Proprioceptive Reflexes in the Legs of Carcinus Maenas (L.)

Abstract

The reflex responses of the motor and inhibitor axons of the four distal muscles in the thoracic limbs of Carcinus maenas to passive movements of the dactylus and propus in the same limb are described. 1. Passive ‘opening’ (extension of the dactylus), both at steady speeds and in a stepwise manner, elicits a strong reflex discharge in the inhibitor axon specific to the opener muscle and in the ‘slow’ motor axon to the closer muscle. Passive ‘stretching’ (re-duction of the propus) evokes similar responses in the specific inhibitor axon of the stretcher muscle and in the ‘slow’ motor axon of the antagonistic bender muscle. Rapid opening (or stretching), in a fresh preparation, also excites the ‘fast’ motor axon of the closer (or bender) muscle. During passive closing or bending (propus pro-duction), the motor axon which innervates both opener and stretcher muscles discharges. No activity was observed in any of the branches of the ‘common inhibitor’. 2. The frequencies of discharge of the two ‘slow’ motor axons and the two inhibitor axons during opening and stretching do not vary with displacement at constant speeds, at least within the range of speeds used (10-500°/sec.). Frequencies of 10-60 impulses per second are common in these axons. The response frequency of the opener-stretcher motor axon during closing, however, and to a lesser extent during bending, increases with displacement at the lower speeds (10-100°/sec.). 3. For all these axons, but in particular for the ‘slow’ motor axons and also the inhibitor axons, the frequency of discharge increases with speed of movement. 4. The reflex responses to dactylus movement vary with the position of the propus, whether stretched, median or bent, in a way which indicates that the separate specific inhibitor axons to the opener and stretcher muscles do make independent action of these muscles possible, despite the fact that both muscles are supplied by one and the same motor axon. 5. The reflex responses to opening and closing can be separately abolished by cutting the requisite one of the two groups of afferent fibres leaving the ‘PD-organ’. Elimination of the receptor organ ‘CP1’ abolishes all reflex response to stretching, and ‘CP2’ elimination abolishes the reflex responses to bending. These proprioceptive motor and inhibitor ‘resistance reflexes’ are discussed in relation to control and co-ordination of the propo-dactylopodite (PD) and carpopropodite (CP) joints, and to the types of afferent fibres involved.