Compartmental and topographical specificity of reinnervation of the glutaeus muscle in the adult toad (Bufo marinus).

The extent to which the compartmental and topographical innervation patterns were re-established in the mature glutaeus muscle after nerve cut was determined by HRP retrograde labelling procedures and single motor unit glycogen depletion experiments. Glutaeus muscle axons normally cluster together along the length of the sciatic nerve and enter the triceps femoris nerve along with axons to the cruralis and tensor fasciae latae muscles. Glutaeus axons did not specifically reinnervate the glutaeus muscle after cutting the triceps femoris nerve. Axons within the glutaeus muscle nerve are grouped according to the primary nerve branch, and therefore muscle compartment, for which they are destined. When the glutaeus nerve was cut, regenerating axons lost their compartmental organization within the glutaeus nerve and nonspecifically reinnervated both glutaeus muscle compartments. In marked contrast, the topographical projection of spinal motoneurones to the ventral and dorsal halves of the glutaeus muscle was largely re-established after glutaeus nerve cut; that is, rostral motoneurones mostly reinnervated ventral muscle fibres, whereas caudal motoneurones reinnervated dorsally located muscle fibres. The relatively confined caudal projection was closely related to the sharp fibre type divisions within the glutaeus muscle. The results suggest that factor(s) present during development that determine the correct innervation of a muscle and its compartments no longer operate in the adult, although cues that direct motor terminals to their appropriate muscle region do, and these are most likely related to the topographical distribution of fibre types.