THEORY OF THE LOCOMOTION OF NEMATODES - CONTROL OF THE SOMATIC MOTOR-NEURONS BY INTERNEURONS

The only animal of which the complete neural circuitry is known at the submicroscopical level is the nematode Caenorhabditis elegans. This a natomical knowledge is complemented by functional insight from electro physiological experiments in the related nematode Ascaris lumbricoides , which show that Ascaris motor neurons transmit signals electrotonica lly and not with unattenuated spikes. We developed a mathematical mode l for electrotonic neural networks and applied it to the motor nervous system of nematodes. This enabled us to reproduce experimental result s in Ascaris quantitatively. In particular, our computed result of the velocity nu congruent to 6 cm/s of neural excitations in the Ascaris interneurons supports the simple hypothesis that the so-called rapidly moving muscular wave is produced by a neural excitation traveling at the same speed in the interneuron as the muscular wave. In C. elegans, the computed velocity nu congruent to 8-30 cm/s of signals in the int erneurons is much larger than the observed velocity nu congruent to 0. 2 cm/s of the body wave. Therefore, the hypothesis that the muscular w ave is produced by a synchronous neural excitation wave cannot hold fo r C. elegans. We argue that stretch receptor control is the most likel y mechanism for the generation of body waves used in the locomotion of C. elegans. Extending the simulation to larger groups of neurons, we found that the neural system of C. elegans can operate purely electrot onically. We demonstrate that the same conclusion cannot be drawn for the nervous system of Ascaris, because in the long (l congruent to 30 cm) interneurons the electrotonic signals would be too strongly attenu ated. This conclusion is not in contradiction with the experimental fi ndings of electrotonic signal propagation in the motor neurons of Asca ris because the latter are shorter (l congruent to 5 cm) than the inte rneurons.