This positive feedback ensures the baby has sufficient milk during feeding. As the baby feeds, its suckling stimulates the breast, promoting further release of prolactin, resulting in yet more milk production. At birth, when the placenta is released from the uterus, progesterone levels drop. Prolactin normally stimulates milk production, but during pregnancy, progesterone inhibits milk production. During pregnancy, levels of the hormone prolactin increase. After birth, the stretching stops and the loop is interrupted.Īnother example of positive feedback occurs in lactation, during which a mother produces milk for her infant. The feedback increases the strength and frequency of the contractions until the baby is born. The contractions are initiated as the baby moves into position, stretching the cervix beyond its normal position. In these cases, the positive feedback loop always ends with counter-signaling that suppresses the original stimulus.Ī good example of positive feedback involves the amplification of labor contractions. As noted, there are some physiologic processes that are commonly considered to be positive feedback, although they may not all have identifiable components of a feedback loop. Without a counter-balancing or “shut-down” reaction or process, a positive feedback mechanism has the potential to produce a runaway process. Common terms that could describe positive feedback loops or cycles include “snowballing” and “chain reaction”. In a positive feedback mechanism, the output of the system stimulates the system in such a way as to further increase the output. If blood glucose gets too low, the body releases glucagon, which causes the release of glucose from some of the body’s cells. Insulin causes the body’s cells to take in and store glucose, lowering the blood glucose concentration. If glucose levels get too high, the body releases insulin into the bloodstream. For example, negative feedback loops involving insulin and glucagon help to keep blood glucose levels within a narrow concentration range. Negative feedback loops, in conjunction with the various stimuli that can affect a variable, typically produce a condition in which the variable oscillates around the set point. Negative feedback loops are inherently stable systems. Although some may consider this a positive feedback loop, such terminology is not universally accepted. But if we just consider the effects of thrombin on itself, it is considered a positive feedback cycle. It should be noted that there are other aspects of blood clotting that keep the overall process in check, such that thrombin levels don’t rise without limit. This latter step leads to a positive feedback cycle, where an increase in thrombin leads to further increases in thrombin. One of the enzymes in the pathway, called thrombin, not only acts on the next protein in the pathway but also has an ability to activate a protein that preceded it in the cascade. For example, during blood clotting, a cascade of enzymatic proteins activates each other, leading to the formation of a fibrin clot that prevents blood loss. In most cases, positive feedback is harmful, but there are a few instances where positive feedback, when used in limited fashion, contributes to normal function. The term positive feedback is typically used as long as a variable has an ability to amplify itself, even if the components of a loop (receptor, control center and effector) are not easily identifiable. Because a change in an input causes responses that produce continued changes in the same direction, positive feedback loops can lead to runaway conditions. Positive feedback loops are inherently unstable systems.
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