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 ele gans, is now considered a promising model for the study of the segment al motor system in the generalized tetrapod. To facilitate such studie s we have examined the location, soma geometry, soma size, and number of motoneurons innervating the external gastrocnemius (EG) muscle in t his species, as this muscle is ideally suited to the study of interrel ations between the neuronal and muscular components of the segmental m otor system. Motoneurons were retrogradely labeled following applicati on of horseradish peroxidase to the EG muscle nerve. In both horizonta l and transverse planes, labeled motoneurons innervating the EG muscle were concentrated in the S1 lumbosacral segment, and extended rostral ly and caudally as far as the exits of the D-10 and S-2 spinal nerves, respectively. In the transverse plane, motoneurons were arranged in a longitudinal column which occupied the dorsolateral quadrant of the v entral horn. EG motoneurons are fusiform in shape and present their la rgest dimension in the transverse plane with their long axis oriented in the ventromedial to dorsolateral plane. The soma diameters of EG mo toneurons were normally distributed, reflecting the absence of separat e fusimotor innervation in reptilian species. In individual turtles, t here was a two- to threefold range in soma diameter while soma surface area extended over a seven- to tenfold range. Based on cell counts fr om five animals, the EG motor pool was composed of approximately 75 mo toneurons. Taken together, the results of this study provide valuable information for interpreting the results of future studies on the segm ental motor system of this species under both normal and pathophysiolo gical conditions. (C) 1996 Wiley-Liss, Inc.