Positive Feedback Definition
Positive feedback is a process in which the end products of an action cause more of that action to occur in a feedback loop. This amplifies the original action. It is contrasted with negative feedback, which is when the end results of an action inhibit that action from continuing to occur. These mechanisms are found in many biological systems. An important example of positive feedback is the process of labor and childbirth.
Parts of a Positive Feedback Loop
A stimulus is something that disrupts the body’s homeostasis, which is the tendency toward equilibrium in all body systems. A bodily injury or an infection are examples of stimuli. They disrupt normal processes in the body.
A sensor detects the change in homeostasis. For example, nerve cells in the cervix detect pressure placed on it from the head of the fetus during labor. Nerve impulses from a sensor will travel to the control center.
A control center is the part of the body that responds to the change and takes action. The pituitary gland, located near the brain, is the control center in many feedback loops; it produces many different hormones, such as oxytocin, growth hormone, and anti-diuretic hormone (ADH), in response to stimuli.
An effector is any organ or cell that ultimately responds to the stimulus. For example, in labor, the end result of the positive feedback loop is that the uterus contracts. In this case, the uterus is the effector organ.
These four parts are also found in negative feedback loops, but the end result is different because in negative feedback the effector organs work to hinder the process that caused them to activate. Positive feedback loops do not go on forever; they are ultimately stopped by negative feedback loops once the process they were used for is complete.
Examples of Positive Feedback
When a part of the body is injured, it releases chemicals that activate blood platelets. Platelets are responsible for stopping bleeding by forming clots. An activated platelet in turn activates more platelets, which group together to form a blood clot. (In individuals with hemophilia, the blood lacks enough blood-clotting proteins, causing excessive bleeding after an injury.)
The Menstrual Cycle
Before a woman ovulates, the hormone estrogen is released by the ovary. The estrogen travels to the brain, which causes gonadotropin-releasing hormone (GnRH) to be released from the hypothalamus and luteinizing hormone (LH) to be released from the pituitary gland. LH causes more estrogen to be released from the ovary, which in turn causes an increase in GnRH and LH in the bloodstream through positive feedback. The rise in these hormones, along with follicle-stimulating hormone (FSH), causes ovulation to occur.
Labor and Childbirth
The process of labor and childbirth is perhaps the most-cited example of positive feedback. In childbirth, when the fetus’s head presses up against the cervix, it stimulates nerves that tell the brain to stimulate the pituitary gland, which then produces oxytocin. Oxytocin causes the uterus to contract. This moves the fetus even closer to the cervix, which causes more oxytocin to be produced until childbirth occurs and the baby leaves the womb. Breastfeeding is also a positive feedback loop; as the baby suckles, the mother’s pituitary gland produces more of the hormone prolactin, which causes more milk to be produced.
The stomach uses the molecule pepsin to digest proteins. It first secretes pepsinogen, which is an enzyme in an inactive form. When food is taken into the body and needs to be digested, pepsinogen is converted to pepsin. The conversion triggers a positive feedback loop that changes other pepsinogen molecules in the stomach to pepsin, so that the stomach accumulates enough to it to be able to digest proteins.
Nerve impulses work through action potentials, which are changes in electrical potential between the inside and outside of the nerve that propagates signaling. Action potentials are caused by an influx of sodium ions in the nerve cell. If a small amount of sodium enters the nerve, it causes more channels to open which causes more sodium to rush in, creating a positive feedback loop that causes a large amount of sodium to enter the nerve and create an action potential.