Nervous control of walking in the crab, Cardisoma guanhumi

Abstract

1. Discharge patterns in the motoneurons innervating the propodite stretcher and bender muscles and dactylopodite opener and closer muscles of the legs of the land crab, Cardisoma guanhumi, were recorded during sideways walking on a styrofoam wheel (Fig. 1). 2. Rhythmic bursts of activity in these motoneurons can be correlated with leg movements during walking in both leading and trailing legs (Figs. 2, 3A, B, D). Similar patterns of activity can be recorded in animals freely walking on a bench (Fig. 3C). 3. These motor output patterns are quite different from those occurring in the same motoneurons during resistance reflexes. In particular, the shared openerstretcher excitor and the stretcher inhibitor show no preferential phase relationship during walking (Figs. 4, 5). It appears that the normal movements of the propo-dactylopodite (P-D) joint during walking do not give rise to resistance reflexes. 4. Opening and closing movements of the P-D joint, manually imposed during walking, induce resistance reflexes (Figs. 6, 7). Reflexes tend to be stronger, both in terms of number of impulses and impulse frequency, when the imposed movement opposes rather than reinforces ongoing P-D movements. 5. Animals walking sideways on the wheel with the P-D joint clamped rigidly in a stable position show only slight changes in their motor output patterns from those seen during normal walking (Fig. 3E). 6. A model is discussed in which the proprioceptive input resulting from the normal movements of the P-D joint during walking is inhibited centrally and so does not give rise to a resistance reflex. 7. These results clearly indicate that resistance reflexes do not play an important role in coordination of antagonistic muscles during walking. Instead it is likely that their primary function is to compensate for changes in the load applied at a joint as would occur on steep inclines and irregular surfaces.