Internal Anal Sphincter Parasympathetic-Nitrergic and Sympathetic-Adrenergic Innervation: A 3-Dimensional Morphological and Functional Analysis

BACKGROUND: Little detailed information is available concerning morphological and functional autonomic nerve supply to the internal anal sphincter. However, denervation of the sphincter potentially affects anal function after rectal surgery for cancer. OBJECTIVE: The aim of this study was to identify the location and type (nitrergic, adrenergic, and cholinergic) of nerve fibers in the internal anal sphincter and to provide a 3-dimensional representation of their structural relationship in the human fetus. MATERIALS AND METHODS: serial transverse sections were obtained from 14 human fetuses (7 male, 7 female, 15-31 weeks of gestation) and were studied histologically and immunohistochemically; digitized serial sections were used to construct a 3-dimensional representation of the pelvis. MAIN OUTCOMES MEASURES: The location and type of internal anal sphincter nerves were assessed qualitatively. RESULTS: Posteroinferior fibers originating from the inferior hypogastric plexus posteroinferior angle projected to the posterolateral and posterior rectal wall and internal anal sphincter, forming the inferior rectal plexus. The inferior rectal plexus contained vesicular acetylcholine transporter-positive (cholinergic), tyrosine hydroxylase-positive (adrenergic/sympathetic), and neural nitric oxide synthase-positive (nitrergic) fibers. The intrasphincteric vesicular acetylcholine transporter-positive fibers included both neural nitric oxide synthase-negative fibers and neural nitric oxide synthase-positive fibers (nitrergic-parasympathetic). LIMITATIONS: The study focused on topographic and functional anatomy, so that quantitative data were not obtained. A small number of fetal specimens were available. CONCLUSIONS: We report the precise location and distribution of the autonomic neural supply to the internal anal sphincter. This description contributes to the understanding of neurogenic postoperative sphincteric dysfunction. Three-dimensional cartography of pelvic-perineal neurotransmitters provides an anatomical and physiological basis for the selection and development of pharmacological agents to be used in the treatment of primary or postoperative continence and evacuation disorders.