LM 3.1: Mechanism of Erection and Ejaculation

Question 1

what is the structure of the penis?

Answer 1

three columns of erectile tissue, each enclosed by its own dense, fibrous connective tissue capsule, the tunica albuginea

two columns towards the top dorsal side are known as the corpora cavernosa and connect intermittently with each other

the bottom, ventral side contains the corpus spongiosum which surrounds the urethra and does not get as firm during erection

the erectile tissue within the corpus spongiosum and corpora cavernosa consists of variably shaped sinusoidal spaces lined by endothelium and surrounded by a meshwork of connective tissue, smooth muscle cells and coiled arteries

within the tunica albuginea like the dorsal vein, artery and nerve and the deep dorsal vein while the central cavernous artery runs through the corpus cavernosum

Question 2

which arteries supply the penis?

Answer 2

1. deep artery of the penis
2. dorsal artery of the penis

both arise from the internal pudendal artery

Question 3

which veins drain the penis?

Answer 3

1. superficial dorsal vein which drains to the external pudendal vein
2. deep dorsal vein which drains to the prostatic venous plexus

Question 4

what is the innervation of the penis?

Answer 4

1. pudendal nerve provides somatic innervation
2. hypogastric nerve provides sympathetic innervation
3. pelvic nerve provides parasympathetic innervation

in an erection you need the somatic and parasympathetic nervous systems aka the pudendal nerve and pelvic nerve activation –> activation of the parasympathetic nervous system is critical for the shift in blood flow to cause an erection and also leads to lubrication of the male urethra by inducing secretions from the bulbs-urethral glands

Question 5

when the penis is flaccid, arterial blood mostly bypasses the sinusoidal spaces of the erectile tissue and drains through which vein?

Answer 5

deep dorsal vein

Question 6

what are the steps of an erection?

Answer 6

1. parasympathetic impulses trigger local release of NO which dilates the smooth muscles in the deep, dorsal and helicon arteries of the penis leading to increased blood flow
2. blood flow through helical arteries fill the sinusoidal spaces in the corpa cavernosa
3. the volume of the corpora cavernosa increases pressing up against the inelastic tunica albuginea surrounding it
4. the subtunical venular plexus and emissary veins are compressed, preventing outflow of blood

during an erection, relaxation of the trabecular smooth muscle and vasodilatation of the arterioles results in a severalfold increase in blood flow, which expands the sinusoidal spaces to lengthen and enlarge the penis –> the expansion of the sinusoids compresses the subtunical venular plexus against the tunica albuginea

also, stretching of the tunica compresses the emissary veins, thus reducing the outflow of blood to a minimum

in the flaccid state, inflow through the constricted and tortuous helicine arteries (coming from the cavernous artery) is minimal, and there is free outflow via the subtunical venular plexus

Question 7

what is the biochemical process of an erection?

Answer 7

parasympathetic fibers directly innervating the smooth muscle and sinusoidal endothelial cells release ACh

NO produced locally in the smooth muscle cell or reaching it by diffusion from the adjacent endothelial cells is the major mediator of smooth muscle relaxation through activation of guanylate cylase and increased production of cGMP

these actions decrease intracellular free Ca+2 concentration

using the drug sildenafil to inactivate cGMP phosphodiesterase, the enzyme that degrades cGMP, preserved smooth muscle relaxation and prolongs the erection period

Question 8

what is the mechanism of ejaculation?

Answer 8

1. emission

smooth muscle of the epididymis, ductus deferens, seminal vesicle, and prostate glands contract propelling the sperm and fluid unto the urethra

at the same time, the smooth muscle of the internal urethral sphincter contracts to prevent reflux of semen into the bladde

2. expulsion/propulsion of semen from the penis

caused by contraction of the skeletal bulbospongiosus muscle and relaxation of the skeletal muscle of the urethral sphincter

Question 9

what branch of the nervous system is responsible for ejaculation?

Answer 9

sympathetic nervous system

norepinephrine release in the penis also leads to increased intracellular calcium concentrations, smooth muscle contraction and vasoconstriction to induce detumescence

Question 10

which branch of the nervous system is responsible for controlling the skeletal muscles involved in expulsion?

Answer 10

somatic nervous system

Question 11

which branches of the nervous system control the different parts of ejaculation?

Answer 11

erection –> parasympathetic

emission –> sympathetic

expulsion –> somatic