The secretion of all endocrine glands is termed as hormone (Gr. Hormoeine-to set in motion).
It was first discovered by Bayliss and Starling in 1902. A hormone can be defined as “a chemical substance produced in one part of the body and carried by blood to a target organ where it regulates certain process.”
Therefore, hormones are informational molecules or chemical messengers that regulate different biological processes of the body.
Hormones as Messengers and Regulators:
Hormones as Messengers:
On the ventral side of diencephalon (last part of fore brain) hypothalamus is present. It consists of number of scattered masses of grey matter in the white matter. Masses of grey matter containing neurons form hypothalamic nuclei. The neurons (neuro secretory cells) of hypothalamic nuclei secrete several hormones called neuro hormones (releasing factors) into the blood. Blood carries these neuro hormones to the anterior pituitary where the neuro hormones stimulate pituitary to release various hormones. Thus, neuro hormones act as messengers.
Hormones as Regulators (Feed Back Control):
This mechanism is known as homeostasis. Blood carries hormones from the endocrine gland to the target organ. Decrease or increase in the amount of hormone in the blood has an effect on the concerned gland (secreting gland) to reduce or increase the secretion of the specific hormone. This mechanism is known as feedback control.
It is of two types:
(i) Positive feedback control and
(ii) Negative feedback control.
To understand the mechanism involved in feedback control, we can take case of thyroxine secretion.
(i) Positive Feed Back Control:
Thyroxine hormone is secreted by thyroid gland. The thyrotropin releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to secrete thyroid stimulating hormone (TSH). TSH in turn stimulates the tyroid gland to secrete thyroxine. If the level of thyroxine in blood is less than normal, this low thyroxine level stimulates hypothalamus to secrete more TRH. This results in increased secretion of TSH which in turn stimulates increased secretion of thyroxine. Such regulatory mechanism is called positive feedback control.
(ii) Negative Feed Back Control:
If the level of thyroxine in blood is more than the normal, this high thyroxine level produces an inhibitory effect on hypothalamus. As a result less TRH and then less TSH are produced by hypothalamus and anterior pituitary respectively. This results a decrease in thyroxine. This is called negative feedback control.