Mobilization of a Coordinated Escape Response by Giant Axons in the Ophiuroid, Ophiopteris Papillosa

Abstract

ABSTRACT The escape response of Ophiopteris papillosa to contact by a predatory asteroid consists of a fast withdrawal of the stimulated arm, completed in less than 1 s, followed by rapid locomotion carried out by coordinated rowing of the two arms opposite the one stimulated. Electrical activity recorded from the radial nerve cord (RNC) during the locomotory phase consists of small-amplitude spikes (<5μV). The initial arm jerk response is mediated by sequential activation of segmental intervertebral muscles, and the onset of activation progresses centrally at a rate of <10cms-1 commencing 100-200ms after stimulation. Electrical activity recorded from the RNC immediately after tubefoot stimulation consists of a burst of large-amplitude spikes (50-100 μV) that propagate centrally at approximately 50cms-1. Electrical activity in the RNC with the lowest threshold to direct electrical stimulation consists of large spikes propagating at a mean velocity of 55cms-1 at 13 °C, and this activity persists in a Ca2+-free medium. Electrical and tubefoot stimulation of the arm tip in the same preparation both trigger a burst of large action potentials that propagate at approximately 50cms−1. The identity of the giant axons activated by tubefoot stimulation, the path-ways they follow and their role in mobilizing the coordinated escape response are discussed.