Properties of Ruffini afferents revealed by stress analysis of isolated sections of cat knee capsule.

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Abstract
Sensory neurons innervating joint capsule receptors were studied using isolated, innervated sections of joint capsule from the posterior region of the cat knee subjected to controlled loads and displacements in a stretching device. Afferents were exquisitely sensitive to capsule stretching. Discharge could be produced by stretching along either or simultaneously along both axes in the plane of the specimen. No unique sensitivity to the components of plane stress was seen. Some neurons were most sensitive to stress along the femur-to-tibia axis, others to stress along the medial-to-lateral axis, and still others were most sensitive to the magnitude of the shear stress on a 45.degree. plane. The response of most afferents could be accounted for in terms of a linear relationship involving these 3 components of plane stress. Variations in this linear sensitivity of afferents may be related to gross variations in the orientations of the fibrous material comprising the capsule. Most afferents were relatively insensitive to compressive stress in a direction perpendicular to the plane of the specimen. Some afferents were studied 1st in intact capsule in situ; then the capsule was excised and those afferents were studied in isolated capsule. Neurons sensitive to extension movements in situ had properties like those described when the capsule was excised and studied as isolated tissue. In histological experiments; the region of capsule from which afferents were sampled was populated almost exclusively with receptors of the Ruffini type. Ruffini afferents in the knee posterior articular nerve are sensitive to linear combinations of the components of plane stress and that their sensory role is to signal proximity of the joint to its limit of rotation in extension.