Heidi Felix

A model of the electrically excited human cochlear neuron I. Contribution of neural substructures to the generation and propagation of spikes

Differences in neural geometry and the fact that the soma of the human cochlear neuron typically is not myelinated are reasons for disagreements between single fiber recordings in animals and the neural code evoked in cochlear implant patients. We introduce a compartment model of the human cochlear neuron to study the excitation and propagation process of action potentials. The model can be used to predict (i) the points of spike generation, (ii) the time difference between stimulation and the arrival of a spike at the proximal end of the central axon, (iii) the vanishing of peripherally evoked spikes at the soma region under specific conditions, (iv) the influence of electrode positions on spiking behavior, and (v) consequences of the loss of the peripheral axon. Every subunit of the cochlear neuron is separately modeled. Ion channel dynamics are described by a modified Hodgkin--Huxley model. Influence of membrane noise is taken into account. Additionally, the generalized activating function is introduced as a tool to give an envision of the origin of spikes in the peripheral and in the central axon without any knowledge of the gating processes in the active membranes. Comparing the reactions of a human and cat cochlear neuron, we find differences in spiking behavior, e.g. peripherally and centrally evoked spikes arrive with a time difference of about 400 mus in man and 200 mus in cat.

A model of the electrically excited human cochlear neuron. II. Influence of the three-dimensional cochlear structure on neural excitability

A simplified spiraled model of the human cochlea is developed from a cross sectional photograph. The potential distribution within this model cochlea is calculated with the finite element technique for an active scala tympani implant. The method in the companion article [Rattay et al., 2001] allows for simulation of the excitation process of selected elements of the cochlear nerve. The bony boundary has an insulating influence along every nerve fiber which shifts the stimulation condition from that of a homogeneous extracellular medium towards constant field stimulation: for a target neuron which is stimulated by a ring electrode positioned just below the peripheral end of the fiber the extracellular voltage profile is rather linear. About half of the cochlear neurons of a completely innervated cochlea are excited with monopolar stimulation at three-fold threshold intensity, whereas bipolar and especially quadrupolar stimulation focuses the excited region even for stronger stimuli. In contrast to single fiber experiments with cats, the long peripheral processes in human cochlear neurons cause first excitation in the periphery and, consequently, neurons with lost dendrite need higher stimuli.

On the Pathogenesls of Bell's Palsy

The data obtained from the anatomical study of the Fallopian canal, the intraoperative evoked electromyography and the histopathological evaluation of specimens from the greater petrosal nerve support the entrapment neuropathy concept in Bells palsy.

A Comparative Study of the Effect of Age on the Human Cochlear and Vestibular Neuroepithelia

A semi-quantitative and semi-qualitative comparative analysis was performed on the maculae and cochleas of 3 patients. The cochleas were examined by SEM and the maculae by TEM and light microscopy. The surface features of the cochleas were minimally affected by autolysis. Hearing loss and increasing age correlated well with inner and outer hair cell counts. In the labyrinth, the sacculi were more resistant to autolysis than were the utriculi and the type 2 cells better preserved than the type 1 cells. The pattern of cellular degeneration in the utriculus and sacculus varied with both age and functional deficit. Lipofuscin was present in the sensory cell of all 3 patients but was most pronounced in the oldest. Long-spaced collagen, laminated bodies and membrane-bound inclusions were seen in all maculae.

Abnormal basement membrane in the inner ear and the kidney of the Mpv17-/- mouse strain: ultrastructural and immunohistochemical investigations

The loss of the function of the peroxisomal Mpv17-protein and associated imbalanced radical oxygen species (ROS) homeostasis leads to an early onset of focal segmental glomerulosclerosis and sensorineural deafness associated with severe degeneration of cochlear structures. An excessive enlargement of basal laminae of the stria vascularis capillaries and glomeruli indicates numerous changes in their molecular composition. The basement membrane (BM) of the glomeruli and the stria vascularis are simultaneously affected in early stages of the disease and the lamination, splitting of the membrane and formation of the “basket weaving” seen at the onset of the disease in the kidney are similar to the ultrastructural alterations characteristic for Alportȁ9s syndrome. The progressive alteration of the BMs is accompanied by irregularity in the distribution of the collagen IV subunits and by an accumulation of the laminin B2(γ1) in the inner ear and B(β1) in the kidney. Since Mpv17 protein contributes to ROS homeostasis, further studies are necessary to elucidate downstream signaling molecules activated by ROS. These studies explain the cellular responses to missing Mpv17-protein, such as accumulation of the extracellular matrix, degeneration, and apoptosis in the inner ear.

Quantitative Analysis of Cochlear Sensory Cells and Neuronal Elements in Man

The myelinated radial fibres in the osseous spiral lamina and the myelinated fibres in the cochlear nerve in the internal auditory canal as well as the sensory cells were counted in cochleae from 15 dissected temporal bones from 8 patients. Light microscopy was carried out on semithin sections of epoxy resin embedded tissue. Audiometry had been performed within 6 months prior to death. Three patients had normal hearing for their age group, 2 had slight presbyacusis and the remaining 3 had sustained noise injury. All specimens clearly had fewer fibres in the spiral lamina than in the internal auditory canal. The cochleae from patients with normal hearing for their age group had a difference in the nerve fibre counts of up to 34%. A case of sensorineural presbyacusis showed 31%, and a case of neural presbyacusis, 47% difference. The greatest difference was found in a case of acoustic trauma, the range in this group being between 25% and 55%. The lower the number of fibres in the spiral lamina, the greater was the difference in all but two specimens. A slow retrograde degeneration, i.e. beginning in the peripheral process of the cochlear nerve, could be an explanation for these findings.

Light and Electron Microscopic Investigation of Cochlear Nerve Specimens from Profoundly Deaf Patients

Surgical specimens of cochlear nerves from patients suffering from sudden deafness or congenital deafness have been evaluated with regard to the number of nerve fibres and the morphological appearance. The meatal segment of cochlear nerves from 6 patients with sudden deafness shows a variable number of nerve fibres and different morphological changes. The duration of deafness does not correlate with the number of nerve fibres. In the case of congenital deafness for more than 17 years the cochlear nerve presented an apparently normal number of myelinated fibres. The presented study reveals that the retrograde degeneration of the human cochlear nerve is unpredictable.

Methodological Aspects of Quantitative Study of Spiral Ganglion Cells

The literature contains little numerical information on the population of spiral ganglion cells from normal-hearing adults. We present a method for quantitative analysis of spiral ganglion cells. We used not only the total counts of the neurons but also divided the spiral ganglion into four segments as described by Otte et al. Nine perilymphatic perfused temporal bones from 5 adult patients with normal hearing for age were examined. The largest population of spiral ganglion cells was always in segment II and fewest in segment I. There is no correlation between the total count and the length of the cochlea.

Kartagener's syndrome: Clinical presentation and cardiosurgical aspects

BACKGROUND Long-term observations in patients with Kartageners syndrome (situs inversus, bronchiectasis, and sinusitis) are rare. The role of additional cardiac malformations and their surgical repair is not well known. METHODS Nine patients (5 female and 4 male) with Kartageners syndrome were identified and followed. Four patients had associated cardiac anomalies; 4 underwent total surgical repair at the ages of 4 (2 patients), 7, and 34 years. RESULTS The postoperative period was uneventful, and these 4 patients are doing well 7 months and 2, 9, and 19 years after repair. The other patients are being treated with conservative therapy and are in relatively good condition. CONCLUSIONS This disease can be temporarily benign when treated with antibiotics and physiotherapy. Associated cardiac anomalies seem to be quite common, and such patients need careful cardiologic follow-up. Surgical intervention can be safely performed in patients suffering from Kartageners syndrome associated with a congenital cardiac malformation and produces good long-term results. Bilateral lung transplantation seems to be the therapy of choice in patients with respiratory insufficiency but without concomitant cardiac anomalies.

Observations on the Pattern of Sensorineural Degeneration in the Human Cochlea

Cochlear sensory and neural degeneration was examined in nine pairs of human temporal bones fixed by perilymphatic perfusion, using phase-contrast and electron microscopy. Four pairs, three from females, had only slight sensorineural degeneration, limited to the very basal end of the cochlea. A predominantly neural degeneration was identified in a 54-year-old male. The process was bilateral, asymmetrical, uneven, and involved the entire length of the cochlea with several circumscribed areas of severe nerve degeneration. One case had mild, diffuse sensorineural degeneration in the lower half of the basal turn characteristic of presbyacusis. The other three pairs, all from males, revealed sensorineural degeneration patterns associated with noise injury and were remarkably similar to or almost identical with cochleas described previously. There was a good correlation between the presence of supporting cells and the survival of nerve fibres in the osseous spiral lamina in the corresponding area. In one cochlea, however, the degeneration of only inner hair cells in a small area was associated with complete nerve degeneration in the corresponding sector of the spiral lamina. Giant cilia were frequently seen in the apical turn.