Introduction.
The Endocrine System consists of endocrine glands separated from each other by a wide distance.
Endocrine glands consist of secretory cells present between a network of capillaries.
Secretion of the endocrine gland is called “Hormone”, the hormones secreted diffuse in blood directly and transfer to the target organs.
Endocrine glands are also called “Ductless Glands”.
It plays a vital role in maintaining homeostasis along with the Nervous system.
The branch of science that deals with the study of Endocrine glands is called Endocrinology.
Important Endocrine Glands:
Pituitary Gland.
Thyroid Gland.
Parathyroid Glands.
Adrenal Glands.
Pineal Gland.
Definition and Classification of Hormones:
The chemicals secreted by Endocrine Glands are called Hormones.
Hormones are classified on the basis of the chemical structures as follows,
A) Amine Hormones:
The hormones that are derived from single amino acids with modified groups in their structures.
E.g.
Noradrenaline.
Melatonin
Thyroxin
B) Peptide Hormones:
The hormones derived from multiple amino acids and contain shorter peptide chains than proteins are called peptide hormones.
E.g.
ADH (AntiDiuretic Hormone)
Oxytocin.
Calcitonin
C) Protein Hormones:
The hormones derived from multiple amino acids and contain longer peptide chains than peptide hormones are called protein hormones.
E.g.
Insuline.
Growth Hormone.
Glucagon.
D) Steroid Hormones:
Hormones containing steroidal nuclei in their structure are called steroidal hormones.
E.g.
Testosterone
Progesterone.
Estrogen.
Mechanisms of Hormone Action.
The message sent by the endocrine gland through hormones is received in the cell by hormone receptors.
The receptor then processes the message by initiating various cellular mechanisms to produce target cells' response.
The processing is done by one of the either pathways,
Pathway involving intracellular hormone receptors.
Pathway involving extracellular hormone receptors.
Pathway involving intracellular hormone receptors:
The lipid soluble hormones like steroidal hormones easily pass through the cell membrane.
They then attach to the hormone receptor present in the cytoplasm or nucleus of the cell.
The hormone receptor complex then attaches to a chromatin and generates a mRNA (messenger RNA) which comes out into cytoplasm.
The information written in mRNA is used to synthesize the proteins which in turn produce the target cell’s response.
Pathway involving extracellular hormone receptors:
Hydrophilic hormones such as Angiotensin II having lesser lipid solubility cannot pass easily inside a cell.
Such hormones attach with a receptor located on a cellular membrane to form a “Hormone - receptor complex”.
The formed hormone receptor complex then initiates a cascade of events by forming a second molecule (Second Messenger), resulting in the target cell’s action.
The Hormone receptor complex works from either of the following pathways,
cAMP
Phospholipase C.
Cyclic Adenosine Mono Amino Phosphate (cAMP) Pathway:
The hormone receptor complex activates a cellular component called “G Protein”.
The activated G Protein then activates an enzyme “Adenylate cyclase” which causes conversion of ATP (Adenosine Triphosphate) to cAMP.
cAMP activates a type of enzyme called “Protein kinase” which causes a cascade of phosphorylation events of many proteins activating different enzymes resulting in target cell’s action.
The cAMP action is checked by an enzyme “Phosphodiesterase”.
E.g. Calcitonin, Glucagon.
Phospholipase C pathway:
The hormone receptor complex activates a cellular component called “G Protein”.
The activated G Protein then activates an enzyme “Phospholipase C” which causes cleavage of a phospholipid from the cellular membrane to produce,
diacylglycerol (DAG)
inositol triphosphate (IP3)
Diacylglycerol activates a protein kinase like cAMP while inositol triphosphate causes mobilisation of calcium ions from the endoplasmic reticulum, which activates many proteins or enzymes.
E.g. Angiotensin II
Commonly Asked Questions.
Define and classify Hormones.
Discuss in detail various mechanisms of action of hormones.