Single-unit responses and reflex effects of force-sensitive mechanoreceptors of the dactyl of the crab

1. This paper examines the responses and reflex effects of force-sensitive mechanoreceptors of the most distal leg segment, the dactyl, of the leg of the crab, Carcinus maenas. The goals of these studies are to establish the potential activities and functions of these receptors in posture and locomotion. 2. The responses of force-sensitive mechanoreceptors to imposed mechanical stimuli depended upon their location on the dactyl. A distal group of receptors is located on a specialized region, the dactyl tip, which is composed solely of epicuticle. Another group of receptors is distributed throughout more proximal regions of the dactyl where the cuticle is completely calcified. 3. Both groups of receptors showed vigorous responses to imposed bending forces. When bending forces were applied as step functions at the dactyl, tip distal receptors showed only phasic responses to all levels of force application. Receptors located at more proximal positions on the dactyl showed phasic responses to low levels of step applied forces and phasicotonic discharges at higher levels of force. Increasing levels of force produced a sigmoid increase in the tonic firing of these units. 4. When bending forces were applied using ramp functions, receptors of the distal group responded with an intense initial charge followed by firing at a constant rate throughout both force application and release. This response was not related to the velocity of force application. In contrast, receptors located more proximally responded directionally to force application and release. Proximal receptors also effectively encoded the velocity of force application. 5. Responses of these two groups of receptors also differed when vibrations were applied at the dactyl tip: proximal receptors only followed vibrational stimuli up to 50 Hz, whereas distal receptors showed 1:1 responses at vibrations as high as 95 Hz. 6. Mechanoreceptors of the dactyl also responded intensely to bending forces resulting from resisted contractions of the animal's own muscles. No responses were obtained from unresisted movements of the leg. 7. Stimulation of force-sensitive mechanoreceptors of the dactyl produced intra- and interleg reflex discharges in motor neurons to leg muscles. Mechanical bending of the dactyl or electrical stimulation of dactyl nerves in which force-sensitive mechanoreceptors were recorded produced strong tonic excitation of motor neurons to the levator muscles of the same leg. The same stimuli produced excitatory discharges in depressor muscles of adjacent ipsilateral legs. These reflexes form a negative feedback system that could limit the forces applied to the leg during the stance phase of walking and aid in the coordination of leg movements. 8. Force-sensitive mechanoreceptors of the dactyl could thus provide proprioceptive inputs about mechanical forces applied to the leg and aid in the adaptation and coordination of locomotion.