Conduction Along the Ventral Nerve Cord of A Hemichordate Worm

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Abstract
Compound and all-or-none pulses have been recorded from the ventral nerve cord of a hemichordate worm. Compound pulses are composed of at least four smaller pulses, suggesting that four conduction pathways may be present in the cord. Conduction velocities in the anterior ventral cord are as high as 1.1 m/s but fall to about half this value in the posterior cord. In both parts of the cord the passage of the first pulse facilitates the passage of the second so that conduction velocities are increased by 10 to 20%. Paired or multiple stimuli produce repetitive firing and facilitation of size and number of pulses in the cord. Although strong or repeated shocks evoke local, graded muscle potentials, no correlation has been found between the size of these potentials and the number of pulses in the cord. Integration occurs in the hepatic region of the cord. It is the most labile to through-conduction, output from this region does not have a one-to-one relation to input, and pulses originating in the posterior cord and travelling toward the collar are usually blocked in this area.

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