Characteristics of hip joint mechanoreceptors in the cat.

2. Capsule afferents in the hip discharged only when the joint was rotated into its limit of movement along a single axis of rotation. 2. The optimal axis of rotation for an afferent was determined by the part of the capsule that was loaded by the rotation and by the location of the receptor in the capsule. Afferents were activated by rotations along the axes of abduction, adduction, and internal and external rotation. 3. Less than 3% of the afferents sampled responded to flexion or extension of the hip, even using extreme rotations. 4. Neurons located in the posterior and anterior regions of the capsule were studied quantitatively using as stimuli axial rotations of the femur, which load those regions of the capsule. Responses of afferents are described in relation to joint angles and torques. Thresholds for activation and saturation of slowly adapting responses of neurons are described. 5. Evidence is presented that “full-range” joint afferents, described by others in the hip joint, are afferents from the gemellus inferior muscle. 6. It is concluded that capsule afferents serve as “limit detectors,” which signal proximity of the joint to its limit of rotation.