Distinctive Innervation Patterns of Thigh Extensor Muscles in Anura: A Morphogenetic Hypothesis of Nerve Re‐Routing Within the Hindlimb Plexus in the Tetrapod Evolutionary Process

In mammals, thigh muscles are compartmentalized into three groups: extensor, flexor, and adductor muscles, and these are innervated by the femoral nerve, sciatic nerve (tibial and common peroneal nerve divisions), and obturator nerve, respectively. However, amphibians show a different pattern of innervation. In this study, the extensor and flexor muscles, their innervation, and relation to the muscle compartment (by the lateral femoral intermuscular septum) were examined and noted in bullfrog thighs. Ten thighs of five bullfrog specimens were examined macroscopically and partially histologically. The extensor and flexor muscles were identified, and these two muscle groups were separated by the lateral femoral intermuscular septum in the dorsal thigh of bullfrogs. However, the nerve from the common peroneal nerve division of the sciatic nerve coursed dorsal to the ischiopubis and innervated both most of the extensor muscles (vastus lateralis, rectus femoris, and vastus intermedius muscles) and the flexor muscles, unrelated to the muscle compartment by the septum. Additionally, the vastus medialis muscle was innervated by the crural nerve (mammalian femoral nerve-like nerve) that coursed ventral to the ischiopubis. Thus, the extensor quadriceps femoris muscle is innervated by two types of nerves that take different routes to reach the muscles from the plexus. The innervating branches of the three extensor muscles are speculated to have been re-routed from the dorsal to the ventral side of the ischiopubis and merged into the crural nerve, thereby giving rise to the mammalian femoral nerve in the tetrapod evolutionary process. This nerve re-routing of the branches to the extensor muscles is presumably attributed to interspecific differences in hindlimb position and septum structure.