The function of the statocyst sensory cells inAplysia timacina

Abstract

1. Single statocyst sensory cells ofAplysia limacina were studied electrophysiologically. Special attention was paid to the implications of the morphology of the statocyst for the function of the sensory cells. Attention was focussed on cells which were activated by positional changes from the horizontal position of the animal. 2. Measurements on freshly isolated statocysts show that the statocyst is only partly filled by a viscous mass of statoconia. Positional changes of the statocyst result in very slow movements of the mass of statoconia. 3. Because the statocyst is only partly filled by the mass of statoconia a single sensory cell can be in two initial states (the state of the cell just before stimulation): it can be free from the mass of statoconia or it can be covered (partly) by the mass of statoconia. 4. By turning the preparations over 360 ° in the most responsive vertical plane (see 5) of the relevant sensory cell it is estimated that the cells extend (in the direction of their most responsive plane of tilt) over 70 ° of the statocyst (Fig. 4). The responsiveness of the cells to tilts in different vertical planes indicates that the cells are regularly shaped (Figs. 10 and 11). Assuming that the cells are circular it was calculated that their diameter is about 100 μm. 5. When a sensory cell is, in the initial position, just free from the mass of statoconia the response to a maintained stepwise change in position is phasic-tonic (Fig. 5). Moreover, the cell responds differently to tilts of the preparation in different vertical planes (Fig. 10) and the response (phasic and tonic part) is linearly related to the amplitude of the positional change (in the most responsive plane of tilt) over a range of 70 ° (the dimension of the cell) (Fig. 8). During sinusoidal oscillation around the horizontal position the responses are in phase with position (Fig. 12, Table 1). In the upside-down position the responses of such a single cell to tilts of the statocyst in different vertical planes reverse as compared to responses to similar tilts (as related to the anatomy of the animal) from the horizontal position (Fig. 11). The other properties of the response remain unchanged. 6. When a particular sensory cell in the initial position and during stimulation is entirely covered by the mass of statoconia, the response to a stepwise positional change is a phasic increase of the firing level (Fig. 6). In this position the cell does not respond differently to tilts of the preparation in different vertical planes (Fig. 12) and the response is not related to the amplitude of the positional change (Fig. 9). During sinusoidal stimulations the responses are in phase with velocity (Fig. 12, Table 1). 7. On the basis of measurements of 1. the position of the preparation in which single sensory cells just touch the statolith, 2. the responses of the sensory cells to tilts of the preparation in different vertical planes, and 3. the dimensions of the sensory cells, a reconstruction was made of the location of the sensory cells in the statocyst wall. The present study concentrated on 5 sensory cells which are located near the equator of the statocyst. In this position these cells just or nearly touch the mass of statoconia when the animal is in the horizontal position (Fig. 13). 8. It is postulated that in the intact animal a ring of sensory cells is located at the equator of the statocyst and that these cells have a special function in the initiation of compensatory reflexes. 9. The implications of the present findings for the understanding of the function of the statocysts of other gastropods are discussed.