Transfer properties of the slowly adapting stretch receptor of the crayfish abdomen.

The slowly adapting stretch receptor in the abdomen of freshwater crayfish (Astacus fluviatilis) was investigated to determine its properties under dynamic conditions. An in situ preparation was used; the necessary dissection did not involve the receptor organ or its immediate surroundings. Sinusoidal variations in the angle of flexion in the joint to which the receptor organ was connected, were generated by a feed-back controlled stretcher. Nerve spiked recorded from the axon of the receptor neurone and information about angle of flexion in the joint obtained by position transducers, were fed into a computer. Fourier transforms were performed on both input and output data to determine the amplitude of the 0. and 1. harmonic together with the phase of the 1. harmonic. The receptor organ was investigated for linearity up to 1.5 degrees input amplitude, and proved to be surprisingly linear within this range. In addition, the transfer function of the receptor organ was determined by stimulating it with small-amplitude sinusoidals with different frequencies. With a steady flexion of 35-40 degrees in the joint, the gain of the receptor organ increased 5-6 times when the modulation frequency of the input signal was increased from 0.1 to 5 cycles/s. A maximum in gain was constantly found at about 5 cycles/s, with a rapid fall towards 0 when the modulation frequency was increased further. A change in phase lead from positive (leading output) to negative with change in sign about 1 cycle/s was also found. These results resemble the results found by investigators of isolated preparations. A "hold" property is probably a part of the overall property of the receptor organ together with an element of Maxwell type. An element of the form h(s) = ksn with n approximately 0.45 is also a part of the transfer function of the receptor organ, although the physiological parallel to this element is uncertain.