Rami motor neurons and motor control of the swimmeret system ofHomarus gammarus

Summary

• 1. In intactHomarus gammarus, the swimmeret motor patterns present during the startle response and in gravid females, are very different. Kinetics and electromyograms (EMGs) reveal that the major difference is that the curler muscles of the rami (endopodite and exopodite) are active in the first response and silent in the second (Figs. 2, 3, 4).

• 2. An in vitro preparation has been used (Fig. 1) to analyze the activity of the curler motor neurons (MNs). In this preparation, all peripheral structures have been kept intact. This allowed us to show that the curler MNs axons run through root Ia (Fig. 5).

• 3. The in vitro preparation is sometimes spontaneously rhythmic. An individual curler MN is active either during the powerstroke (PS) or the returnstroke (RS) of the swimmeret rhythm (Fig. 6). There are more ‘PS’ curler MNs than ‘RS’ curler MNs.

• 4. Curler MNs active in the same stroke are often electrically coupled regardless of the ramus that they innervate (Fig. 8).

• 5. Sensory activity from a swimmeret ramus strongly modifies the activities of the curler MNs of the two rami as well as those of other swimmeret MNs (Fig. 9). In rhythmic preparations, periodic electrical stimulation can entrain the MN rhythm within a certain range of stimulus frequencies (Figs. 10, 11).

• 5. Thus, the curler MNs from both rami are linked centrally (electrical coupling) and peripherally (sensory afferents activated by movement). These two mechanisms synchronize the activities of the two rami.