The dynamic responses of the hearing organ to acoustic overstimulation were investigated using the guinea pig isolated temporal bone preparation. The organ was loaded with the f luorescent Ca 2؉ indicator Fluo-3, and the cochlear electric responses to low-level tones were recorded through a microelectrode in the scala media. After overstimulation, the amplitude of the cochlear potentials decreased significantly. In some cases, rapid recovery was seen with the potentials returning to their initial amplitude. ] changes were not seen in preparations that were stimulated at levels that did not cause an amplitude change in the cochlear potentials. The overstimulation also gave rise to a contraction, evident as a decrease of the width of the organ of Corti. The average contraction in 10 preparations was 9 m (SE 2 m). Partial or complete recovery was seen within 30-45 min after the overstimulation. The [Ca 2؉ ] changes and the contraction are likely to produce major functional alterations and consequently are suggested to be a factor contributing strongly to the loss of function seen after exposure to loud sounds.Noise-induced hearing loss is a common condition that leads to considerable communication problems for affected individuals. Recent research on the physiology of this condition (reviewed in ref. 1) has been mainly focused on damage to the stereocilia (SC) of the sensory cells in the inner ear, important because this is the location of the ion channels converting mechanic vibrations into electric currents. Damage to the SC correlates well with alterations of the tuning curves of auditory nerve fibres (2). A capacity for repair of the SC after acoustic trauma also has been implicated (3), but the mechanisms underlying the stereociliary changes as well as the repair process remain unknown.Acoustic trauma also may cause degeneration of the sensory cells, resulting in an irreversible elevation in hearing thresholds (4). The degeneration most likely involves not only stereociliary changes but also alterations at the cell body level. The events taking place in these cells during and after overstimulation remain largely obscure. Cody and Russell (5) have shown that sustained depolarizations of the outer hair cells (OHCs) occur after moderately intense acoustic overstimulation and that repolarization parallels the recovery of auditory sensitivity. The underlying mechanisms are unclear.In isolated OHCs, mechanical overstimulation results in cytoplasmic [Ca 2ϩ ] increase (6). To investigate how this finding relates to reduced hearing sensitivity after acoustic trauma, the guinea pig isolated temporal bone preparation (7) was used to perform simultaneous investigations of calcium-dependent fluorescence, stimulus-evoked cochlear potentials and cochlear morphology. The sensory cells were visualized in situ in an almost native environment, and the cochlear electric responses were recorded. The preparation was used previously to study changes of organ of Corti mechanics following acoustic trauma (8) and has now been ...

Receptor cells in the ear are excited through the bending of sensory hairs which project in a bundle from their surface. The individual stereocilia of a bundle contain filaments about 5 nm in diameter. The identity of these filaments has been investigated in the crista ampullaris of the frog and guinea pig by a technique of decoration with subfragment-1 of myosin (S-l). After demembranation with Triton X-100 and incubation with S-l, "arrowhead" formation was observed along the filaments of the stereocilia and their rootlets and also along filaments in the cuticular plate inside the receptor cell. The distance between attached S-1 was 35 nm and arrowheads pointed in towards the cell soma. It is concluded that the filaments of stereocilia are composed of actin. KEY WORDS actininner ear 9 hair cell sensory hairs crista ampullaris sensory transduction Auditory and vestibular receptor cells are equipped with sensory hairs, the bending of which causes excitation of the sensory cells (2). A signal is generated in response to this stimulus and is in turn coded into an impulse message in the auditory nerve. The bending of the hairs is brought about by displacement of auxiliary sensory structures to which the tips of the sensory hairs are attached. The physical displacement of the sensory hairs is thus the first step in the cellular excitation process, and it is of obvious importance to obtain information about the mechanical properties of the sensory hairs and about the arrangement and nature of the responsible structures.The sensory hairs are tubular projections of the cell membrane which arise from the cell surface, 50-100 in number. One of these, the kinocilium (29), resembles an ordinary flagellum and is present in the vestibular system and the hearing organs of lower vertebrates; all the others are called stereocilia and are the only ones present in the mammalian hearing organ. The present paper deals with the stereocilia. Inside each stereocilium is a core of fine filaments which run down its length and collect to a narrow bundle at the slender base of the cilium, continuing as a rootlet into a cuticular plate which occupies the cell apex. The filament core has recently been found to exhibit considerable stiffness, as determined by micromanipulation of sensory hairs where the membrane has been removed by Triton X-100 (10).There are certain structural similarities between stereocilia and microvilli of the intestine as described by Mooseker and Tilney (20). In particular, we were interested in testing the possibility that the core of the stereocilium is composed of actin filaments as is the case in gut microvilli. For this purpose, we used the technique of decoration of actin with the S-1 fragment of myosin (14). MATERIALS AND METHODS PreparationThe crista ampullaris of frogs of the species Rana

Isolated outer hair cells were found to slowly shorten when subjected to a solution that would induce contraction in a muscle fibre. Two possible mechanisms underlying this behaviour emerge from ultrastructural and immunocytochemical investigations. Antibody labelling at the electron microscopic level demonstrates that actin is present not only in the stereocilia and in the cuticular plate but also along the wall of outer hair cells, between the plasma membrane and the subsurface fenestrated cisternae. The latter are interconnected by regularly spaced pillars, resembling those seen between the T-tubules and sarcoplasmic reticulum in muscle fibres. Contraction also results from the application of positively charged macromolecules to the bathing solution. This implies sensitivity of the membrane-associated complex (the cortex system) to an electrical current. A second contractile system may reside in the cytoplasm, where calmodulin is present in contracted hair cells. This protein is a calcium-binding control protein for contraction-like events in smooth muscle and non-muscle cells. The unique presence of the cortex system in outer hair cells, and its absence in inner hair cells, indicates a functional significance that relates to a motor function of outer hair cells in hearing.

Intracellular recordings were made from outer hair cells in the third turn of the guinea pig cochlea, and the electrical characteristics of the cells were compared to those of inner hair cells, supporting cells, and extracellular spaces from the same recording region. Outer hair cells have higher membrane potentials than do inner hair cells, but they produce smaller a-c receptor potentials. The frequency response characteristics of both types of hair cells are probably not significantly different. In the frequency region where tuning is optimal, both cell types produce depolarizing d-c receptor potentials, but outer hair cells also generate hyperpolarizing responses at low frequencies.

No abstract