Ventral neurons in the anterior nerve cord of amphioxus larvae. II. Further data on the pacemaker circuit

Previous serial EM studies of the anterior nerve cord of amphioxus larvae implicate the third pair of large paired neurons (LPN3s) as key components of the pacemaker responsible for oscillatory premotor output in somites 1 and 2. Here the synaptic relationship between the LPN3s and a fourth such pair (LPN4s), located in somite 3, is examined from a second series of sections. Because of limited overlap between the two series, fiber identity can only be inferred in most instances. To act as pacemakers, the LPN3s must inhibit each other; the current data show similar patterns of synaptic contact, presumably also inhibitory, with the LPN4s. The oscillatory signal appears therefore to be relayed from one LPN pair to the next. Despite their evident importance in the anterior cord, the role these cells play more caudally in signal propagation is not clear. J. Morphol. 257:212–218, 2003. © 2003 Wiley‐Liss, Inc.