinations of neuronal RNA levels were made in selected brain areas (auditory cortex, optic cortex, and hippocampus), in adrenal cortical cells, and in cochlear receptor elements from adult male albino rats exposed to a 60-min noise (100–120 dB re 0.0002 μbar, 350–20 000 Hz). Control animals were maintained at ambient sound pressure levels (ca. 40 dB). Animals were sacrificed at 1-, 4-, 8-, and 24-hour postexposure intervals. RNA was quantified in azure-B-stained tissue sections which were pretreated with DNase. A transient RNA elevation was evidenced in neurons from the three brain regions analyzed during the 1- to 8-hour postexposure interval. Adrenal cortical cells also exhibited increased RNA levels at corresponding postexposure time intervals, whereas no appreciable stimulation of RNA synthesis was evidenced in spiral ganglion neurons of the cochlea during a 24-hour period following cessation of auditory stimulation. The latency of RNA activation in brain elements, coupled with the similarity in the pattern of adrenal stimulation suggests that nucleic acid responses of brain neurons may be hormonally mediated and not a direct manifestation of excitation via auditory neural pathways. [Research aided by the Office of Environmental Quality Programs and the Center for Air Environment Studies.]

Fixation effects on the relation between the tectorial membrane and the cilia of the hair cells.

The observation of unusual specimens induces one to suggest that, <i>in vivo, </i>the cilia of the hair cells are deeply immersed in the tectorial membrane. The cilia show their terminal parts free only when, due to fixation effects, they are drawn out from the tectorial membrane. This relationship induces one to reconsider the modality of excitement of the cochlear receptors.

Ototoxicity of quinine in experimental animals.

Quinine is a protoplasmotic poison which, in cases of cinchonism, produces tinnitus, hearing loss, dizziness, blurring of vision, photophobia, and hypotension. Administration of a large dose may produce delirium, unconsciousness, respiratory failure, and death. Ototoxicity of quinine seems to be the result of administration of a large dose or prolonged treatment, or it may be a matter of idiosyncrasy. Hearing loss of conductive type is said to occur, by Falbe-Hansen, 1 after several clinical and experimental observations, while Forbes 2 and Ruedi et al. 3 believed it is of perceptive type. The opinions concerning the location and nature of cochlear lesions as observed in experimental animals may be divided in two groups. The first group claims that quinine produced changes in the primary cochlear neurons and hypotonic degeneration in the cochlear receptor. The features and explanation of this phenomenon were put forward by Wittmaack, 4 and have been supported by

Effect of hypoglycemia on auditory cortex and cochlear receptor in the cat.

Effect of hypoglycemia induced by insulin and/or functional evisceration on auditory cortex and cochlear responses to sound stimuli was studied in cats. Both responses remained unchanged at blood sugar levels as low as 5 mg %. They diminished in size whenever blood pressure was allowed to fall to low values, and returned to nearly normal when it was restored. The observations suggest that carbohydrates are not essential for cochlear function or that the structure contains a large store of them. The presence of responses from auditory cortex during medullary stage of coma suggests that this area does not conform well to the concept of phyletic organization of the central nervous system.