Partial denervation of the rat soleus muscle at two different developmental stages.

Rat soleus muscles were partially denervated at two developmental stages. The L5 ventral ramus was sectioned in rats which were 4-6 days old, when the motor unit size of soleus muscles was still large, and at 17-19 days, when motor unit territory reached its adult value. The response of axons in the L4 ventral ramus to this procedure was then investigated. The removal of the L5 ventral ramus at 4-6 days results in an initial brief increase of motor unit size, after which the motor units retain the territory they occupied at 4-6 days. After removal of the L5 ventral ramus at 17-19 days, the L4 ventral ramus is able to expand to occupy a territory comparable in size to that of animals operated at 4-6 days. In both cases the final percentage of mean motor unit tension is two- to three-fold greater than that in normal muscles. Although the final motor unit territory is similar for both groups, it is achieved by different mechanisms. In animals operated on at 4-6 days the normal elimination of terminals does not occur, and the large neonatal motor units are retained, whereas in animals operated on at 17-19 days the peripheral field of L4 axons expands by axonal sprouting.