We know the nervous system and the endocrine system are the two controlling systems of the body. The endocrine system controls the body with the help of hormones they release. These hormones bind with specific receptors on target cells which further proceed to trigger intracellular signals either by altering membrane permeability, acting through second messenger system or activating specific genes to form new proteins.
To better understand the mechanism of hormone action, it is crucial to comprehend various definitions.
A Hormone is a tiny chemical messenger, travelling along with the bloodstream. It exists to maintain internal balance or in other words, keeps the human body in a state of homeostasis.
This is an extremely rudimentary definition of hormones, as in reality there are numerous hormones that perform an immense number of functions for various purposes.
Starting with receptors, they are specific for every hormone. Their sensitivity and responsiveness are dependent on both, the number of receptors and its affinity. Also, the site of the receptor might be:
- Present in or on the surface of the cell membrane. g. protein or peptide hormones and catecholamines.
- In the cytoplasm. E.g. steroid hormones; Or
- In the cell nucleus. E.g. thyroxine.
Furthermore, hormones can be divided into groups on the basis of their chemical nature:
- Peptide, polypeptide, protein hormones.
- Iodothyronines (thyroid hormone).
- Amino acid derivatives.
The hormones that interact with membrane-bound receptors generally don’t enter target cells, rather generate second messengers that proceed to regulate cellular metabolism.
On the other hand, hormones which interact with intracellular receptors mostly regulate gene expression or chromosome function by the interaction of hormone receptor complex with the genome. The sum total of all the biochemical actions results in physiological and developmental effects.
Also Read: Hormones- A Chemical messenger
Mechanism of Hormone Action
The mechanism of hormone action is grouped into two classes:
- Fixed membrane receptor mechanism
- Mobile receptor mechanism
Fixed Membrane Receptor Mechanism
This type of mechanism is shown by the water-soluble hormones that are amines or proteins in composition such as the growth hormone, oxytocin, ADH, etc.
These hormones can’t pass through the lipid membrane. They have their target receptor on the cell membrane to which the hormone binds.
When the hormone binds on the specific target receptor, the enzyme adenyl cyclase in the cell membrane is activated. This helps in the production of cyclic AMP (cAMP).
cAMP acts as the secondary messenger. It diffuses through the cell membrane and activates several enzymatic reactions to cause biochemical changes.
The target cell responds to these changes and cAMP is deactivated by the enzyme phosphodiesterase.
Mobile Receptor Mechanism
This type of mechanism is shown by lipid soluble hormones such as fatty acids and steroids that can easily pass through the plasma membrane.
They possess intracellular receptors. The hormones bind to the target receptor that activates the enzymatic activity of the cell to bring about biochemical changes.
Transcription of DNA is initiated by the hormone-receptor complex.
The mRNA is translated into protein. This protein causes biochemical changes inside the cell.
Hormones as Regulators
Hormones help in maintaining the internal environment of the body. When the secretion of hormones is under the control of other hormones, it is known as feedback control. It can be of two types:
- Positive Feedback Control: In this process, the end products of action cause more of the action to occur in a feedback loop. For eg., blood clotting, menstrual cycle.
- Negative Feedback Control: In this type of regulation the end product of a process reduces the stimulus of that same process. For eg., temperature regulation, regulation of blood sugar.
Hormones as Messengers
The neurosecretory cells of the hypothalamus secrete hormones known as neurohormones into the blood. These neurohormones are carried to the pituitary gland and stimulate it to release various hormones. hence they are also known as “releasing factors”.
Also Read: Feedback mechanism of hormones
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