Sensory-synaptic interactions in crayfish stretch receptor neurones

Abstract

1. This communication describes, in the tonic stretch receptor organ (RM1) of crayfish, the inhibitory fibre's influence upon sensory modulated discharges. Periodic trapezoidal length changes were imposed, and rate plots of the afferent discharges were compared without inhibition (C) and with inhibition, either irregular (P) or regular (R), and at different rates. 2. Inhibition changed all sensory response components. Changes were dependent on issues as inhibitory rates and patterns, RM 1 discharge modulations, and pre-post-synaptic rate ratios. The most common effect was rate reduction, usually non-uniform along the cycle and with little evidence of proportionality. Saturation, i.e., inefficacy of shifts around extreme inhibitory rates was apparent. Rate increases occurred also. Accelerations were manifest by increased phasic lengthening response slopes and heights, by “faster inhibitor-faster inhibited” relations, or by postinhibitory rebounds. 3. Irregular inhibitory discharges (i) favoured variability along individual and average cycles, (ii) favoured monotonicity, (iii) rarely silenced the RM 1, and (iv) reduced without eliminating nonproportionality and saturation. 4. Regular inhibitory discharges showed the most clear-cut nonmonotonicities and saturations silencing the RM 1 effectively. Furthermore, they included characteristic epochs where (i) the RM 1 spike slid across the invariant interinhibition intervals, (ii) intervals recurred in stereotyped sequences and (iii) rate ratios had special values (e.g., 1:1, 1:2). Thus, the gradually decaying slope of the control adaptation after the lengthening transient was changed into a staircase profile or a sudden drop to a constant plateau. 5. Inhibition changed phasic and tonic sensitivities, usually refucing them (phasic decreasing less than tonic); increases, joint or individual, occurred also. The fidelity with which the rate plot reproduced the sensory stimulus was modified in many ways (e.g., by conversion into a phasic prototype, or into a system with perfect reproduction, etc.). Changes depended on the inhibitory discharge, and on the stimulus features. 6. These experiments have implications in two fields. In that of synaptic rate effects, (i) they confirm that the inhibition repertory includes slowings (predominant here) and accelerations, plus special effects, and (ii) they demonstrate extensively their dependence on the post-synaptic features. In the field of sensory control, they note sensory-synaptic interactions that, in intact animals, must arise but whose characteristics and roles can only be conjectured about.