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At least 397 records · Page 22Linked to original sources
PubMed1994

The antidromic compound action potential of the auditory nerve.

1. The antidromic compound action potential (ACAP) of the auditory nerve was evoked by shocks to the auditory nerve root and recorded at the round window of the cochlea in anesthetized guinea pigs. The goal of this study was to determine the characteristics of the ACAP and compare these characteristics with those of the orthodromic, sound-evoked compound action potential (CAP). 2. The ACAP consists of an initial complex of a positive peak (p1) followed by a negative peak (n1). In contrast, the CAP consists of a negative peak (N1) followed by a positive peak (P1). These differences in waveform are likely due to the differences in conduction direction, antidromic for the ACAP vs. orthodromic for the CAP. 3. After the initial complex, the ACAP has a second complex of peaks (p2, n2) at a latency of approximately 1 ms; this complex is much smaller in amplitude than the initial complex (p1, n1). It is likely that the initial ACAP complex reflects firing of auditory-nerve fibers whereas the second complex reflects firing of neurons further centrally, perhaps in the cochlear nucleus, that are activated by orthodromic firing of auditory-nerve fibers. 4. Experiments with shock pairs are consistent with the idea that for auditory nerve fibers, the absolute refractory period is < 0.5 ms, and the relative refractory period is between 0.5 and at least 5 ms. 5. Experiments with click-shock pairs indicate that a shock interferes with the response to a click when the click and shock are given at about the same time.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2000

Electrical cochlear stimulation in the deaf cat: comparisons between psychophysical and central auditory neuronal thresholds.

Cochlear prostheses for electrical stimulation of the auditory nerve ("electrical hearing") can provide auditory capacity for profoundly deaf adults and children, including in many cases a restored ability to perceive speech without visual cues. A fundamental challenge in auditory neuroscience is to understand the neural and perceptual mechanisms that make rehabilitation of hearing possible in these deaf humans. We have developed a feline behavioral model that allows us to study behavioral and physiological variables in the same deaf animals. Cats deafened by injection of ototoxic antibiotics were implanted with either a monopolar round window electrode or a multichannel scala tympani electrode array. To evaluate the effects of perceptually significant electrical stimulation of the auditory nerve on the central auditory system, an animal was trained to avoid a mild electrocutaneous shock when biphasic current pulses (0.2 ms/phase) were delivered to its implanted cochlea. Psychophysical detection thresholds and electrical auditory brain stem response (EABR) thresholds were estimated in each cat. At the conclusion of behavioral testing, acute physiological experiments were conducted, and threshold responses were recorded for single neurons and multineuronal clusters in the central nucleus of the inferior colliculus (ICC) and the primary auditory cortex (A1). Behavioral and neurophysiological thresholds were evaluated with reference to cochlear histopathology in the same deaf cats. The results of the present study include: 1) in the cats implanted with a scala tympani electrode array, the lowest ICC and A1 neural thresholds were virtually identical to the behavioral thresholds for intracochlear bipolar stimulation; 2) behavioral thresholds were lower than ICC and A1 neural thresholds in each of the cats implanted with a monopolar round window electrode; 3) EABR thresholds were higher than behavioral thresholds in all of the cats (mean difference = 6.5 dB); and 4) the cumulative number of action potentials for a sample of ICC neurons increased monotonically as a function of the amplitude and the number of stimulating biphasic pulses. This physiological result suggests that the output from the ICC may be integrated spatially across neurons and temporally integrated across pulses when the auditory nerve array is stimulated with a train of biphasic current pulses. Because behavioral thresholds were lower and reaction times were faster at a pulse rate of 30 pps compared with a pulse rate of 2 pps, spatial-temporal integration in the central auditory system was presumably reflected in psychophysical performance.

Age Factors↗
PubMedDate not supplied

Pathological features of healing of a ruptured human round window membrane.

Pathology of a round window membrane rupture was demonstrated in a human temporal bone from a case in which labyrinthotomy had been performed through the round window membrane. Proliferation of mesothelial cells was seen in the inner layer of the membrane, and it appeared to be reinforced from the inside by these reactive cells. The middle layer of the membrane was thickened by increased collagen and elastin. The pathologic changes which take place during healing of the ruptured round window membrane are discussed.

Adult↗
PubMedDate not supplied

Current issues in cochlear gene transfer.

Cochlear gene therapy represents a potential experimental and therapeutic tool to understand and treat deafness. In designing cochlear gene transfer studies, the chosen route of delivery of vector and the choice of gene therapy vector have to be given careful consideration. Several different routes of delivery have been tested in our laboratory including infusion with osmotic minipump, direct microinjection into the cochlea and application of vector-transgene complex-soaked Gelfoam((R)) into the direct contact with the round window membrane. In our experience, the latter is an easy, safe and atraumatic technique to deliver gene into the cochlea. A number of different gene transfer vectors have been investigated in vivo for their efficacy, utility and safety in intracochlear gene transfer. Vectors successfully studied include cationic liposomes, adeno-associated virus, adenovirus, lentivirus, herpes simplex virus and vaccinia virus. While the viral vectors offer clear experimental advantages, human gene therapy in the future will likely utilize nonviral vectors to maximize safety. Finally, safety issues regarding dissemination of gene transfer vectors beyond the target cochlea will need to be adequately addressed.

Animals↗
PubMed2006

A frequency-position function for the human cochlear spiral ganglion.

Greenwood's frequency-position function for the organ of Corti (OC) is commonly used to estimate represented frequencies for cochlear implant (CI) electrodes, both in temporal bone studies and in imaging studies of living CI recipients. However, many contemporary CIs position stimulating electrodes near the modiolus, directly targeting the spiral ganglion (SG) cells within Rosenthal's canal. At the extreme base and apex, the SG does not extend as far as the OC, and the radial nerve fibers take a tangential course into the modiolus resulting in a potential offset between the frequency maps of the OC and SG. In this investigation, human cadaveric cochleae (n = 7) were studied in surface preparations after osmium staining. The OC and SG lengths were measured and radial fiber trajectories traced to identify frequency-matched points on each structure. These data allowed derivation of a mathematical function correlating represented frequency along the OC to position along the SG. A cubic function fit the data with a very high intersubject correlation. Better knowledge of the human SG 'neural frequency map' may help to refine electrode design, and to more accurately map CI channel filter bands to the appropriate cochlear place along the SG, which may be advantageous for more sophisticated CI outcomes, such as music appreciation. These data also could be valuable for electroacoustic stimulation, by defining the insertion distance of a CI electrode required to reach specific frequencies (based upon preoperative imaging) in an individual subject, thus helping to avoid trauma to cochlear regions with residual hearing.

Acoustic Stimulation↗
PubMed2006

Comparison of round window and cochleostomy approaches with a prototype hearing preservation electrode.

INTRODUCTION: Preservation of residual hearing in cochlear implant recipients has been demonstrated to be possible and provides the potential benefit of combined electric and acoustic auditory stimulation. A prototype 16-mm multichannel array has been designed to facilitate placement of 22 electrodes without damage to intracochlear structures. The electrode array is suitable for insertion via the round window membrane (RWM) or a small cochleostomy. AIM: To evaluate the insertion trajectory and the presence of trauma to intracochlear structures with the prototype electrode inserted by either the RWM or a scala tympani cochleostomy. MATERIALS AND METHODS: Eighteen fresh frozen human temporal bones were prepared for cochlear implantation using a standard transmastoid facial recess technique. Twelve electrodes were implanted at the University of Melbourne and 6 at the Medizinische Hochschule Hannover. In Melbourne fluoroscopy was used to monitor the insertions. Twelve prototype electrodes were inserted via the RWM. A further 6 electrodes were inserted via a small scala tympani cochleostomy. The cochleostomy was sited inferior to the RWM to avoid trauma to the basilar membrane and spiral ligament. Specimens were embedded and fixed with acrylic resin and the cochleae then examined histologically at 200-mum intervals using a grinding and polishing technique. RESULTS: Full insertion of the electrode was achieved without significant resistance in all RWM and cochleostomy specimens. In two RWM specimens fold-over of the electrode tip occurred, and in one specimen the electrode penetrated the spiral ligament to lie in an 'endosteal 'position. In one cochleostomy specimen the electrode was rotated within the cochlea to face laterally rather than towards the modiolus. The final electrode position differed for the two groups, with the electrodes inserted via the RWM lying in a more perimodiolar position along the first part of the basal turn. The average depth of insertion was 240 degrees for the RWM electrodes and 255 degrees for the cochleostomy electrodes. Histologic examination showed no damage in any specimen to the modiolus, osseous spiral lamina or basilar membrane. CONCLUSIONS: A prototype hearing preservation electrode array was inserted by either a RWM or a scala tympani cochleostomy without evidence of significant intracochlear trauma.

Basilar Membrane↗
PubMed1997

Developmental study of the round window region.

A silicone impression method to study the anatomy of the round window region was used in 102 temporal bones belonging to individuals aged from 4 months of fetal life to 3-year-old children as well as adults. A total of 2,142 measurements of the round window, oval window and round window fossula was made in the molds. The data demonstrate that the round window shows a diameter from 1.21 mm (average for the short axis) to 1.74 mm (average for the long axis) and the circular shape was present in 55% of the adult temporal bones and in 18.18% of the fetal and infantile ones. The adult dimensions of the measured structures are reached during fetal development.

Adult↗
PubMed1978

The ultrastructure of the round window membrane of the cat.

The ultrastructure of the round window membrane of the cat was investigated. Three distinct layers were recognised and described. It was established that the outer epithelial layer is non-ciliated, but often contains microvilli. The middle layer, under the basement membrane, contains a large amount of collagen and elastic fibres in addition to fibrocytes. The inner layer is a continuation of the epithelial layer of the perilymphatic space. The thickness of the membrane has been measured as being 15--20 micrometer. The latest human audiological and clinical observations concerning the function and rupture of the round window membrane emhasise the importance of the electron microscopic study of this membrane.

Animals↗
PubMed1985

Microvasculature of the stria vascularis in the round window area in the rat. A scanning electron microscopy study.

This paper reports the results of a scanning electron microscopy study of corrosion cast preparation of the vessels in the cochlea of the rat. This method confirms the general findings about the cochlear vascularization, but also gives the opportunity to study the detailed pattern of the vascular supply of the stria vascularis in the round window area. The basal end of the stria vascularis has a remarkably different vascular anatomy compared with the other areas of the cochlea. The possible role of the vessels in the round area is discussed.

Animals↗
PubMed1986

Perilymph fistulas caused by myringotomy.

Two cases of rupture of the round window membrane verified by operation are presented. Both cases arose in connection with myringotomy and were presumably caused by a violent force acting directly on the tympanic membrane and the ossicles. This would result in a strong movement of the stapes and an acute change in the perilymph pressure, thereby causing an indirect injury to the round window. Symptoms and treatment are briefly discussed.

Acute Disease↗
PubMed1990

Sealing of the round window reversibly affects auditory brainstem response latencies.

The round window (RW) niche was sealed in 12 adult female albino Sprague-Dawley rats. Electrophysiological auditory function was measured with a computerized auditory brainstem response (ABR) recording technique, using 1- and 6 kHz stimuli. ABR measurements were performed prior to closure of the niche, immediately after the sealing, and 4 h, 24 h and 7 days later. After sealing, the latencies elicited by a 6 kHz stimulus increased, but returned to normal between 24 h and 7 days after closure. Stimulation with 1 kHz did not influence the latencies in such a clear-cut manner as when using 6 kHz. It is conceivable that sealing of the RW deprives a minor fraction of oxygen supplied to the basal part of the cochlea through the RW membrane, a procedure which by us is monitored as transient electrophysiological changes.

Animals↗
PubMedDate not supplied

Alpha-bungarotoxin inhibits outer hair cell motility in situ.

The effect of two substances (alpha-bungarotoxin, alpha-BGTX, a small protein, and the local anesthetic bupivacaine hydrochloride) with an assumed effect on outer hair cell (OHC) motility were analyzed after exposing the cochlea via the round window membrane. Electrophysiological measurements were performed with a very narrow frequency-specific gating (+/- 100 Hz) technique to determine auditory brainstem response (ABR) thresholds, including ABR-based frequency tuning curves. Exposure to alpha-BGTX gave a minor improvement in thresholds, interpreted as a facilitation of OHCs, i.e. releasing their efferent inhibitory control, whereas exposure to bupivacaine hydrochloride impaired ABR thresholds, possibly due to immobilization of OHC motility via the lateral cell membrane. Our results are consistent with the hypothesis that efferent influence on the cochlea may be linked with a modulation of the mechanical function of OHCs. We can now postulate that there is in vivo evidence that acetylcholine exerts its effect at the OHCs via an alpha-BGTX alpha-BGTX binding acetylcholine receptor.

Acetylcholine↗