Coitus and Conception
Table of contents
Coitus and conception are the initial stages that occur in humans that allow for the establishment of pregnancy. Sexual intercourse that results in the deposition of sperm in the vagina at the level of the cervix is known as coitus. In order for coitus to occur humans need to be sexually aroused. Sexual arousal is then followed by a series of phases. These phases are the excitement phase, plateau phase, orgasmic phase and resolution phase. After sperm has been deposited at the cervix it is transported to the uterus where it fertilises the ovum and implants in the uterine stroma. This is known as conception.
In this article we shall look at the various phases of coitus, conception and some clinical conditions.
The changes that occur to allow coitus involve genital and systemic alterations in both sexes. The first part of sexual stimulation in a male results in the production of an erection. An erection may be produced due to psychogenic or somatogenic stimuli; this means it may be caused by stimulation of efferent nerves to the penis via the limbic system in the brain due to sensory cues such as images or it may be via somatogenic stimuli such as touching the penis.
The plateau phase in males results in an increase in the penile erection, increase in size of testicles and a rise in heart rate and blood pressure.
Following on from the plateau phase there is the orgasmic phase. Firstly, several structures contract in order to mix the contents of the ejaculate and this is called emission. The second part of the orgasmic phase is ejaculation and is the result of muscular contractions to expel semen from the prostatic urethra.
The final part of the sexual response cycle is the resolution phase and in males this is split into two stages. The first stage results in the penis reducing from full erection to 50% larger than its unstimulated size and the second, longer phase, results in the penis reducing to its normal unstimulated size. Below is a table summarising the stages in the male:
|Males||Nervous system stimulated||Consequence|
|Excitement phase||Sacral parasympathetic neurons||Arteriolar vasodilation in corpora cavernosa which increases penile blood flow
Penile filling (latency)
Penile tumescence (erection)
|Plateau phase||Sacrospinous reflex||Contract ischiocavernosus
Venous engorgement and decreased arterial inflow
Testes become completely engorged and elevated
Stimulate secretion from accessory glands which will make up 5% of ejaculate
Lubricate distal urethra, neutralise acidic urine in urethra
|Orgasmic phase||Emission||Thoracolumbar sympathetic reflex||Contract smooth muscles in ductus deferens, ampulla, seminar vesicle and prostate
Internal and external urethral sphincters contract
Semen pooled in urethral bulb
|Ejaculation||Spinal reflex with cortical control (L1,L2 sympathetic)||Contract glands and ducts
Contract urethral sphincter
Filling of internal urethra stimulates pudendal nerve which contracts genital organs, ischiocavernosus and bulbocavernosus resulting in expulsion of semen
|Resolution phase||Thoracolumbar sympathetic pathway||Contraction of arteriolar smooth muscle in corpora cavernosa
Increased venous return leads to detumescence flaccidity which results in refractory period
Fig 1 – Summary of the male phases of coitus
The autonomic nervous system that controls the four parts of coitus is the same in males and females however the responses of the genital organs are different.
Within females, the excitement phase results in anatomical changes that increase the circumference of the vaginal diameter at the level of the pelvis and lubricate the vagina for penile entry.
Following the excitement phase, the plateau phase results in the labia minora becoming red in appearance as well as the respiratory rate, heart rate and blood pressure increasing.
Within females, the orgasmic phase does not include emission and ejaculation. The orgasm includes contraction of the lower third of the vagina and can extend to whole vagina and uterus. The final phase of resolution encompasses several changes that result in the return of the structures to normal unstimulated states. Below is a table summarising the changes in the female.
|Females||Nervous system stimulated||Consequence|
|Excitement phase||Sacral parasympathetic neurons||Vasocongestion
Clitoris engorges with blood
Increase in muscle tone, heart rate and BP
Inner two-thirds of vagina lengthens and expands
|Plateau phase||Sacrospinous reflex||Further increase in muscle tone, heart rate and BP
Labia minora deepends in colour
Clitoris withdraws under hood
Bartholin gland secretion in order to lubricate vestibule for entry of penis
Uterus fully elevated and organismic platform forms in lower one third of vagina
|Orgasmic||Spinal reflex with cortical control (L1,L2 sympathetic)||Orgasmic platform contracts rhythmically 3-15 times
Uterus contracts, anal sphincter contracts
|Resolution phase||Thoracolumbar sympathetic pathway||Clitoris descends and engorgement subsides
Labia return to unaroused colour and size
Uterus descends to unaroused position
Vagina shortens and narrows back to unaroused state
No refractory period and so multiple orgasms are possible
Fig 2 – Summary of the female phases of coitus
There are several differences in the physiological sexual response of males and females:
- Emission and ejaculation do not occur in the female
- Females are capable of several orgasms and the orgasm may last relatively longer than that of a male
- Females are able to reach the plateau phase and sustain it then return to an unstimulated state without orgasm
Ageing and the sexual response
There are several changes that occur in females as a result of ageing:
- Reduced vasocongestion response, causing reduced vaginal lubrication
- Vaginal and urethral tissue lose elasticity
- Length and width of vagina decrease which reduces the expansile ability of inner vagina during arousal
- Number of orgasmic contractions is often reduced
- More rapid resolution
Sexual dysfunction can be caused by a change in desire or arousal. The most common type of sexual dysfunction is when there is a problem with desire. Below are several types of sexual dysfunctions caused by an abnormality in desire:
- Hypoactive- little of no interest in sex
- Aversion- revulsion or fear of sex
- Kluver Bucci syndrome
When the arousal stage is abnormal in females this is due to a lack of ability to maintain or attain the lubrication response and is often seen in menopause. In males, a problem with arousal is called impotence. Impotence can be caused by descending inhibition of spinal reflexes due to psychological problems, tears in fibrous tissue of corpora cavernosa, vascular abnormalities (atherosclerosis and diabetes) and certain drugs. Viagra can be prescribed for males and works by inhibiting cGMP breakdown and results in an increased nitric oxide action that produces an erection.
Sperm abnormalities may be another reason for sexual dysfunction and this could be a low level of sperm within the ejactulate (oligoospermia) or sperm morphological defects.
Following coitus, a small percentage of the millions of sperm deposited at the cervix will reach the site of fertilisation. In order for conception to occur these sperm must travel to the ampulla in the fallopian tube and undergo a series of steps that will allow it to then penetrate the cell layer surrounding the oocyte.
Oxytocin stimulates uterine contraction, which along with the sperms own propulsive activity transports the sperm allowing it to travel. In order for sperm to penetrate the oocyte the sperm undergoes capacitation. Capacitation reorganises the sperm cell membrane and results in the tail movement changing from a beat-like action to a thrashing whip-like which propel the sperm forward. The changes that occur in capacitation induced by the removal of protein coat of sperm exposes acrosome enzymes. Therefore, capacitation is vital to allowing the acrosome reaction to occur- which will allow penetration of the zona pellucida.
The zona pellucida specific cell surface glycoproteins (ZP3 proteins) interact with the capacitated sperm and allow calcium to enter the spermatozoa which results in an increase in intracellular cAMP. The acrosome then swells and its outer membrane fuses with the sperm plasma membrane resulting in the release of enzymes from the acrosome into the space surrounding the head of the sperm. The inner cell membrane of the acrosome is then exposed and another glycoprotein (ZP2) holds the sperm near the egg. The proteolytic enzymes released from the acrosome then allow for penetration of the zona pellucida by the whiplashing sperm. Penetration of the zona pellucida allows the sperm and oocyte cell membranes to fuse almost immediately, calcium to enter to oocyte and the sperm cell stop moving. The rise in intracellular calcium results in 3 changes in the oocyte:
- The egg cell membrane depolarises in order to prevent polyspermy- this is the primary block.
- The cortical reaction which is where cortical granules that lie just beneath the egg cell membrane fuse with the membrane and release their contents into the zona pellucida- this is a secondary block to polyspermy as it hardens the zona.
- Completion of the second meiotic division of the egg- this forms a polar body that is subsequently released. Following fertilisation the zygote undergoes several changes which will result in the implantation within the wall of the uterus in a successful pregnancy.