Recurrent inhibition: Its influence upon transduction and afferent discharges in slowly-adapting stretch receptor organs

The crayfish stretch receptor organs provide the opportunity to study recurrent inhibition (RI) in pure experimental conditions. The slowly adapting organ (SAO) from the cephalothoraxabdomen joint of Procambarus darkii was isolated and its length set by anchoring one end and clamping the other with a movable forceps. Each impulse from the SAO triggered a shock to the common inhibitory fiber, after an adjustable delay called “phase”. Thus, RI acted upon the SAO. Under steady length conditions, the discharge rate with RI was proportional to the spontaneous discharge rate (i.e. to that without RI), and negatively correlated to the ratio of the phase to the spontaneous interspike interval. Under dynamic conditions, with the SAO submitted to under 0.3 mm length variations, sine-like with or without “white” noise, and periodic at 0.2, 1.0, or 3.0 cps, the effect of RI was basically the same in all conditions. While the lower rates of discharge were unchanged, the higher rates were reduced: thus the overall sensitivity of the SAO was decreased. The RI also decreased the discharge variability at the low rates; variability remained unaffected at the high rates. The general form of the response of the SAO was unaffected by RI, as indicated by displays of rate versus length, of rate versus velocity, or of change in rate versus velocity: only the widness of the loops changed. The effects of RI were compared to those of randomly arriving, Poisson-like IPSP's at the same overall rate: the latter did not exhibit the same dependence on the SAO firing rate.