Electrical Activity in the Radial Nerve Cord and Ampullae of Sea Urchins*

Abstract

A single shock applied through wick electrodes to the isolated radial nerve cord of a sea urchin produces a recordable potential in the cord. The potential is conducted along the cord at a velocity of between 14 and 20 cm./sec. The potential is complex and graded. Two components of the potential can be identified and have different thresholds to stimulation, conduction velocities and amplitudes. They are believed to represent two classes of fibres. The potential is conducted decrementally along the cord and normally cannot be recorded at distances greater than 60 mm. from the stimulus. The amplitude of the potential decays logarithmically falling to half after 7 mm. spread. There is no facilitation of amplitude or distance of spread. Potentials initiated simultaneously at either end of the isolated nerve cord collide and partially occlude each other. Stimulation of a side branch of the nerve cord evokes potentials recordable from only ipsilateral neighbouring side branches and the whole cord. However, contractions of the contralateral ampullae following stimulation of lateral branches reveal spread of the excitation beyond the region of recordable potentials. A single shock to a cord still attached to the test causes contraction of the associated ampullae. One ampulla will contract several times after a single shock, a period of relaxation following each contraction. Electrical activity recorded from the ampullae, and lasting many seconds after the single shock, corresponds with their contractions. The activity is believed to be muscle action potentials. Evidence of a feedback from damaged tube feet to the cord, suppressing ampulla response to cord stimulation, was found.