The vas deferens packs more muscle for its size than any other hollow organ
The vas deferens is a narrow tube, only 2 to 3 millimetres wide yet 30 to 35 centimetres long in humans, that carries sperm away from the testis. Its thick wall holds more muscle relative to its channel than any other hollow organ, and that dense wiring of nerves has made it a favourite tool for pharmacologists.
Its Latin name means carrying-away vessel. Sperm made in the seminiferous tubules pass into the epididymis, whose tail continues as the vas deferens. The duct starts out tightly convoluted for its first 2 to 3 centimetres, runs up inside the spermatic cord, leaves the abdominal wall region through the inguinal canal into the pelvis, then widens into a coiled ampulla and merges with the seminal vesicle's outlet, the pair becoming the ejaculatory duct. From there sperm enter the urethra and mix with fluids from the seminal vesicles, prostate and bulbourethral glands, which make up most of semen.
The wall has three layers of smooth muscle, a circular one sandwiched between two running lengthwise, lined inside by cells bearing non-moving stereocilia. During ejaculation the muscle contracts in a reflex wave that drives sperm forward. Sympathetic nerves releasing noradrenaline dominate its supply, and ion channels in its lining help regulate fluid and salts inside the tube.
That heavy sympathetic innervation makes the organ a convenient lab system. Researchers have used it as a bioassay in discovering enkephalins, the body's own opiates, to show that nerve terminals release transmitter in discrete packets, and to make the first direct measurement of free calcium inside a postganglionic nerve ending.
Clinically, a vasectomy permanently cuts the ducts as contraception, sometimes reversibly, and a newer approach injects material to block them instead. Damage during hernia repair can cause infertility, and some men are born without the ducts, a condition that can accompany cystic fibrosis; sperm can then be retrieved surgically. Across vertebrates the arrangement varies: sharks and amphibians carry sperm through a duct that also handles urine, in reptiles, birds and mammals it carries only sperm, the duct loops over the ureter in placental mammals but not marsupials, and jawless fishes lack one altogether.
Source: Vas deferens
