Stretch-sensitive afferent neurons in cat knee joint capsule: sensitivity to axial and compression stresses and strains.

1. Experiments were performed to determine whether the response of stretch-sensitive mechanoreceptors to tissue deformation is caused by the axial stretching of the tissue or by the associated transverse compression of the tissue caused by the Poisson effect. 2. Single, stretch-sensitive mechanoreceptors were recorded in vitro in a preparation of innervated, isolated capsule from the cat knee. Afferents were isolated in a ligamentous capsule thickening that has a uniform geometry and parallel collagen fibers. The tissue was loaded axially while simultaneously stretching it around the surface of a cylinder to produce compression stresses and strains. Axial stresses and strains were measured or estimated. 3. By altering the diameter of the cylinder, given axial stresses and strains produced different levels of compression stresses and strains. It was possible to compare the neuronal response to pure uniaxial tension with the response when both axial stretching and transverse compression was applied. 4. In 8 of 11 experiments, transverse compression did not significantly change the response (P > 0.05). In one experiment, the response was decreased by compression. In the other two experiments, the response was increased but was not a function of the magnitude of the compressive stress. 5. Compressive strain was not significantly correlated with neuron response in any experiment. 6. The data do not sustain the model that the responses of stretch-sensitive neurons are due to local compression of the afferent ending.