What is negative feedback and what is positive feedback? Give an example for each of where it is important in regulation?

Negative feedback: occurs where the feedback causes the corrective measures to be "switched off", and in doing so returns the system to its original level.

Example: Temperature regulation in mammals.

A rise in blood temperature leads to thermoreceptors in the hypothalamus sending nervous impulses to the heat loss centre, which in turn sends impulses to the skin. Vasodilation, sweating and lowering of body hairs is stimulated, leading to heat loss from the blood. The thermoreceptors detect that blood temperature is at its normal set point again and they cease to send impulses to the heat loss centre, which in turn ceases impulses to the skin. The cooling mechanisms are no longer stimulated and so the blood temperature remains at a normal level instead of continuing to decrease.

Positive feedback: occurs when the feedback causes the corrective measures to remain "switched on" and in doing so causes the system to deviate even more from the original level.

Example: Generation of an action potential in a neurone.

A stimulus leads to a small influx of sodium ions through opened sodium ion channels into a neurone. This in turn causes more sodium ion channels to open, allowing more ions to diffuse into the neurone, which in turn leads to a further increase in permeability to sodium ions. This positive feedback leads to a very rapid build-up of an action potential, and so a rapid response to a stimulus.