Previous studies of the responses of joint efferent neurons, and of reflexes mediated by their activation, have employed primarily passive externally generated changes in joint angles and joint torque as stimuli. Yet, a number of investigators have demonstrated that joint afferents may discharge in response to activation of muscles acting at the joint, in the absence of changes in joint angle. Previous work on the present project has indicated that a number of variables may influence discharge in knee joint afferents; the effect of each of these variables may be mediated by changes in the mechanical status of the joint capsule. The object of the proposed research is to further investigate the mechanical factors involved in the initiation of afferent discharge in joint neurons. Extrinsic factors such as force in muscles acting at the joint will be systematically investigated, and the study of responses to passive changes in joint angle and joint torque will be continued using the method of single unit recording in dorsal root filaments. The role of intrinsic factors relating to the specific properties of capsullar tissue will be investigated by studying the properties of the capsule either in vitro as isolated nerve-capsule system, or in situ by passively stretching the capsule with passive movements. The results of the above experiments will be applied to the study of joint-mediated reflexes. Joint afferents will be activated using conditions which have been demonstrated to be optimal in activating joint afferents and reflex changes in motoneuron excitability will be tested using monosynaptic reflex testing.