The following points highlight the three ways to control secretions of hormones. The three ways are: 1. Neural Control 2. Endocrine Control 3. Feedback Control.
Secretion of Hormones: Way # 1. Neural Control:
Some endocrine secretions are solely controlled by nerve impulses. Secretion of adrenal medullary hormones secretion of neuro-hypophysial hormones and various releasing hormones of hypothalamus are under this category.
For instance, in mammals, the act of suckling of baby stimulates tactile receptors in the nipple of mother and this impulse stimulates hypothalamic cells through sensory nerve and spinal cord. Latter, hypothalamic neuro-secretion stimulates neurohypophysis for secretion of oxytocin. Oxytocin helps in secretion of milk.
Secretions of Hormones: Way # 2. Endocrine Control:
Some endocrine secretions are controlled by other endocrine glands. For instance, different releasing hormones of hypothalamus control the secretion of anterior pituitary hormones. TSH-RH of hypothalamus controls the secretion TSH from anterior pituitary.
Similarly, TSH and ACTH secretion of anterior pituitary stimulate thyroid gland and adrenal cortex respectively for secretion of thyroid hormone and adrenal cortical hormone.
Secretions of Hormones: Way # 3. Feedback Control:
The process of inhibiting or stimulating the first step by the final step in a hormonal reaction pathway, is called feedback regulation. The secretion of a hormone may be stimulated or inhibited by the feedback effect of some other hormone or metabolite.
This feedback control can be divided into two ways:
(a) Negative feedback mechanism:
In this type, rising concentration of a hormone inhibits the release of second hormone from other gland, called negative feedback control (Fig. 3.2).
Example:
(i) Pineal hormone is anti-gonadotropic in nature. Increased secretion of pinealin of pineal gland inhibits the anterior pituitary for secretion of gonadotropin.
(ii) High secretion of Cortisol of the adrenal cortex inhibits the secretion anterior pituitary corticotrophin.
(b) Positive feedback mechanism:
In this mechanism, rising concentration of a hormone acts on another gland to release second hormone, which further stimulates the first hormone, called positive feedback mechanism (Fig. 3.3).
Example:
At pre-ovulatory phase, a gonadal hormone estrogen increases the release of pituitary LH, which in turn stimulates ovarian estrogen production. Thus estrogen and LH levels go on increasing continuously.