Responses of fine medial articular nerve afferents to passive movements of knee joints

Abstract

The responses of single fine afferent units of the medial articular nerve to passive movements of the [cat] knee joint were recorded from filaments of the saphenous nerve of the cat''s right hindlimb. All units included in this study were sensitive to local mechanical probing of the medial and anteromedial aspects of the knee joint capsule. The units were identified by conduction velocity as belonging either to group III (2.5-20 m/s, 36 units) or group IV (< 2.5 m/s, 46 units). According to their response behavior to passive nonnoxius and noxious joint movement, they could be classified in 4 distinct categories. Eleven group III (30.5%) and 6 group IV units (13%) were activated by nonnoxious movements of the knee joint. Most often flexion and/or extension excited these units, but rotations of the joint in the long axis of the tibia startng from various angles of the knee joint were more effective. All units of this type were excited by many different movements. Seven Group III (19.5%) and 8 group IV units (7.5%) were only weakly activated by nonnoxious movements. These movements elicited only a few impulses in 1 or a few joint positions (usually outward rotations of the tibia in half or full extension). Noxious movements led to pronounced discharges of these units. Ten Group III (28%) and 12 group IV units (26%) did not respond to any nonnoxious joint movements but fired clearly and consistently to noxious ones. The discharges showed little adaptation during the maintenance of noxious joint positions. The most effective stimuli were forceful rotations in the long axis of the tibia. Units with receptive fields on the medial aspect of the joint responded to rotations in extension and flexion, whereas those with receptive fields anterior or posterior to this area were activated only by rotations of the extended joint. Eight Group III (22%) and 20 group IV units (43.5%) could not be excited by any joint movement although 1 or more local receptive fields could be clearly identified. Most of these units had their receptive fields on the anterior aspect of the joint, on or next to the patellar ligament. Articular mechanoreceptive units with fine afferent fibers contribute to deep pressure sensation and nociception. They seem to be mainly engaged in signaling that the joint is about to leave its normal working range. These warning signals are presumed to induce motor reflexes counteracting the excess movement, thus preventing joint damage.