Medicograspers | Reproductive system | Physiology | Notes on Erection and Ejaculation |

      Erection and Ejaculation

In this article, you will come to know about the physiology of erection and ejaculation. 

Erection and ejaculation

Parasympathetic – point – erection

Sympathetic – shoot – ejaculation

 

Erection:

            The stimulus for an erection is via the site, thoughts, tactile stimulation, and even the smell.

            The parasympathetic nervous system from the sacral spinal cord S2 – S4 comes to the penis via the pelvic nerve. This pelvic nerve has a connection to the blood vessels present inside the corpora cavernosa. The corpora cavernosa is filled with more and more blood vessels and the meshwork of smooth muscles and it’s an erectile tissue in the penis.

 

Mechanism of erection (the zoomed portion of corpora cavernosa):

            The pelvic nerve releases acetylcholine (ACH) to the receptors present in the endothelial cells of the blood vessels present inside the corpora cavernosa.

This activates the nitric oxide synthase inside the cell and this converts the arginine into citrulline and Nitric oxide.

This nitric oxide stimulates the Guanyl cyclase which in turn converts the GTP to cGMP. This cGMP triggers the K+ ions to exit the smooth muscles causing the relaxation of the smooth muscles presents around the penile artery.

This relaxation causes the penile artery to vasodilate. This vasodilation compresses the corpora cavernosa and this, in turn, compresses the veins of the penis. So, the veins of the penis are blocked and results in the erection of the penis due to blood in the penile artery.

Clinical: some people have erection problems due to poor vasodilation. These patients have the enzyme in the smooth muscle of the penis. The enzyme called Phosphodiesterase type – 5 is eating the cGMP responsible for the exit of K+ ions causing vasodilation. Doctors prescribe Viagra to those patients. Viagra inhibits the phosphodiesterase type – 5 and results in an erection.

 

Ejaculation:

                        Once the tactile stimulation gets into the threshold that is the parasympathetic stimulation attains the threshold, this is inhibited by the cerebral cortex. Now, the cerebral cortex stimulates the sympathetic nervous system. The nerves from the spinal cord level T12 to L2 reaches the inferior mesenteric ganglion and synapse there.

The nerves from the inferior mesenteric ganglion join with the receptors present in the epididymis of the testes, vas deferens, seminal vesicle, internal urethral sphincter.

These nerves stimulate the above-said areas and cause the contraction of the epididymis, vas deferens. As a result, the sperms get pushed and comes to the duct of the seminal vesicle. The seminal vesicle releases the seminal fluid which mixes with the sperm cells and forms the semen. This semen now travels through the common ejaculatory duct and reaches the prostatic urethra.

The sympathetic stimulation from the inferior mesenteric ganglion contracts the internal urethral sphincter and closes the urinary bladder.

Now, the somatic (pudendal nerve) from the sacral region of the spinal cord stimulates the muscle called bulbospongiosus (two more muscles are also involved). This muscle contracts and pushes the semen out of the urethra. This also gives pleasure while releasing.

Semen = sperms + seminal fluid.

When the ejaculation is completed, the sympathetic stimulation cuts off and results in a flaccid penis.  The blood in the penis is drained back via the veins back to the circulation.