Hormones as messengers [Hypothalamo-hypophysial (pituitary) Axis]
Hypothalamus is a part of the fore brain. Its hypothalamic nuclei — masses of grey matter containing neurons, are located in the white matter in the floor of the third ventricle of the brain. The neurons (neurosecretory cells) of hypothalamic nuclei secrete some hormones called neurohormones (releasing factors) into the blood. The neurohormones are carried to the anterior lobe of the pituitary gland (= hypophysis) by a pair of hypophysial portal veins. In the pituitary gland (hypophysis) the neurohormones stimulate it to release various hormones. Hence the neurohormones are also called “releasing factors”.
Hormones as regulators (Feed back control) Homeostasis means keeping the internal environment of the body constant. Hormones help in maintaining internal environment of the body. When the secretion of hormones is under the control of factors or other hormones it is called feed back control. The regulation of secretion of thyroxine from the thyroid gland is an example of such feed back control mechanism. Feed back control is of two types.
(i) Positive feed back control: If the level of thyroxine is less than normal limits in the blood, thyroxine level stimulates the hypothalamus to secrete more of TRH which results in increased secretion of TSH which in turn stimulates increased secretion of thyroxine. Such regulatory effect is called positive feed back control.
(ii) Negative feed back control: The thyrotropin releasing hormone (TRH) from the hypothalamus stimulates the anterior lobe of the pituitary gland to secrete the thyroid stimulating hormone (TSH). The TSH in turn stimulates the thyroid gland to secrete thyroxine. A high amount of thyroxine in the blood exerts an inhibitory effect on hypothalamus in such a way that less of TRH and TSH is produced respectively. This eventually results in a decrease in thyroxine. This is called negative feed back control.
Negative feedback control
It occurs at three levels.
(i) Long loop feedback: Peripheral gland hormones and substances arising from tissue metabolism can exert what is called long-loop feedback control on both the hypothalamus and the anterior lobe of the pituitary gland. Long-loop feedback usually is negative but occasionally can be positive and is particularly important in the control of thyroidal, adrenocortical and gonadal secretions.
(ii) Short loop feedback: Negative feedback also can be exerted by the anterior pituitary trophic hromones on the synthesis or release of the hypothalamic releasing or inhibiting hormones, which collectively are called hypophysiotropic hormones e.g., GHRH, GHIH; PRF, PIF; TRH; CRH and GnRH.
(iii) Ultrashort loop feedback: The hypophysiotropic hormones may inhibit their own synthesis and secretion via a control system referred to as ultrashort loop feedback.