Motor pool organization of the external gastrocnemius muscle in the turtle, Pseudemys (Trachemys) scripta elegans

The spinal cord of the adult turtle, Pseudemys (Trachemys) scripta elegans, is now considered a promising model for the study of the segmental motor system in the generalized tetrapod. To facilitate such studies we have examined the location, soma geometry, soma size, and number of motoneurons innervating the external gastrocnemius (EG) muscle in this species, as this muscle is ideally suited to the study of interrelations between the neuronal and muscular components of the segmental motor system. Motoneurons were retrogradely labeled following application of horseradish peroxidase to the EG muscle nerve. In both horizontal and transverse planes, labeled motoneurons innervating the EG muscle were concentrated in the S1 lumbosacral segment, and extended rostrally and caudally as far as the exists of the D10 and S2 spinal nerves, respectively. In the transverse plane, motoneurons were arranged in a longitudinal column which occupied the dorsolateral quadrant of the ventral horn. EG motoneurons are fusiform in shape and present their largest dimension in the transverse plane with their long axis oriented in the ventromedial to dorsolateral plane. The soma diameters of EG motoneurons were normally distributed, reflecting the absence of separate fusimotor innervation in reptilian species. In individual turtles, there was a two- to threefold range in soma diameter while soma surface area extended over a seven- to tenfold range. Based on cell counts from five animals, the EG motor pool was composed of approximately 75 motoneurons. Taken together, the results of this study provide valuable information for interpreting the results of future studies on the segmental motor system of this species under both normal and pathophysiological conditions.