The response of theOctopus angular acceleration receptor system to sinusoidal stimulation

Summary

The nine crista/cupula sections (C1–C9) that form the angular acceleration receptor system ofOctopus vulgaris can be divided into two subsystems on the basis of cupula size. Electrophysiological recordings were made from representatives (C1 and C2) of these two sub-systems to determine their functional response characteristics.

• 1. With the statocyst stationary in the normal upright position, 62% of the afferent units from erista section C2, which has a large cupula, displayed a resting level of activity. None of the afferent units from erista section C1, which has a small cupula, showed resting activity.

• 2. The dynamic responses of the C1 and C2 units were examined during sinusoidal oscillations of the statocyst in the pitch plane at stimulus frequencies between 0.01 and 1.6 Hz, and with amplitudes between 3° and 35°. Both the C1 and C2 units showed linear increases in their responses with increasing stimulus amplitudes (Fig. 5). The threshold frequency at which a unit first responded was found to be an order of magnitude lower for the C2 units in comparison to the C1 units (Fig. 9). At a stimulus frequency of 1.6 Hz, 40% of the C2 units showed a saturation in their responses. None of the C1 units showed signs of response saturation at this frequency.

• 3. The gain values of the C1 and C2 units decreased with increasing stimulus frequency; the C1 values were higher than the C2 values (Fig. 7). The phase lag values, with respect to peak angular acceleration, were lower for C2 units compared with C1 units (Fig. 8). The phase lag values of both types of unit increased with increasing stimulus frequency but, in the middle frequency range, showed a plateau level at 100° for the C1 units and between 80° and 90° for the C2 units (Fig. 8). This indicates that in the middle frequency range the units code angular velocity. Both phase and gain values were affected by changes in the orientation of the statocyst with respect to gravity (Fig. 12).

• 4. All of the units analysed in this report originated from ventrally polarized receptor cells. However, during large amplitude oscillations, responses from dorsally polarized receptor units were also observed (Fig. 13).

• 5. These results demonstrate that the octopus angular acceleration receptor system is divided into two sub-systems of differing sensitivities and that the dynamic response characteristics of the system show close parallels with those of the vertebrate semicircular canal system.