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

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Abstract
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.