Tracing of frog sensory-motor synapses by intracellular injection of horseradish peroxidase.

Monosynaptically connected primary afferent fibres and motoneurones of the isolated spinal cord of the frog were injected with horseradish peroxidase (HRP). Six labelled afferent fibre-motoneurone pairs were reconstructed and subjected to detailed analysis. Frog motoneurones possess eight to twelve dendritic arrays displaying some dorso-ventral asymmetry. Dorsal dendrites exhibit a rostro-caudal extent of 1.7-2.6 mm (average 2.2 mm). Primary afferent fibres bifurcate in the dorsal funiculus. First-order collaterals emanate from the main ascending and descending branches, at an average distance of 407 micron. The average number of boutons per collateral is 670. To reach a contacting bouton the presynaptic spike must pass on average five bifurcations and then zero to twelve boutons en passant, attached to a single terminal collateral branch. The structural equivalent of the axon cylinder of the collateral tree roughly preserves cross-sectional area. The branch power ranged between 1.15 and 3.35 (average 2.06). Primary afferent fibres usually form clusters of contacting boutons (contact regions). Connexions between an afferent fibre and a motoneurone comprise from five to twenty-three contact regions (average 12.5). Each contact region contains one to twelve contacting boutons (average 3.3). In two of three experiments contacting boutons were found to be significantly larger than non-contacting boutons. The average diameter of the former was 2.6 micron (range 1.2-4.0). In five out of six cases more than one collateral belonging to the same fibre participated in the connexion with a given motoneurone. The average number of contacting boutons per motoneurone and collateral is 19.1. It was estimated that each collateral could supply not more than thirty-five motoneurones. This would be less than 8.5% of the motoneurones with their dendrites which cross the termination space of a single collateral. The average number of contacting boutons forming one primary motoneurone connexion was 41.5 (range 21-72).