Measurements of Turtle Head and Semicircular Canal Orientation Using a Modified Small Animal Stereotaxic Device

INTRODUCTION The vestibular system detects and encodes changes in angular and linear accelerations of the head. This information is then used in a broad repertoire of vertebrate motor behaviors essential for survival. We are trying to understand some of the underlying mechanisms of vestibular function by studying components of the vestibulocollic reflex (VCR), one of the most basic mechanisms by which adjustment of head position is achieved. The function of the VCR is to stabilize head and cranial sense organs in space and is elicited by perturbations of the head. These perturbations stimulate angular and linear labyrinthine receptors and result in compensatory counter-rotations of the neck. Mammals have been typical subjects for such studies but in our work we chose turtles because this animal offers a number of theoretical and practical advantages for analyzing vestibular function in general and the VCR in particular. One advantage is that since turtles have a rigid shell and no back muscles, the vestibular control of head position must be achieved entirely by neck movements. Therefore the turtle might be considered a natural 'isolated head-neck' preparation. A stimulus evoking the VCR is angular acceleration of the head and this stimulus is detected by a total of six semicircular canals: one horizontal and two vertical canals on each side of the head. The commonly held view regarding the spatial orientation of these canals is that the three ipsilateral canal planes are mutually perpendicular and this is perfectly mirrored by the contralat-eral canals. Closer examination of mammalian species has shown significant deviations from the 'ideal' arrangement suggesting that stimulation of one canal will result in some stimulation of another and the total pattern of activity set up by head rotation will depend on the angular relations between canals. As a consequence, it is necessary to know the planar relations between semicircular canals so that we may understand the labyrinthine output caused by head movement of any spatial form. What follows is a brief summary of a study we conducted to determine canal orientation in awake, resting turtles (Brichta et al., 1988).