Efferent Regulation of the Abdominal Stretch Receptors of the Crayfish

1. Efferent regulation of the activities of the slowly adapting and fast adapting stretch receptors (the MRO1 and MRO2, 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 MRO1 and the MIN. The activity of the MIN was determined by the activity of the MRO1 to a higher degree than vice versa; the activity of the MIN was influenced not only by the MRO1 impulses, but by the cessation of those impulses as well (silent periods). The MRO1 could inhibit or excite the MIN, whereas the MIN could inhibit the activity of the MRO1. 4. Interaction between the MRO1 and the MIN could occur during background activity of the MRO1 or of the MIN. In both cases the activity of the initially non-active neurone appeared only after stimulation had been applied to the MRO1. 5. Often each impulse of the MRO1 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 MRO1. 6. During interaction between the MRO1 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 MRO1 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 MRO2. 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 more complicated connexion between the receptors and the AIN than between the receptors and the MIN.