The overlapping innervation of the two sides of the external anal sphincter by the pudendal nerves.

Unilateral pudendal neurectomy in the monkey results in histological changes consistent with partial denervation on both sides of the external anal sphincter muscle, but much more marked ipsilaterally than contralaterally. Histological features consistent with reinnervation developed during the 9-13 weeks after unilateral neurectomy in the anterior parts of the muscle on both sides. These observations suggest that there is a substantial overlap in the pudendal innervation of the external anal sphincter muscles on the 2 sides. This derives from interdigitation of muscle fascicles across the midline in this circular muscle. This functionally overlapping innervation enables reinnervation to be partially accomplished from the contralateral side. This is important in understanding the results of electrophysiological and histopathological studies of this muscle in patients with idiopathic faecal incontinence, itself a neurogenic disorder.