Activation of groups III and IV sensory units in medial articular nerve by local mechanical stimulation of knee joint.

Abstract

The activity of single fine joint afferent units of the medial articular nerve was 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 and the medial collateral ligament. The units were identified by their conduction velocity as belonging either to group III (2.5-20 m/s, 55 units) or group IV (< 2.5 m/s, 51 units). Resting activity has been observed in .apprx. 1/3 of the fine afferents. Most units discharged with < 1 impulse/s. The resting discharges were irregular. The receptive fields were small, often spotlike. In some units 2-4 receptive fields several millimeters apart from each other were found. The minimum thresholds of freely accessible receptive fields were determined with von Frey hairs. They ranged from 0.3-22.5 g and sometimes above. In both fiber groups high- and low-threshold units were found in the capsule and the medial collateral ligament. The stimulus-response relationships of the low-threshold group III units were often steep, giving in some units maximum responses at < 3 g. With higher stimulus strength the peak frequecies declined. The other group III units as well as practically all group IV units had well-graded responses over a wide stimulus range. Peak discharge frequencies in group IV units were rarely > 20 Hz; those in group III units were usually > 20 but < 50 Hz. The results document that the medial articular nerve, in addition to the well-known mechanoreceptors with fast afferent fibers, contains numerous mechanosensitive fine afferent units with small receptive fields on the medial aspect of the knee joint. The mechanosensitive properties of these units suggest that some of them are activated by everyday nonnoxious local mechanical stimuli, whereas the others probably serve as nociceptors.