Overwork-induced axonal hypertrophy in the soleus nerve of the rat.

Effect of overwork, induced by synergistic tenotomy (bilateral tenotomy of the gastrocnemius and the plantaris muscles) of the soleus muscle, was evaluated on areas of the large myelinated axons (LMA) (greater than 19.5 micron 2), in the soleus nerve (the tibial nerve branch innervating the soleus muscle) of the rat. Three lots (n = 12-16) of young adult male (body weight of 275 to 325 g), Sprague-Dawley rats were used. One lot of 16 animals was kept as normal, while the other two underwent bilateral L4 or L5 spinal nerve sectioning. Six to seven animals each from the normal, L4-, and L5 neurectomized animals underwent bilateral synergistic tenotomy of the soleus so that the soleus was retained as the sole plantar-flexor of the ankle joint. The remaining animals were kept as normal or neurectomized control groups. Thus, the six groups studied were normal control (n = 9); normal synergistically tenotomized (n = 7); L4 neurectomized control (n = 8); L4 neurectomized and synergistically tenotomized (n = 6); L5 neurectomized control (n = 8); and L5 neurectomized and synergistically tenotomized (n = 7). Seven days after surgery, the soleus muscles were used to evaluate the muscle weights. The soleus nerves were evaluated for the number plus areas of the LMA by computerized planimetric procedures. The data revealed a 13.9% (p less than 0.05) increase in areas of the LMA in the L5 neurectomized control group; whereas the L5 neurectomized and synergistically tenotomized group showed a 39.6% (p less than 0.01) increase in areas of the LMA.(ABSTRACT TRUNCATED AT 250 WORDS)