Sensory transduction in a locust multipolar joint receptor: The dynamic behaviour under a variety of stimulus conditions

1. The dynamic behaviour of a multipolar mechanoreceptor in the locust femoro-tibial joint has been studied by linear systems analysis using both sinusoidal and pseudo-random mechanical stimuli. Experiments were performed at different mean positions of the joint and with a range of stimulating signal strengths. 2. The joint sensilla respond to increasing joint extension with increasing tonic firing of action potentials. Movements of the joint position produce modulation of the firing rate around the mean level which allows faithful reproduction of the input signal in the probability of action potential occurrence. However, at frequencies of about 1 Hz and above the response increasingly phase-locks to a repetitive signal. 3. For sinusoidal stimuli up to 1 Hz and for random stimuli up to 10 Hz the dynamic behaviour can be very well characterized as fractional differentiation. In the frequency domain this corresponds to a frequency response function which can be represented by the gain at a frequency of 1 radian/s and a fractional exponent of frequency. 4. The dynamic behaviour of these sensilla is discussed in terms of models of transduction for cuticular and multipolar sensilla.