Positive feed back mechanism is seen in all EXCEPT
Correct Answer: Moderate hemorrhage
Description: [B1 (Moderate hemorrhage): (303-304-Ganong 24th edition; 680, 838, 857; A.K. Jain 5th; 7-8- Guyton 10th)NEGATIVE FEED BACK - is the one to which the system reacts in such a way as to arrest the change or reverse the direction of changeExamples(i) TSH released from pituitary stimulates thyroid gland which intum secretes thyroxine(ii) Maintenance of water balance in the body(iii) In the regulation of the C02 concentration(iv) In the arterial pressure -regulating mechanism a high pressure causes a series of reactions that promote a lowered pressure or a low pressure causes a series of reactions, that promote an elevated pressure. In both instances, these effects are negative with respect to the initiating stimulusPOSITIVE FEED BACK - is the one to which the system reacts in such a way as amplify (increase the intensity of) the same directionExamples(i) Blood coagulation(ii) Milk ejection reflex(iii) ParturitionInjury to blood vessels and bleeding Stoppage of bleeding| Formation of prothrombin activatorConversion of fibrinogen into fibrin||Conversion of prothrombin into thrombinConversion of prothrombin into thrombinRole of oxytocin in parturitionMore than 100 fold increase in oxytocin receptor in the myometrium* Another important use of positive feedback is for the generation of nerve signals. That is when the membrane of a nerve fiber is stimulated this causes slight leakage of sodium ions through sodium channels in the nerve membrane to the fiber's interior. The sodium ions entering the fiber than change the membrane potential. Which in turn causes more opening of channels, more change of potential still more opening of the channels and so forth. Thus, a slight leak becomes an explosion of sodium entering the interior of the nerve fiber which creates the nerve action potential. This action potential in turn causes electrical corrent to flow along both the outside and the inside of the fiber and initiates additional action potential. This process continue again and again until the nerve signal goes all the way to the end of the fiber.* Oestrogen surge has a positive feedback effect* Positive feed back can sometimes cause vicious cycles and death
Category:
Physiology
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