(a) Tropic hormones  stimulate other endocrine glands to secrete their hormones. In feedback mechanism, inhibitory effect is shown by Hypothalamus and Pituitary gland. When the secretion of hormones is under the control of factors or other hormones it is called feedback control e. g. regulation of secretion of thyroxine from the thyroid gland.

Feedback control is of two types :

(i) Positive feedback control: If the level of thyroxine is less than the normal limits in the blood, thyroxine level stimulates the hypothalamus to secrete more of TRH which results in the increased secretion of thyroxine. Such regulatory effect is called positive feedback control.

(ii) Negative feedback control : The thyroxine 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 thyroid gland to secrete thyroxine. A high level of thyroxine in the blood exerts an inhibitory effect on hypothalamus in such a way that less of TRH and TSH is produced. This results a decrease in thyroxine. This is called negative feedback control.

(b) Generation of nerve impulse.

1.    Depolarisation : When a stimulus is recieved by the nerve fibre, there is a local increase in the membrane permeability. This happens because of the opening Na gate at the site of stimulus. This permits more sodium ions to move into the cell.

2.    Action Potential : The changed electric potential of the neurolemma is called action potential. The initial change produces an ionic inbalance in the membrane on either side of the point of stimulus. This area of negative depolarisation in turn initiates changes in the membrane adjacent to it.

3.    Repolarisation : With the increase of positive change inside, further entry of Na⁺ ions is prevented and permeability of membrane decreases and Na⁺ ions are pushed out.

(c)