A horseradish peroxidase study of motorneuron pools of the forelimb and hindlimb musculature of the axolotl

Motorneuron pools innervating axolotl limb muscles have been investigated by using the retrograde neuronal tracer horseradish peroxidase. Four muscles in the forelimb (biceps, anconeus, flexor digitorum and extensor digitorum) and four functionally equivalent muscles in the hindlimb (puboischiotibialis, iliotibialis, flexor digitorum and extensor digitorum) were studied. Motorneuron pools were characterized by using four criteria: position in the rostrocaudal axis; position of the median in the rostrocaudal axis; number of labelled cells; position of cells in the transverse plane of the spinal cord. Each pool was uniquely defined by the four characteristics, although overlap was found between pools. Two types of motorneuron were found in each pool, distinguished on the basis of size, shape and position in the spinal cord. The first type constituted the majority of cells in a pool, and occupied different positions in the transverse plane for each muscle. The second type was less common and always occupied a characteristic medial ventral position. These data will allow an assay of correct or incorrect innervation in experiments on the regeneration of specific neuromuscular connections in these animals.