Anatomical study of the innervation pattern of the sonic muscle of the oyster toadfish.

The sonic muscle of the oyster toadfish Opsanus tau contracts in almost perfect synchrony along its rostrocaudal extent. We explored some of the anatomical mechanisms underlying this synchronization. Cleared and stained wholemounts demonstrate that axons innervating the rostral portion of the muscle run caudally in the main nerve trunk before branching in the rostral direction, thus increasing their path length and potentially equalizing conduction time to different parts of the muscle. Differential axon diameter is not clearly important for synchronization because the mean diameter of axons innervating the caudal portion is not larger than that of axons from the entire nerve as it enters the muscle. Restricted injections of horseradish peroxidase (HRP) into different regions of the sonic muscle result in similar uniform distributions of labeled motor neurons throughout the ipsilateral sonic motor nucleus (SMN), indicating that the SMN and sonic muscle each form a single compartment. Although there is a tendency for fewer labeled neurons to occur in the rostral and caudal poles of the nucleus, HRP labeling provides no evidence of a somatotopic pattern.