Force-sensitive mechanoreceptors of the dactyl of the crab: single-unit responses during walking and evaluation of function

The activities of individual force-sensitive mechanoreceptors of the dactyl (terminal leg segment) of the crab, Carcinus maenas, have been recorded during free walking. These receptors have also been mechanically and electrically stimulated in freely moving animals to directly evaluate their function in locomotion. All force-sensitive mechanoreceptors fired during the stance phase of walking and were silent during swing. Receptor discharges showed regular phase relationships to bursts in motor neurons of leg muscles. Crabs walk laterally and use the legs of one side either in trailing to actively push the animal to the opposite side, or in leading, to less forcefully pull the animal in that direction. Individual force-sensitive mechanoreceptors differed in their patterns of activity during trailing or leading according to their location on the dactyl. Units of proximal receptors fired more vigorously when used in trailing than in leading. Discharges in trailing were also increased by loading of the animal. In contrast, distal receptors near the dactyl tip fired equally intensely during walking in either direction. Proximal receptors thus encode forces and loads applied to the leg. Distal receptors do not encode loads but can signal leg contact and, potentially, exteroceptive vibrations. Sensory stimulation of force-sensitive mechanoreceptors was produced during walking by a device that imposed continuous mechanical bending of the dactyl and by electrical stimulation of dactyl nerves. Intra- and inter-segmental reflexes were evaluated by myographic recordings from leg muscles. Continuous mechanical deformation of the dactyl increased the activity of the levator and decreased firing in the depressor muscles of the homonymous leg during walking. The same stimulus produced enhanced activity in depressor muscles of adjacent legs. The latter effect was not due to simple mechanical coupling resulting from reflexes in the stimulated leg. These reflexes can function to limit forces applied to a leg and provide compensatory adjustments in other legs. Brief low-threshold electrical stimuli applied to nerves in which the activities of force-sensitive mechanoreceptors were recorded produced reflex effects similar to those obtained by mechanical stimulation. These stimuli also reset the rhythm of motor neuron bursting in both homonymous and adjacent legs during walking. These studies confirm the importance of force-sensitive mechanoreceptors in adapting walking patterns and in determining leg coordination in locomotion.