Question

4. Explain the difference between a positive and negative hormonal feedback loop. How is the testis (i.e. testosterone) regulated ? Explain why giving exogenous testosterone has little effect on serum levels but dramatic lowering of testicular testosterone concentrations. (10 points)

Answer 1

- A positive feedback loop occurs when the product of a reaction lead to an increase in the reaction. In a system of homeostasis positive feedback moves a system away from target of equilibrium. It is done by amplifying the effect of a product or event and occurs when something needs to happen quickly.

A negative feedback occurs when the product of reaction lead to decrease in reaction. It brings a system closer to a target of stability. Negative feedback is responsible for the stabilisation of a system and maintain a steady stable state. The response of this mechanism is opposite to the output of the event.

Example for positive feedback are fruit ripening, child birth

Example for negative feedback are temperature regulation, blood pressure regulation

- Negative feedback mechanism occurs on the rising level of testosterone acting on the hypothalamus and anterior pituitary to inhibit release of GnRH, FSH and LH. The sertoli cell produces the hormone inhibin, releases when the Sperm count is too high. This then inhibit the release of GnRH and FSH leading to slow down of spermatogenesis. When sperm count reaches 20 million per ml, sertoli cell release inhibin, causing sperm count to increase.

- when exogenous testosterone is given, it inhibits gonadotropin releasing hormone GnRH production, which then inhibit lutenising hormone LH, and decrease endogenous testosterone production by the Leydig cells, decreasing Intra testicular testosterone concentration.