The labyrinth of Myxine : anatomy, ultrastructure and electrophysiology

Abstract

The anatomy of the labyrinth of Myxine is described. A torus-shaped cartilaginous capsule contains the membranous labyrinth. There are two ampullae, each with a ring-shaped crista, and the ventro-medial wall of the toroidal space is covered by a single macula communis with an otolith consisting of statoconia in a porous matrix. The characteristic ciliated chambers found in the labyrinth of the lamprey are absent. The sensory cells differ strikingly from those of the lamprey and those of all gnathostomes by having a kinocilium lacking the two central filaments and the basal foot. There are also differences in the topographic arrangement of the kinocilium vis-a-vis the stereocilia. The hair-cell map of the macula communis resembles that of the lamprey maculae at either end, the hair cells pointing their kinocilia forward at the anterior and backward at the posterior end. The orientation is rather confused in the middle portion, and an area corresponding to the saccular macula of the lamprey is missing. There is no macula-neglecta. Recordings from parts of the eighth nerve innervating the cristae of the two ampullae show sensitive and characteristic responses to angular accelerations in all planes of space including the horizontal yawing plane. By dint of their orientation within the labyrinth and the uniform topographic arrangement of the hair cells on them, the two cristae are differentially sensitive to roll, pitch, and yaw. Positional responses can be recorded from nerve fibres innervating the macula communis. These resemble those obtained from the labyrinth of the lamprey (Lowenstein 1970) and cover tilts about the longitudinal and transverse body axes. No responses to vibrational stimuli were obtained from any branch of the eighth nerve, and this result may well fit in with the absence of a well-defined sacculus portion of the macula communis and also with the absence of a macula neglecta. Rhythmic discharges as recorded from the labyrinth of the lamprey (Lowenstein 1970) were not encountered.