Golgi‐staining of “primary” and “secondary” motoneurons in the developing spinal cord of an amphibian

Abstract

The Golgi technique was used to study the morphology of spinal motoneurons at various stages in the early development of swimming behavior in embryos and larvae of the palmate newt, Triturus helveticus (Razoumowsky). The earliest motoneurons stained appeared to be Associated with the Mauthner‐cell system. The overall morphology of these “primary” motoneurons seems to be similar throughout the lower vertebrates and the distinctive characteristics found in earlier descriptions of those from caudate amphibia were probably due to misinterpretation. At about the time of hatching and the development of low‐frequency swimming behaviour, other motoneurons were found to innervate the axial musculature, cells with a central morphology different from those of the “primary” type. It was found likely that these “secondary” motoneurons innervate a separate muscle system concerned with tonic and “slow phasic” activity, while “fast phasic” activity in rapid swimming is supplied by “primary” cells.