RESPONSE OF FAST MUSCLE INNERVATION TO HYPOTHYROIDISM

The early period of motor innervation development is characterized by multiple innervation of muscle cells. This transitory state in rat ext ensor digitorum longus (edl) muscle is normally concluded at weaning w hen a 1:1 ratio between nerve endings and muscle cells is reached. Mot or innervation of edl muscle in rats made hypothyroid after weaning wa s studied in three ways: electrophysiology (intracellular recordings o f muscle postsynaptic potentials) was carried out to study neuromuscul ar transmission; silver impregnation of terminal axons to observe spro uting; force production in twitch and tetanus following direct muscle stimulation and nerve stimulation. A number of multiply innervated mus cle cells was found in hypothyroid rats following two months of treatm ent, This finding seems to be related to the appearance of nodal sprou ting in motor axons. No sign of denervated end-plates was found. Twitc h and tetanus tension were smaller than in controls, but they were big ger when referred to unitary muscle mass. Time course of twitch, parti cularly half relaxation, was slowed in muscles of hypothyroid rats. Th ese findings suggest that plastic processes occur in muscle innervatio n of rats made hypothyroid after weaning. Therefore, thyroid hormones play a role in stabilizing motor innervation not only during developme nt, but also in adults.