Localization of Neuron Somata Innervating the External Anal Sphincter Muscles Encircling Canine Anal Sac

Purpose of this study was to localize the neurons innervating the external anal sphincter muscles (EAS) encircling canine anal sac using the retrograde neuronal tracing technique with horseradish peroxidase (HRP). In the dorsal root ganglia (DRG), HRP-positive afferent neurons were found predominantly and ipsilaterally at Si to S3 levels, with the highest number at S2, while a few of them were found contralaterally at S2 and S3 levels. In the paravertebral sympathetic ganglia, HRP-positive neurons were presented mainly at ipsilateral L6 to S3 levels with the highest number at Si and S2; while a few HRP-positive neurons were presented contralaterally at L6 to S2 levels. In addition, certain number of HRP-positive neurons were found in the caudal mesenteric ganglia, right pelvic ganglia, and sacral intermediolateral nucleus. Furthermore, HRP-positive neurons might be observed in the Onuf's nucleus of the spinal ventral horn at Si and S2. This study is the first to reveal the origin of sensory, autonomic, and motor innervation of the EAS encircling the canine anal sac.