Signal transduction and nonlinearities revealed by white noise inputs in the fast adapting crayfish stretch receptor

Summary Input-output relations were investigated in the fast adapting stretch receptor organ (RM2) of the crayfish by matching gaussian white noise (GWN) length inputs, with the resulting spike output. The analysis revealed the expected sensitivity to lengthening velocity, a behavior termed phasic. It also disclosed a sensitivity to sustained elongation, a performance termed tonic and previously not recognized in the RM2. Spectral analysis indicated the properties of a low-pass filter, confirming the tonic sensitivity. A variety of individual length trajectories could lead to a spike. The average trajectory consisted in a biphasic shortening-lengthening wave. The range of possible trajectories and their averages changed with stimulus prestretch and GWN amplitude, indicating that system properties depended on the input characteristics; i.e., a nonlinear operation. Length waveforms in the GWN were isolated by computing methods and the corresponding responses were calculated. Symmetric stimuli led to responses that reflected magnitudes and velocities asymmetrically. Nonlinear interactions between responses in the past and present were negligible. In conclusion, depending on the input, the RM2 modifies its operation to enhance the detectability of the functionally relevant signal in each natural situation.     

Potential-dependent potassium currents in the slowly adapting stretch receptor neuron of the crayfish

The outward current in the stretch receptor neuron of the crayfish Pacifastacus leniusculus was analysed using a two-micropipette potential-clamp technique. The outward current was shown to be carried by K+. When the sodium-dependent inward current was blocked by tetrodotoxin a fast-activating potassium current was revealed, resembling the delayed rectifier. The time-course of activation (Tau n) was dependent on potential and had a mean value of I ms at potential steps of to mV. The activation followed a second-order process according to the Hodgkin-Huxley model. The potential dependence of activation (n infinity) followed a sigmoid curve, n infinity = I/(I + exp [(E-En)/a]) with half-maximal activation potential En = -31 mV and a = -13 mV. When long pulses were applied, the potassium current showed marked inactivation with a fast time constant of 0.5 s that was potential independent and a slow component that was slightly potential dependent. The minimum value for the slow time constant was 4 s for steps to about 0 mV. The potential dependence of inactivation followed a sigmoid function k infinity = I/(I + exp [(E-Ek)/a]) with Ek = -39 mV and a = II mV. No transient potassium outward current (IA) was found in the crayfish stretch receptor neuron. In experiments on tail currents after depolarizing potential steps of different duration, it was found that the reversal potential changed in the positive direction when the duration of the pre-pulse increased. This could be due to K- accumulation in a space close to the neuronal membrane. The potassium current during depolarizing potential steps in the crayfish stretch receptor is similar to the delayed current found in other cells, for example the frog myelinated nerve, but different from many other invertebrate neurons.