Neuromuscular strategies underlying different behavioral acts in a multifunctional crustacean leg joint

Abstract

1. The movements of the merocarpopodite (M-C) joint of the rock lobster were recorded simultaneously with electromyograms from the extensor, flexor and accessory flexor muscles during unrestrained walking in all directions. In optimal recordings it was possible to distinguish the activity of several identifiable motor neurons. 2. During lateral walking, the M-C joint makes vigorous movements (Table 1), due in large part to strong alternating discharge in the antagonistic muscles (Fig. 2, Table 2). During forward and backward walking, the movements of the M-C joint occur over a more restricted angular range (Table 1). Here the antagonistic muscles are co-activated (Fig. 2, Table 2), so that the joint functions more as a rigid strut. 3. Extreme variability was observed in the timing of the discharge of several identified motor neurons between steps during walking in a given direction (Table 3). Implications with regard to the nature of the pattern generator are discussed. 4. Major differences were observed in the discharge frequency of these identified motor neurons between walking modes when the corresponding muscles served a power stroke, return stroke or postural function (Fig. 8, Table 4). Variability occurred in the discharge of some units between steps in some walking directions, but was not directly related to step period (Figs. 9, 10; Table 4).