Physiology of the Inner Ear

Summary

Myxine glutinosa has a single semicircular canal on each side of the head. Each canal is oriented so that it projects onto all three major planes of rotation. The unusually large internal diameter of the canal enhances its mechanical sensitivity to rotation while the absence of a cupula enables the diameter of the torus to be kept relatively small without detracting from its function of a rotational velocity transducer. Each canal contains three sensory epithelia; two cristae and one macula communis. The hair cells of the cristae have their kinocilia directed away from the midline of the fish. Records obtained from SCC nerves generate velocity-dependent, phase-locked responses to sinusoidal oscillations in the horizontal plane between 0.25 and 2.0 Hz. The acuity of the response is less than that obtained from similar vertebrate preparations. The orientation of the kinocilia results in a situation in which only rotation in the horizontal plane generates unambiguous responses. This is also the plane in which mechanical sensitivity is greatest. Ambiguity of response from the cristae in rolling and pitching is resolved by the gravitational responses of the hair cells within the macula communis. Mole rats lack eyes which form a sharp visual image, yet have semicircular canals with enhanced sensitivity. Neither Myxine nor mole rats need a vestibularocular reflex (VOR) to stabilize the eyes, yet each has responded differently to visual deprivation. This suggests that the relativly poor acuity of the semicircular canals in Myxine is not dictated by the absence of a VOR but may be sufficient to orientate on the sea floor.