Bioelectric Activity During the Startle Response of a Hemichordate Worm

1. When the proboscis of a hemichordate worm is prodded, two or more nerve pulses travel along the ventral cord to trigger the large, compound muscle potential that precedes the startle response. 2. Nerve pulses also may be compound or they may be all-or-none spikes. Compound pulses decay in size as they travel from their point of origin, but spikes are generally through-conducted to the posterior end and are seen most often during the repetitive discharge evoked by a strong stimulus. 3. Repetitive discharge produces facilitation of conduction velocity so that a burst of closely spaced spikes is generated. 4. The same pulses that trigger the startle response initiate waves of retreat peristaltic contractions, but in the latter activity there is less summation of the spike-like muscle potentials. 5. Although conduction of the waves toward the anterior end depends on the presence of the ventral cord, waves that follow the initial contraction are not preceded by nerve spikes.