Additional Stuff about Hormones

Hi all,
Please do download the exercises GEP 1 and 2 under GEP on the right.
Just to clarify negative and positive feedback control.

Negative feedback mechanisms act like a thermostat in the home. As the temperature rises (deviation from the ideal normal value), the thermostat detects the change and triggers the air-conditioning to turn on and cool the house. Once the temperature reaches its thermostat setting (ideal normal value), the air conditioning turns off.

An Example of negative feeback, is the lowering of insulin production when insulin has produced the effect of lowering blood glucose level. Insulin is secreted when blood glucose level rise. The lowering of blood glucose level by insulin is fedback to the pancreas, which thus decreases its secretion of insulin. This is negative feedback.

Positive feedback mechanisms control self-perpetuating events that can be out of control and do not require continuous adjustment. In positive feedback mechanisms, the original stimulus is promoted rather than negated. Positive feedback increases the deviation from an ideal normal value. Unlike negative feedback that maintains hormone levels within narrow ranges, positive feedback is rarely used to maintain homeostatic functions.

An example of positive feedback can be found in childbirth. The hormone oxytocin stimulates and enhances labor contractions. As the baby moves toward the vagina (birth canal), pressure receptors within the cervix (muscular outlet of uterus) send messages to the brain to produce oxytocin. Oxytocin travels to the uterus through the bloodstream, stimulating the muscles in the uterine wall to contract stronger (increase of ideal normal value). The contractions intensify and increase until the baby is outside the birth canal. When the stimulus to the pressure receptors ends, oxytocin production stops and labor contractions cease.