Visco-elastic properties of the rapidly adapting stretch receptor muscle of the crayfish

The visco-elastic properties of the receptor muscle associated with the rapidly adapting stretch receptor organ of crayfish (Pacifastacus Leniusculus) were studied by recording the tension responses to various length changes. Steady-state length changes resulted in a non-linear tension development in the receptor muscle. The tension increased slowly for small extensions and more rapidly when extension increased. Muscle tension responses to ramp-and-hold extension were characterized by a transient peak followed by a gradual non-exponentional decline in tension. At the onset of the ramp the tension increased rapidly, similar to what has been observed in the muscle of the slowly adapting receptor (SM). The steeper rise in tension during the first part of the ramp indicating higher initial stiffness, resulted in a 'hump' when large extensions (> 15%) were applied. The results show that the rapidly adapting receptor muscle has a more pronounced dynamic component; the ratio between the amplitude of the peak and the steady state response was larger in the rapidly than in the slowly adapting receptor muscle. Accordingly, different values for the elements of a visco-elastic model of the muscle had to be set for the two types of receptors. The different properties of the rapidly and slowly adapting receptor muscles are in line with the differences in the overall adaptive behaviour of the organ and give further support to the idea that mechanical factors contribute to the adaptive properties.