Negative Feedback

Negative feedback occurs when the output of a system acts to oppose changes to the input of a system, acting to stabilize it. The classic example is a central heating system which cuts off when a (suitably placed) temperature sensor hits a pre-set mark. The negative feedback part is the thermostat. Negative feedback is a basic concept of cybernetics (the theory of communication and control based on regulatory feedback in animals and machines).

In biology negative feedback is known as homeostasis (the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition). Virtually all aspects of living systems involve homeostasis (e.g. blood pressure, glucose level, liver functions, cell division). The disruption of feedback loops can lead to undesirable results: in the case of blood glucose levels, if negative feedback fails, the glucose levels in the blood may begin to rise dramatically, thus resulting in diabetes.

Negative feedback (also known as degenerative feedback) occurs when information about a gap between the actual value and a reference value of a system parameter is used to reduce the gap. In many physical and biological systems, qualitatively different influences can oppose each other. For example, in biochemistry, one set of chemicals drives the system in a given direction, whereas another set of chemicals drives it in an opposing direction. If one, or both of these opposing influences are non-linear, equilibrium point(s) result. In biology, this process (generally biochemical) is often referred to as homeostasis; whereas in mechanics, the more common term is equilibrium. Common terms for the points around which the system gravitates include: attractors, stable states, eigenstates/eigenfunctions, equilibrium points, and setpoints. In contrast, positive feedback is feedback in which the system responds so as to increase the magnitude of any particular perturbation, resulting in amplification of the original signal instead of stabilization. Any system where there is positive feedback together with a gain greater than one will result in a runaway situation. Both positive and negative feedback require a feedback loop to operate.

The concept of negative feedback came from that of ‘milieu interieur’ (the ‘environment within,’ the extra-cellular fluid environment, and its physiological capacity to ensure protective stability for the tissues and organs of multicellular living organisms), which was coined by French physiologist Claude Bernard in 1865. If the overall feedback of the system is negative, then the system will tend to be stable. Most endocrine hormones are controlled by a physiologic negative feedback inhibition loop, for example the hypothalamus secretes corticotropin-releasing hormone, which directs the pituitary gland to secrete adrenocorticotropic hormone, which directs the adrenal cortex to secrete glucocorticoids (which not only perform their respective functions throughout the body but also negatively affect the release of further stimulating secretions of both the hypothalamus and the pituitary gland, effectively reducing the output of glucocorticoids once a sufficient amount has been released.

Negative feedback was first implemented in the 16th Century with the invention of the centrifugal governor. Its operation is most easily seen in its use by James Watt to control the speed of his steam engine. Two heavy balls on an upright frame rotate at the same speed as the engine. As their speed increases they swing up and outwards due to the centrifugal force. This causes them to lift a mechanism which closes the steam inlet valve and the engine slows. When the speed of the engine falls too far, the balls will fall by gravity and open the steam valve. In the example of a thermostat, when the temperature in a heated room reaches a certain upper limit the room heating is switched off so that the temperature begins to fall. When the temperature drops to a lower limit, the heating is switched on again. Provided the limits are close to each other, a steady room temperature is maintained. Similar control mechanisms are used in cooling systems, such as an air conditioner, a refrigerator, or a freezer.

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