There seem to be many factors concerned in regulation of secretion of hormones. They are: 1. Influence of Higher Centres 2. Feed-Back Mechanism 3. Peripheral Nervous Control 4. Genetic Control.
Factor # 1. Influence of Higher Centres:
Higher centres of brain under influence of environmental factors regulate the secretion and release of various hypothalamic hormones, which have an intrisic diurnal rhythm set on them by the former, Light affects the reproductive cycle of certain animals and birds is well known. Similarly anxiety and fear affects release of gonadotrophins. Both appear to work through higher centres involving hypothalamus (Fig.22.7).
Factor # 2. Feed-Back Mechanism:
The mechanism of mutual regulation of hormonal activity between two glands is called feed-back system (servomechanism).
The feed back mechanism can further be classified into:
(a) Positive feed-back
(b) Negative feed back and
(c) Metabolite feed back systems.
Since trophins influence synthesis and release of hormones from their respective target glands, and certain other hormones influence metabolite changes in the plasma, feed back mechanisms can be explained as below:
(a) Positive Feedback Mechanism:
Increase in plasma concentration of target homone influence the hypothalamus to alter the rate of secretion of the regulatory hormone and reduce the responsiveness of the pituitary gland to the regulatory hormone. The two combined together reduce the secretion of the trophic hormone(s) of the AP.
(b) Negative Feedback Mechanism:
Fall in the plasma concentration of the target hormone initiates more and more release of the corresponding trophic hormone.
(c) Metabolite Feedback Mechanism:
The rate of release of a hormone may be influenced by changes in the plasma level of a non-hormonal chemical substances (metabolic). Insulin reduces the level of glucose in blood while glucagon increases it. An increase in the level of glucose in blood signals for release of insulin while any decrease in the level of the same metabolite signals for release of glucagon.
Factor # 3. Peripheral Nervous Control:
It is difficult to separate it entirely, since environment plays its stimulatory/inhibitory role in it, which may affect by way of other means also. Any fearful sight affecting visual cortex signals to the cerebral cortex, which through its intricate fibre in the descending tract may affect the preganglionic sympathetic fibre to the adrenal medulla, and consequent release of adrenaline.
Factor # 4. Genetic Control:
Genes also play their role in hormonal regulation is evident in animals, since fast growing animals show increased output of pituitary somatotrophins.