First, we will take a look at the male reproductive system. Your average male has a pretty run of the mill system as compared to the complexity of the female reproductive system.
As always, things start in the brain, which releases the hormone LHRH.1 This stimulates the pituitary to release the two hormones leutenizing hormone (LH) and follicle stimulating hormone (FSH). LH stimulates the testes to re¬lease testosterone. Lacking eggs and follicles, in males the FSH stimulates sperm production. Stress disrupts this pro¬cess at each stage. The first casualty is the LHRH. With the onset of stress, the endorphins and enkephalins act to block the release of LHRH from the brain.
Another hormone re-leased during stress reduces the sensitivity of the pituitary to LHRH. This is an attack on both flanks—less of LHRH production and lower sensitivity of the pituitary to LHRH. The LHRH concentrations decline which results in the decline in levels of LH and FSH. LHRH stands for leutenizing hormone releasing hormone.
As a result, the testes stop functioning and the level of testosterone declines. Further, glucocorti¬coids block the response of the testes to LH. The decline in the level of circulating hormone is only part of the problem. The actual problem is that stress causes difficulties in getting erections. Put simply, for the male to have an erection, it is necessary to have the parasympathetic system active. As we saw in chapter 3, stress causes the sympathetic system to be activated and the parasympathetic system to be blocked. This makes it difficult to get erections during periods of stress resulting in impotence. Even if the male can get an erection, it is difficult to sustain the erection during a stressor.
Female Reproductive System
The first step in the female reproductive system is the same as in the males. LHRH is released by the brain, which causes the pituitary to release LH and FSH. In the case of females, the FSH as the name implies, stimulates the fol¬licles. In the first half of the menstrual cycle (to use the scien¬tific jargon—the follicular stage), levels of LHRH, LH and FSH and estrogen build up leading to ovulation. This starts the second phase of the cycle (called the luteal phase). The dominant hormone in this phase is progesterone, which is made in the ovary.
It stimulates the uterine walls to mature, so that if an egg is fertilized, it can implant in the wall and de¬velop into an embryo. Unlike the male reproductive system, the levels of the hormones vary with time and the process is a lot more complicated. Not surprisingly, the portion of the brain that controls the hormone release (hypothalamus) is structurally more complicated in women than men.
One of the interesting facets of reproduction is the eggs and their storage. It is incredible that the lifetime supply of eggs is formed even before birth!3 While the female foetus is developing, between months three and six the eggs are formed. It is only after she grows up and has hit puberty that these eggs are released. Evolution has worked a surprisingly smart way to store these eggs.
Many of the mechanisms that disrupt the male repro-ductive system, also affect the female system. Endorphins and enkephalins will inhibit LHRH release and the glucocor¬ticoids will block pituitary sensitivity to LHRH. This will result in lowered LH, FSH and estrogen release thereby lengthening the follicular stage. This will extend the whole cycle and make it irregular. An extreme version of this is a condition termed anovulatory amenorrhea where the entire ovulatory system is shut down.
Another way in which stress affects the reproductive system is by suppressing progesterone, which disrupts the matu¬ration of the uterine walls. Worse, prolactin, which is released during stress, interferes with the working of progesterone. This is a double whammy. First, the ovulatory hormones are affected, thereby reducing the chances of ovulation. If an egg does ovulate and get fertilized there is very little chance of it to implant normally.
An interesting aside on the hormone prolactin. It is a very versatile hormone and besides stress it is released during breastfeeding and is an effective contraceptive. There is a reflex loop that goes from the nipples to the hypothalamus. If there is nipple stimulation for any reason, the hypothalamus signals the pituitary to secrete prolactin. Have sufficient prolactin over a long period of time in the blood and you have a very effective contraceptive. Hence, the conventional wisdom that breastfeeding your baby is a good contraceptive.
Like all ideas floating around in the garb of conventional wisdom’ it is partially true. In the remark on the contraceptive effects of prolactin, the key point to note is the phrase ‘long period of time’. At the start of breastfeeding, prolactin is released but ceases with the end of feeding. As the average mother nurses her baby only for five or six times during a day, the sustained levels of prolactin are not maintained and the contraceptive effect is not seen at all times.
There is a surprising facet to the female reproductive system and the effect that stress has on it. Typically, there is a small amount of male sex4 hormone in the bloodstream of women. This hormone does not come from the ovaries but from the adrenal glands (hence the not surprising name— adrenal androgens). The amount is a small fraction of that found in males and an enzyme in the fat cells of females usually eliminates these androgens by converting them into estrogen.
In the face of physical stressor when the body weight drops and fat stores are depleted, this conversion is affected and the level of androgens builds up. The androgens disrupt many steps in the female reproductive system. As would be obvious, this happens if you starve voluntarily—as in the case of young teenage girls chasing the impossible dream of being as pencil thin as supermodels. In such cases, the loss of fat cells leading to androgen buildup results in impaired reproduction mechanisms. In young girls puberty can be delayed for years and in older women cycles can become irregular or cease altogether.
Harking back to one of the main themes of this book, the stress response that disrupts the female reproductive system and prevents pregnancies is very logical from the evolutionary point. An average pregnancy costs over 50,000 calories and such an undertaking should not be undertaken during periods of stress or when reserves of energy are not available in the fat cells.