Motor Pattern Analysis in the Shore Crab (Carcinus Maenas) Walking Freely in Water and on Land *

Neuromuscular activity underlying lateral walking was studied in the shore crab Carcinus maenas. Electromyograms (EMGs) were recorded from legs on both the trailing and leading sides during free walking on land and under water in a pool (Figs 1, 2, 6, 7). In a trailing leg, the levator, flexor and closer muscles were active during the return stroke (RS) in alternation with the depressor, extensor and opener muscles which were responsible for the power stroke (PS). In a leading leg a different pattern of activity was observed. The flexor and closer muscles were active during the PS, and the extensor and opener muscles during the RS. Trailing steps were shorter and less variable in duration than leading steps (Figs 2, 3 for walking under water, Fig. 6 for walking on land, see also Fig. 7). A comparison of the activity patterns of the single common motor neurone innervating the opener and the stretcher muscle during trailing and leading showed a difference in burst length and instantaneous frequency (Fig. 2C,D). The discharge of this motor neurone usually lasted longer in leading steps. The discharge frequency started at a high level and then decreased during a trailing step, whereas in a leading step it was irregular (Fig. 8). In all walking situations the stretcher and opener muscles, which share a common excitatory motor neurone, received identical excitatory input (Fig. 4). The discharge frequencies of motor neurones innervating the extensor, the stretcher/opener and the closer muscles were investigated (Fig. 5). For motor neurones active during the PS, the frequency was high at the onset of the burst and then declined gradually. With the exception of the closer muscle, the discharge of motor neurones during the RS was more or less constant during the burst. A comparison between walking under water and on land showed that the overall EMG patterns were essentially similar (Fig. 7). However, on land the PS lasted longer and involved the activation of additional motor neurones in muscles which are innervated by several motor neurones, e.g. the extensor (Fig. 6). During walking on land maximal discharge frequencies up to 350 Hz were recorded. Homologous muscles in the three different walking legs operated similarly during trailing or leading movements without major differences in their EMG patterns. This indicates a similar load distribution on the different legs (Fig. 9).