The urethral sphincters are two muscles used to control the exit of urine in the urinary bladder through the urethra.The two muscles are either the male or female external urethral sphincter and the internal urethral sphincter.When either of these muscles contracts, the urethra is sealed shut.. Retrograde dyes were injected into each end organ in female dogs. We aimed to clarify innervation of the bladder, external urethral sphincter (EUS) and clitoris. Postganglionic axons release acetylcholine to cause detrusor contraction and, via nitric oxide release, internal sphincter relaxation. The internal anal sphincter, IAS, (or sphincter ani internus) is a ring of smooth muscle that surrounds about 2.5–4.0 cm of the anal canal; its inferior border is in contact with, but quite separate from, the external anal sphincter..

Sympathetic innervation to the bladder and urethra originates with preganglionic neurons in the intermediolateral nucleus of the lumbar spinal cord (L2-4). The structure of the striated urethral sphincter, the so-called rhabdosphincter, remains the subject of controversy. In the storage phase, impulses travel to the EUS via the pudendal nerve (nerve roots S2-S4) to nicotinic (cholinergic) receptors on the striated muscle, resulting in contraction of the sphincter.. The sympathetic innervation of the bladder originates in the lower thoracic and upper lumbar spinal cord segments (T10-L2), the preganglionic axons running to sympathetic neurons in the inferior mesenteric ganglion and the ganglia of the pelvic plexus. This picture also contains other parts such as the external urethral sphincter, external urethral orifice, hip bone (pubis), urethra, internal urethral orifice, detrusor muscle and so on. There are two main concepts regarding its structure: either it is a part of the urogenital diaphragm, or it extends from the base of the bladder up to the urogenital diaphragm and is an integral part of the urethra. You will also find peritoneum, trigone, internal urethral sphincter (involuntary), external urethral sphincter in deep muscles of the perineum (voluntary) as well. Somatic Innervation. As described above, the EUS is under voluntary somatic control.

It is about 5 mm thick, and is formed by an aggregation of the involuntary circular fibers of the rectum.

Spinal cord, mid-bladder, and spinal, caudal mesenteric, sympathetic trunk and pelvic plexus ganglia were examined for retrograde dye-labeled neurons.