ACC Hall C
I AIM OF INVESTIGATION: Sensory receptors in the cat's knee joint with fine afferent nerve fibers (groups III ' and IV) have a wide range of thresholds to local mechanical stimuli and to passive joint movements (Schaible & Schmidt, Exp Brain Res Suppl9, 284-297, 1984). On the other hand, the sensory endings of these nerve fibers vary in regard to several ultrastructural parameters. As yet, due to methodical reasons these morphological data could not be directly related to the physiological ones. Now for the first time we present a group III fiber which was characterized by electrophysiological recordings followed by electron microscopical identification of its sensory ending.
METHODS: A single unit was recorded from the saphenous nerve and characterized by its conduction velocity, its v.Frey threshold in its receptive field in the knee joint, and its response to passive joint movements in the innocuous and noxious range. The receptive field was closely marked off by fine steel needles, the knee joint was fixed by perfusion, and the marked tissue was dissected for histology and electron microscopy. Reconstructions of the marked tissue block and the sensory ending were performed from serial semi-and ultrathin sections.
RESULTS: The group III fiber had a conduction velocity of 6.4 m/s, no resting activity, and a high threshold both to local mechanical stimuli (v.Frey threshold 12.5 g) and to movements of the knee joint (marked activation only by noxious rotation, i.e. a presumed nociceptor). The most powerful response with local stimulation was by stretching the receptive field with a needle in one direction.
The histological analysis revealed that the sensory ending was inserted in a thin ligament on the outer surface of the knee joint capsule near the articular cleft. Presumably it was the stretching of this ligament which gave the high frequency response reported above. A terminal segment of the sensory ending was reconstructed at a length of 30 pm and evaluated by morphometry. Its ultrastructure showed a small diameter group III sensory axon and a medium content of mitochondria.
CONCLUSIONS:
The study shows that it is possible to identify the sensory endings of group III fibers in deep tissue which are previously characterized by electrophysiology. Thus it may be possible to correlate physiological and morphological data, aiming at a new insight in the mechanotransduction of fine afferent nerve fibers.