Absence of a function for the crossed olivocochlear bundle under physiological conditions

Zusammenfassung

Der efferente olivocochleare Trakt (Rasmussen, 1942) ist durch der Versuche von Galambos, 1956; Desmedt et al., 1962/1963 und Fex, 1962/1965 auf vielen Tagungen diskutiert worden. Der Trakt soll eine Hemmung der Haarzellen bewirken. Nach der Ansicht des Autors ist dies jedoch unter physiologischen Bedingung bisher nicht gezeigt worden. Vielmehr ist bisher nur gezeigt worden, daß die gekreuzte, efferente, olivocochleäre Hemmung lediglich unter hochgradig unphysiologischen Bedingungen, und auch dann nur schwach, auftritt. Die Experimente der oben zitierten Autoren unterstützen bei kritischer Diskussion diese Meinung. Ebenso die jetzigen eigenen Versuchsergebnisse, die physiologischere Reizbedingungen verwenden. Schließlich haben Patienten nach einseitiger Durchtrennung des olivicochleären Bündels (House, 1964; Fisch u. Yasargil, 1968) keine entsprechenden Hörstörungen vom Haarzelltyp. keine entsprechenden Hörstörungen vom Haarzelltyp.

Summary

Twenty-eight guinea-pigs were anaesthetized by means of Dial in urethane and immobilized by Curarine HAF. Twenty of the twenty-eight guinea-pigs yielded reliable data: 10 animals in experimental Group 1; 2 animals in Group 2; 5 animals in Group 3, and 3 animals in Group 4.

The left ear of each of these 20 guinea-pigs was prepared and stimulated in an identical manner: Recording of N₁/N₂ and CM by differential electrodes in the basal turn, as evoked by a program of acoustical clicks (repetition rates 1–1,000/s; intensities 0–90 db above the threshold); and recording of amplitude latency, form, and time-course of adaptation of N₁/N₂ on film.

The four groups differed with respect to the preparation and stimulation of the right ear.

Group 1. Stimulation at the right ear by pure tones of all effective frequencies (125–16,000 cps) and intensities (up to 100 db SPL); and measurement of peripheral physical masking of the left N₁/N₂. All reduction of the left N₁/N₂ proved to be due to peripheral masking by the sound at the right. There were no crossed neural effects.

Group 2. Reversible blocking of the right auditory nerve by an electrotonus during stimulation of the left ear, and during pure tone stimulation at the right ear. Anodic blocking neither abolished nor diminished the reduction of N₁/N₂ at the left caused by the sound at the right. Thus the crossed reduction at the left, evoked by sound at the right, proved to be due to peripheral physical masking. There was no contribution by the right auditory nerve, and no crossed neural effect was observed.

Group 3. Permanent blocking of the right auditory nerve by transverse section. Results were negative. There was no crossed neural effect.

Group 4. Electrical tetanic stimulation of the right auditory nerve trunk. No effect of N₁/N₂ by the shock stimulation of the opposite side was observed, and no crossed neural effect from the right auditory input to the left cochlea was demonstrable.

A critical discussion of the experiments of Galambos (1955, 1956), Desmedt et al. (1957, 1962), and Fex (1959, 1962, 1965), reiterates the fact that up to the present date there exists no valid evidence for a crossed function of the crossed olivocochlear bundle under physiological stimulating conditions. We have tried to demonstrate, however, that the cited authors have been working far beyond the physiological limits of the crossed olivocochlear system. The complete absence of olivocochlear inhibition in our own experiments using physiological stimulation is in agreement with Rasmussen's finding (1942, 1946) that, from the anatomical point of view, the compound olivocochlear system has a unilateral function, although in part this organ system courses through the opposite side of the brain stem. And it is in agreement with the observation of Galambos (1962) that in conditioned cats no hearing disorder was demonstrable after destruction of the olivocochlear bundle. Finally it is in harmony with the fact that patients after cutting their vestibular nerve, including the composite olivocochlear bundle (House, 1964) have little if any complaints as far as their directional discrimination is concerned (Fisch and Yasargil, 1968).