Page 73 - 86 human physiology part-2

Chapter 15

vasodilation. This may result from any of various physiological stimuli. The corpus spongiosum is a
single tubular structure located just below the corpora cavernosa, which contains the urethra, through
which urine and semen pass during urination and ejaculation, respectively. This may also become
slightly engorged with blood, but less so than the corpora cavernosa.

Penile erection usually results from sexual stimulation and/or arousal, but can also occur by such
causes as a full urinary bladder or spontaneously during the course of a day or at night, often during
erotic or wet dreams. An erection results in swelling and enlargement of the penis. Erection enables
sexual intercourse and other sexual activities (sexual functions), though it is not essential for all sexual
activities.

Ejaculation

Emission is the term used when sperm moves into the urethra. Ejaculation is the term used when
sperm is forced out of the urethra and penis. These are both stimulated by sympathetic nerves.

Sperm Production

A spermatozoon or spermatozoan (pl. spermatozoa), from the ancient Greek σπέρμα (seed) and
ζ ον (living being) and more commonly known as a sperm cell, is the haploid cell that is the maleῷ
gamete.

Spermatagonia divides several times during the process of sperm development. The entire process
of sperm formation and maturation takes about 9-10 weeks. The separate divisions that take place and
what happens in each are as follows:

• First division: The first division is done by mitosis, and ensures a constant supply of
spermatocytes, each with the diploid number of chromosomes.

• Second division: Spermatocytes then undergo a series of two cell divisions during meiosis
to become secondary spermatocytes.

• Third division: Secondary Spermatocytes finally become spermatids. Spermatids, which
are haploid cells, mature slowly to become the male gametes, or sperm.

The sperm is the main reproductive cell in males. The sperms differ in that each carry a set of
chromosomes dividing each into either a male, or female sperm. The females differ in that they carry a
XX gene, while the male sperm carry a XY gene. The female sperm also differ phenotypically in that
they have a larger head in comparison to the male sperms. This contributes to the male sperm being
lighter, and therefore faster and stronger swimmers than their female counterparts (although
statistically there is still a 50% chance of an either XY or XX embryo forming).

Spermatozoan stream lines are straight and parallel. The tail flagellates, which we now know
propels the sperm cell (at about 1-3 mm/minute in humans) by rotating like a propeller, in a circular
motion, not side to side like a whip. The cell is characterized by a minimum of cytoplasm. During
fertilization, the sperm's mitochondria gets destroyed by the egg cell, and this means only the mother is
able to provide the baby's mitochondria and mitochondrial DNA, which has an important application in

288 | Human Physiology