MECHANICAL STATES ENCODED BY STRETCH-SENSITIVE NEURONS IN FELINE JOINT CAPSULE

1. The sensitivity of group II joint afferents innervating cat knee jo int capsule to in-plane stretch was studied in vitro. Single afferents were recorded from teased filaments of the posterior articular nerve. The capsule was stretched by applying forces through tabs along the e dges of the capsule (3 tabs/edge) with the use of an apparatus that al lowed for independent control of each load. The relationships between the neural responses of these afferents and the local continuum mechan ical state of the joint capsule have been investigated. By appropriate ly loading the tissue margins, it was possible to establish states of uniaxial and biaxial tension, including shear. 2. Plane stress was cal culated from the loads along the tissue margins. Stress at the locatio n of the mechanorecepter ending was estimated by interpolation. Strain was calculated from deformations of the capsule measured by tracking markers on its surface. Full characterization of tissue stress and str ain made it possible to determine strain energy density and the magnit udes of other coordinate invariant mechanical quantities. 3. Individua l afferents (n = 15) exhibited pronounced selectivity to the direction of applied stress and strain. There was no overall preferred orientat ion across neurons, and simple correlation of individual stress or str ain components with the neuronal response revealed no consistent relat ionship between neuronal response and any single tensor component. How ever, linear multiple regression of the combined stress and strain com ponents with the neuronal response revealed high correlation (mean R = 0.91), indicating that the measured mechanical states strongly determ ine the neuronal response. There was a much stronger relationship betw een neuronal response and stress variables than with strain variables. Simple correlation of the first invariant of the stress tensor with n euronal response had the highest mean correlation of the tensor quanti ties (R = 0.51). On average, strain energy density was only modestly c orrelated with the neural response (R = 0.28). 4. These findings indic ate that capsule mechanoreceptors are encoding the local continuum mec hanical state in the joint capsule. The neural response of these mecha noreceptors is more strongly correlated to local stress than to local strain.