Motoneurons of the axial swimming muscles in hatchling Xenopus tadpoles: Features, distribution, and central synapses

Xenopus tadpole motoneurons make cholinergic synapses within the spinal cor d. This excitation changes with longitudinal position and contributes to th e excitation that controls motor activity and its longitudinal spread durin g swimming. To explore the anatomic constraints on this excitation, backfil ling has been used to examine the anatomy and distribution of the whole pop ulation of spinal motoneurons, to define the extent of their central axons and to find where they make synapses. Motoneuron features show considerable variation but do not allow their separation into primary and secondary. Mo st motoneurons have descending central axons and it is likely that central synapses are made from these axons as longitudinal dendritic extent is very limited. Motoneuron density reaches a broad plateau over the mid-trunk reg ion at 12-13 per 100 mu m Soma size does not change with longitudinal posit ion, but the dorsoventral extent of the dendrites decreases caudally, where as the central axon length increases. Motoneuron distribution data were use d to estimate the longitudinal distribution of central motoneuron axons. Th is has a broad plateau at 12-14 per 100 mu m over much of the trunk and onl y decreases significantly caudal to the anus. This distribution correlates with cholinergic excitation during swimming. Transmission electron microsco py of motoneurons backfilled with horseradish peroxidase was used to show t hat central motoneuron axons make en passant synapses with motoneuron dendr ites and the dendrites of other unstained neurons. By using measures of syn apse frequency and total dendrite length, trunk motoneurons are estimated t o each receive 100-200 synapses. (C) 1999 Wiley-Liss, Inc.