The dorsal compartment locomotory control system in amphioxus larvae

Amphioxus myotomes consist of separate sets of superficial and deep muscle fibers, each with its own innervation, that are thought to be responsible for slow swimming and escape behavior, respectively. Tracings from serial EM sections of the anterior nerve cord in the larva show that the motoneurons and premotor interneurons controlling the superficial fibers (the dorsal compartment, or DC pathway) are linked by specialized junctions of a previously undescribed type, referred to here as juxta-reticular (JR) junctions for the characteristic presence of a cisterna of endoplasmic reticulum on each side. JR junctions link the DC motoneurons with each other, with the largest of the anterior paired neurons (LPN3s) and with one class of ipsilateral projection neurons (IPNs), but occur nowhere else. Because of the paucity of synaptic input to the DC system, larval behavior can only be explained if the JR junctions act as functional links between cells. An analysis of the pattern of cell contacts also suggests that the LPN3s are probably pacemakers for both slow and fast locomotion, but act through junctions in the former case and conventional synapses in the latter. The only major synaptic input to the DC system identified in somites 1 and 2 was from four neurons located in the cerebral vesicle, referred to here as Type 2 preinfundibular projection neurons (PPN2s). They have unusually large varicosities, arranged in series, that make periodic contacts with the DC motoneurons. More caudally, the DC motoneurons receive additional input via similar large varicosities from the receptor cells of the first dorsal ocellus, located in somite 5. The overall circuitry of the locomotory control system suggests that the PPN2s may be instrumental in sustaining slow swimming, whereas mechanical stimulation, especially of the rostrum, preferentially activates the fast mode.