What is a Negative Feedback Loop?

The changes that occur during the normal menstrual cycle in women. Encyclopaedia Britannica/Universal Image Group/Getty Images

A negative feedback loop is a type of self-regulating system. In this type of feedback loop, increased output from the system inhibits future production by the system. In other words, the system controls how much product it makes by shutting down manufacturing when levels of output or the amount of accumulated product gets too high. Negative feedback systems are responsible for many types of hormone regulation in the human body. They are good at maintaining relatively constant levels of output.

Also Known As: inhibitory loop

The Body as a Factory - Feedback Loops Manage Production

Imagine that the body is a factory making Product X. Furthermore, imagine that making too much Product X is expensive and wasteful. Therefore, the body needs a way to shut down the factory when enough Product X has been made. It does this through a negative feedback loop.  What that means is that the speed of production is sensitive to the amount of Product X. When it starts to build up, production slows and then stops. The opposite of this would be a positive feedback loop. In that case, the more Product X there was, the faster the plant would make more. This type of system easily gets out of control. In contrast, a negative feedback loop is self-regulating.

Examples of Negative Feedback Loops in the Reproductive System

One very well understood negative feedback loop is the female menstrual cycle. The hypothalamus produces gonadotropin releasing hormone (GnRH).

The GnRH signals the pituitary to produce follicle stimulating hormone (FSH). FSH tells the ovaries to produce estrogen. High levels of estrogen (as well as progesterone and testosterone, which are regulated through similar loops) however, inhibit the production of GnRH. This causes the pituitary to make less FSH, which in turn causes ovaries to make less estrogen.

Another feedback loop regulates vaginal acidity. The normal vaginal pH is approximately 4 - mildly acidic. This helps to prevent the growth of problematic bacteria and STDs. In fact, one of the hallmarks of bacterial vaginosis is a pH of above 5. The lactic acid that maintains this pH is made by lactobacilli - part of the normal vaginal flora. These bacteria grow faster and produce more acid at higher pH. Then, when the pH gets close to 4, they slow down and stop. This is how the pH is regulated in the vagina. It also explains some of the difference in different women's vaginal pH. The pH varies depending on the specific bacteria that are present.

Sources:

Boskey ER, Cone RA, Whaley KJ, Moench TR. Origins of vaginal acidity: high D/L lactate ratio is consistent with bacteria being the primary source. Hum Reprod. 2001 Sep;16(9):1809-13.

Boskey ER, Telsch KM, Whaley KJ, Moench TR, Cone RA. Acid production by vaginal flora in vitro is consistent with the rate and extent of vaginal acidification. Infect Immun. 1999 Oct;67(10):5170-5.

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