Efferent regulations of the abdominal stretch receptors of the crayfish.

1. Efferent regulation of the activities of the slowly adapting and fast adapting stretch receptors (the MROX and MRO2l respectively) of Astacus fluviatilis was studied. 2. It was shown that the receptors activity is under the efferent control of two central inhibitory neurones: the main inhibitory neurone (MIN) and the accessory neurone (AIN). Their activities were determined by the degree of discharge from the receptors. 3. There was close interaction between the MROj and the MIN. The activity of the MIN was determined by the activity of the MROX to a higher degree than vice versa; the activity of the MIN was influenced not only by the MROj impulses, but by the cessation of those impulses as well (silent periods). The MROX could inhibit or excite the MIN, whereas the MIN could inhibit the activity of the MROX. 4. Interaction between the MRO2 and the MIN could occur during background activity of the MROX or of the MIN. In both cases the activity of the initially non-active neurone appeared only after stimulation had been applied to the MRO2. 5. Often each impulse of the MROX resulted in a MIN discharge of from 1 to 361 impulses. Sometimes the responses appeared after burst-discharges, induced by mechanical stimulation, of the otherwise inactive MRC^. 6. During interaction between the MROX and the MIN, the neurone commencing activity had a higher discharge frequency, if only by 1-2 imp./ sec, than the one firing previously. 7. Under the influence of MIN impulses the response of the MROX to stimulation changes from slowly adapting to fast-adapting. 8. The MIN has a slight inhibitory effect on the activity of the MRO2 which' is most clearly seen in the course of background activity of the MROg. The MRO2, however, does not exert much influence on the activity of the MIN. 9. The AIN had much less effect than the MIN on receptors, and the receptors did not naturally evoke AIN impulses. There is, therefore, apparently a morej complicated connexion between the receptors and the AIN than between the receptors and the MIN.