Physiology of Sperm production and Erection

Spermatogenesis

• germinal cells divide to become spermatogonia which divide to become spermatocytes
• Type A spermatocytes undergo meiosis to become secondary spermatocytes
• spermatocytes divide and give rise to mature cell lines (spermatids)
• spermatids (haploid) undergo a transformation into a spermatozoa
• There are six stages of seminiferous epithelium development
• 16 days required for a mature sperm to develop from early spermatogonia
• 72 days until ejaculation
• 1 spermatogonium becomes 215 spermatids

• testosterone and FSH are the hormones that are directed at the seminiferous tubule epithelium
• LH effects spermatogenesis indirectly in that it stimulates endogenous testosterone production
• The physical proximity of the Leydig cells to the seminiferous tubules and the elaboration by the Sertoli cells of androgen-binding protein, cause a high level of testosterone to be maintained in the microenvironment of the developing spermatozoa (i.e. 50X peripheral levels)

Epididymis: involved with maturation, storage and transport of spermatozoa.
• Testicular spermatozoa are non-motile
• Spermatozoa gain progressive motility and fertilizing ability after passing through the epididymis
• The epididymis consists of a fragile single convoluted tubule that is 5-6 meters in length. The epididymis is divided into the head, body, and tail
• Although epididymal transport time varies with age and sexual activity, the estimated transit time is four days.
• In epididymis, sperm develops the increased capacity for progressive motility and also acquire the ability to penetrate oocytes during fertilization
• The epididymis also serves as a reservoir or storage area for sperm

• The extragonadal sperm reservoir
• Is 440 million spermatozoa
• more than 50% of these are located in the tail of the epididymis.
• The sperm that are stored in the tail of the epididymis enter the vas deferens which is a muscular duct 30-35 cm in length
• The contents of the vas are propelled by peristaltic motion into the ejaculatory duct
• Sperm are then transported to the outside of the male reproductive tract by emission and ejaculation.







Erection:

Parasympathetic stimulation leads to penile erection (Eckhard, 1863, in the dog)
Sympathetic pathways important for detumescence and play a role in psychogenic erections (via inhibition of these pathways)

Reflex erection: afferent stimulation via S2-4, with efferent impulses via the same level sacral roots
Psychogenic erections: due to audiovisual, olfactory stimuli, or fantasy, and require the long tracts to be intact.

Afferent impulses via the dorsal penile nerves through pudendal nerves to dorsal roots of S2-4.
Upwards transmission via spinothalamic tracts to the thalamus and sensory cortex

Coordinated in medial pre-optic nucleus, contiguous with the hypothalamus

Efferent impulses via the medial forebrain bundle to the spinal cord. Parasympathetic fibres pass in intermediate lateral bundle and outflow through S2-4 in preganglionic pelvic nerves (nervi erigentes) to the pelvic plexus and then to the erectile tissue via the cavernous nerves

Sympathetic outflow from the spinal cord leads to flaccidity. Coordination in medial pre-optic nucleus, with outflow via T11-L4 via the hypogastric plexus, and thence to the pelvic plexus

Following spinal cord injuries higher than T9, erections can occur due to the reflex arc, and with lower lesions erections may occur as a result of efferent impulses through the thoracic sympathetic outflows, even though sympathetic stimulation normally leads to flaccidity (this is via a negative effect on the sympathetic outflow).

LMN pts unable to obtain a reflex erection, but 25% can get psychogenic erections

6 phases of erection (FFTFRD):
1: flaccid
contracted smooth muscle
2: filling
relaxation of arterial and cavernosal smooth muscle
3: tumescence
continued inflow
4: Full erection
compression of subtunical venous plexuses
decreased venous outflow via emissary veins
5: rigid erection
contraction of ischiocavernosus smooth muscle
6: detumescence
sympathetic stimulation and relaxation of ischiocavernosus smooth muscle
release of NA leads to stimulation of a1 receptors, an increase in intracellular calcium and contraction of cavernous smooth muscle


Ejaculation

Emission: sympathetic control (T10-L2)
Secretions from the seminal vesicles and prostate are deposited into the posterior urethra via contraction of epididymes and vasa and seminal vesicles
Bladder neck closure occurs under sympathetic nervous control.
Ejaculation: parasympathetic control (S2-4): the bladder neck tightens and the external sphincter relaxes with the semen being propelled through the urethra via rhythmic contractions of the perineal and bulbocavernosus and ischiocavernosus muscles.


The seminal vesicles provide 65% of volume fructose, prostaglandins and coagulating substrates.
A recognized function of the seminal plasma is its buffering effect on the acidic vaginal environment.
The coagulum formed by the ejaculated semen liquefies within 20 to 30 minutes as a result of prostatic proteolytic enzymes.
The prostate (30% volume) also adds zinc, phospholipids, spermine, and phosphatase to the seminal fluid.
The first portion of the ejaculate characteristically contains most of the spermatozoa and most of the prostatic secretions, while the second portion is composed primarily of seminal vesicle secretions and fewer spermatozoa.