Reflex antennal movements in the spiny lobster,Palinurus elephas

Abstract

1. Reflex antennal movements in the frontal plane, brought about by movement at the J1 joint, occur in the spiny lobster,Palinurus elephas, in response to tilting movements of a platform held under the animal's legs. Imposed movements of the J1 joint give rise to resistance reflexes. Experiments in which the two stimuli are delivered with different timing relations demonstrate that the two reflex pathways remain simultaneously operative. These results suggest a premotor organisation of motor patterns in the antennal muscles, and identify an integration step in the mechanics of the final movement. 2. On the basis of myographic recordings, the innervation of the two external and three internal rotator muscles that operate the J1 joint involve both the sharing of axons between synergists, and synergists receiving distinct but strongly coupled axonal supplies. 3. Two distinct modes of coordination are observed in the antennal rotator muscles. Alteration of antagonists, apparent not only in the general alternation of bursts but also in burst microstructure, occurs during reflex activity (Fig. 2B) and in the absence of imposed stimulation (Fig. 2A), while co-activation of antagonists occurs following general tactile stimulation of the body (Fig. 2A). Biases in the level of both tonic activity and reflex responsiveness towards either rotator muscle commonly occur under a variety of stimulus conditions (Fig. 2B). 4. The amplitude of the reflex responses to platform tilts is determined both by the amplitude of platform tilt and by the number of legs in contact with the platform. The reflex remains strongly phasic even at low frequencies of stimulation although at such frequencies an antennal muscle sometimes fires during movement of the platform in both directions (Fig. 3). 5. Resistance reflexes occurring in response to imposed antennal movement utilize the same pool of motoneurons as are involved in the responses to platform tilts. 6. Ramp platform tilts delivered during continuous sinusoidal movement of the antenna cause predictable changes in the pattern of resistance reflex discharges. An enhanced response in a muscle occurs when a ramp inducing discharge in a particular muscle is timed to coincide with the burst in that muscle, while a reduced burst together with an extra burst of firing in the antagonist muscle follows the application of a ramp movement in the opposite direction at the same relative time (Fig. 4A). An equivalent series of effects follows application of ramp antennal movements during continuous sinusoidal platform tilts (Fig. 4B). 7. Simultaneous sinusoidal movement of both platform and antenna such that the resistance reflexes are in phase with the responses to platform oscillation produces a pattern of simple reciprocation between the antagonist muscles of the moved antenna with clearly-defined non-overlapping bursts (Fig. 5C). When the two stimuli are arranged to conflict, the muscles are co-activated, each firing two bursts in each stimulus cycle (Fig. 5D). During such antiphase stimulation, reductions in the amplitude of platform tilt lead to enhancement of resistance reflex effects as well as reduction in the size of the bursts due to platform stimulation (Fig. 6).