DEVELOPMENTAL-CHANGES IN INNERVATION OF RAT EXTENSOR DIGITORUM LONGUSMUSCLE

Motor neurons and ventral root motor fibres innervating extensor digit orum longus (EDL) muscle in rats of 0.5-3.5 months of age were studied by HRP-retrograde labelling and in vivo muscle tension recording, res pectively. EDL nucleus size increased with age but motor neurons numbe r and size did not change. Twitch and tetanus tension increased with a ge proportionally to muscle mass. At 0.5 months of age, but not at sub sequent ages, the muscle was incompletely innervated functionally. The contribution to EDL muscle innervation came from L3, L4 (pre-eminentl y), and L5 ventral roots. The number of motor fibres running in L3-L5 ventral roots and innervating EDL muscle increased from 41.7 +/- 2.2 ( mean +/- S.E.M.) at 0.5 months to 68.6 +/- 1.9 at 3.5 months (P < 0.00 1). The greatest changes in m.w., in tension and in number of alpha ne rve fibres innervating the muscle occurred from 0.5 to 1.5 months of a ge; afterwards changes occurred, but at a slower rate. HRP labels all neurons innervating EDL muscle, while tension recordings allow the cou nting of a nerve fibres (not gamma) running in ventral roots; these di fferences may account for the different results obtained. (C) 1996 Els evier Science Ireland Ltd.