Structure of recurrent axon collaterals of frog lumbar motoneurons as revealed by intracellular HRP labelling.

Iontophoretical injection of horseradish peroxidase (HRP) into lumbar motoneurons of the isolated and perfused frog spinal cord allowed to reveal recurrent axon collaterals of motoneurons and to study their structural organization. The axons of about 50% of stained motoneurons gave rise to recurrent collaterals. Motoneurons emitted usually one and in a few cases two recurrent axon collaterals. Recurrent collaterals exhibited a complex ramification pattern with numerous swellings presumably corresponding to synaptic boutons both en passant and terminaux. Swellings of recurrent axon collaterals were observed exclusively in the gray matter. Most of the boutons were located in the neuropil, while some of them were found in close apposition to the soma of ventral horn neurons, in particular, motoneurons. The present data provide direct evidence for structural basis involved in recurrent actions of motoneuronal activity in the amphibian spinal cord.