Pressing the nerve alters muscle fiber types of the peroneus longus in rats: Preliminary evidence for external anal sphincteroplasty

BACKGROUND Studies have demonstrated that anal reconstruction with a gracilis graft pressing the dominant nerve could be used to treat fecal incontinence. However, the detailed mechanism by this remains unknown. Herein, we evaluated the alteration in muscle fiber types and contractility of the peroneus longus muscle in rats after pressing its dominant nerves. MATERIAL AND METHODS The rat soleus and peroneus longus were exposed during surgery. The superficial peroneal nerve was pressed so that the peroneus longus temporarily lost its innervation. The epimysium between the soleus and the peroneus longus was removed. The end point of the soleus was cut off and the epimysium of the contact surfaces of the soleus and the peroneus longus were sutured. Five months later, peroneus longus contractility was recorded by the myograph system, and types of muscle fibers were observed using the myosin ATPase staining method. RESULTS The skeletal muscle fiber type underwent adaptive changes due to double innervations with both fast and slow muscle nerves. Compared with other groups, the percentage of type I fibers in the peroneus longus increased significantly in the group of rats with the pressure on the nerve and removal of the sarcolemma. The maximal contraction and relaxation time at the single twitch and complete tetanus of the peroneus longus were also increased. CONCLUSIONS Our results show that pressing dominant nerves alter the skeletal muscle fiber types of the peroneus longus, which lead to increased maximal contraction and relaxation time, and significantly improve the ability in resistance to fatigue in rats. This study provides a basis for future clinical studies for external anal sphincter reconstruction using gracilis grafts that are doubly innervated by pressing on its dominant nerve.