Sensory pathways in amphioxus larvae II. Dorsal tracts and translumenal cells

Serial and interval EM series were used to examine the dorsal nerve tracts in the anterior nerve cord of a 12.5 day larva of Branchiostoma floridae. Fibres within the tracts derive from peripheral sensory cells and a class of intramedullary sensory neurones known as dorsal (Retzius) bipolar cells. Both form repeated synapses of similar type, apparently with the same targets. The synapses occur at points where, at intervals, the tracts expand to form large synaptic zones. The target dendrites, which form complex tangles, belong chiefly to dorsal translumenal cells, a class of neurone distinguished by their apical processes. The latter range from short extensions of the cell apex that contact the opposite side of the cord via junctions, but go no further, to elongate processes with slender branches that project to the contralateral dorsal tract. The morphology indicates that translumenal cells play the same role in amphioxus as internuncial neurones in vertebrate spinal cord. Their axons can be ipsilateral or contralateral; some synapse with motoneurones directly while others innervate other interneurones, including other translumenal cells. From the circuitry, the cells appear to be chiefly involved in integrating sensory input from peripheral mechanoreceptors. This could include acting as a filter that amplifies some input patterns over others, or that normalizes input, so that CNS circuits are not overloaded as new sensory cells differentiate during development. The functional importance of the translumenal system to the organism is reflected in a massive increase in size and cell numbers during the larval phase. The anterior, brain-like integrative centres of the cerebral vesicle, in contrast, are initially small and change very little.