POLYNEURAL INNERVATION IN THE PSOAS MUSCLE OF THE DEVELOPING RAT

Polyneural innervation was studied in the psoas muscle in developing r ats from P4 till P25 and at adult age, with the combined silver-acetyl cholinesterase technique. Nerve endings were counted, and endplates we re measured. These data were compared with such data in the human. The end of polyneural innervation in the rat (around P20) and in the huma n (around 12 weeks postterm age) in both cases coincides with a transf ormation in motor behavior and postural control. The rat's psoas muscl e at early stages is less heavily innervated than this muscle in the h uman. Up to three axons per motor end-plate were counted at P4, but in the human up to five axons at 25 weeks of post menstrual age. This di fference might be related to the lower percentage of type I muscle fib ers in the rat. (C) 1998 John Wiley & Sons, Inc.