Sprouting of active nerve terminals in partially inactive muscles of the rat.

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Abstract
Certain muscles in the hind foot of rats were partially paralyzed by applying tetrodotoxin to part of their motor innervation. In these muscles motor nerve sprouting occurred from the terminals of the unblocked axons. The extent of sprouting was compared with that seen in totally paralyzed and in partially denervated muscles. Action potentials were blocked in the medial and lateral plantar nerves of adult rats for 5-13 days by continuous superfusion with a solution containing tetrodotoxin. The drug was delivered through a tube and nerve cuff from an osmotic pump placed i.p. Control experiments showed that nerve block was complete and that signs of nerve damage were absent in the animals included in the study. Two muscles (the 2nd lumbrical and flexor digitorum brevis), which received innervation only form the medial plantar nerve, were totally paralyzed by the nerve block. Two different muscles (the 4th lumbrical and flexor digitorum quinti brevis) were only partially paralyzed, since they received their innervation from the lateral plantar nerve and, in addition, from the sural nerve which was not blocked. One day before the final experiment, the lateral plantar nerve was cut, and its terminals degenerated. Thus in the partially paralyzed muscles only the unblocked terminals from the sural nerve remained. These terminals were observed after staining with zinc iodide and osmium tetroxide. Terminals from the medial plantar nerve were examined in the totally blocked muscles from the same animal. In other experiments, muscles were partially denervated by cutting the lateral plantar nerve to compare effects of nerve block and nerve section. Sprouting occurred under all 3 conditions. Active terminals in the muscles partially paralyzed for 5-7 days sprouted to the same extent as terminals in muscles totally locked during the same period: about 35% of the terminals had sprouts, and their average length was about 13 .mu.m. Sprouting was more pronounced in partially denervated muscles: about 65% of the terminals had sprouts and they averaged 24 .mu.m in length. Collateral (preterminal) sprouts were seen only after partial denervation.