Neuromotor Bases of the Escape Behaviour of Nassa Mutabilis

Abstract

ABSTRACT The complex sequence of movements in the escape behaviour of the snail Nassa mutabilis (L.) was described in detail and the neuromotor activity underlying the behaviour was investigated by extra- and intracellular recording. The escape reaction is triggered by a chemical stimulus to the animal ‘s foot, in these experiments either application of KC1 solution or contact with a starfish. It consists of a preliminary phase in which the shell tilts to its side, the actual locomotor phase, and a final righting movement. The snail performs leaps, in which the foot and the shell are repeatedly rotated with respect to one another. EMGs recorded from the columellar muscle during the escape reaction showed that bursts of potentials are coupled to the shell rotations. In the intact animal this burst activity ordinarily began 0 ·6 ± 0 ·3 s after stimulation with KC1. In an animal dissected for recording from the columellar nerve (which supplies the columellar muscle), KC1 stimulation of the dorsum of the foot induced burstlike neuronal activity with a latency of 0 ·5 ± 0 ·3 s. The dorsal foot region, the site at which the escape reaction can be triggered, was found to be supplied by the posterior pedal nerves; electrical stimulation of these nerves elicited bursts in the columellar nerve. The left pleural ganglion, which is known to contain neurones that project into the columellar nerve, was also found to contain neurones responsive to KC1 stimulation of the foot. These findings suggest that the left pleural ganglion contains a motor centre which is involved in control of activity of the columellar nerve, and is also active during the escape reaction.