09 Control and Coordination - part 15 - Endocrine system

 

09 Control and Coordination - part 15 - Endocrine system

Endocrine system :

  • Controls body activities by means of chemical messengers called hormones. 
  • Hormones are released directly into the blood. 
  • The hormone is carried all over the body via blood. 
  • However the message is relayed only to the target organs which are stimulated to carry out specific process which include activities like growth and development.
Chemical nature of hormones

I. Amines : 
  • These are simple amines.
  • Catecholamines secreted by adrenal medulla, epinephrine and non-epinephrine and melatonin from pineal gland. 
  • Some are modified from the amino acids. e.g., Thyroxine.
II. Peptide hormones : 
  • Consist of long or short chains of amino acids. 
  • e.g. Hormones of hypothalamus oxytocin, ADH, GnRH.
III. Protein hormone :
  • Insulin, glucagon TSH, FSH, LTH, GH, relaxin.
IV. Fatty acid derivatives :-
  • Prostaglandin
V. Steroid hormones : 
  • These hormones are lipid soluble.
  • Derived from cholesterol and other steroids. 
  • e.g. estrogen testosterone, aldosterone. 
  • Action of these hormones is concerned with long lasting responses.
VI. Gas :  NO (Nitric Oxide)

Properties of Hormones :
  • Act as chemical messengers.
  • Effective in very low concentration. 
  • Function as regulators that inhibit or stimulate or modify specific processess. 
  • Some hormones interact with receptors present on plasma membrane of target cells where as some enter the necluses to interact with genes.
  • Hypersecretion or Hyposecretion of hormones leads to various disorders.
  • These are metabolised after their function. Thus cannot be reused. 
  • Hormone secretion is regulated by positive or negative feedback mechanism.
Mechanism of hormone action :
  • Hormones are released in a very small quantity. 
  • Produce their effect on the target organs cells by binding to hormone receptors.
  • Hormone receptors may be on the cell membrane or may be intracellular. 
  • A hormone  receptor complex is formed and this leads to biochemical change in the target tissue.
A. Mode of hormone action through membrane receptors :

  • Hormones like catecholamines, peptide and polypeptide hormones are not lipid soluble.
  • Therefore they cannot enter their target cells through plasma membrane.
  • These non steroid water soluble hormones interact with surface receptor, which initiate metabolic activity. 
  • Molecules of amino acid derivatives, peptide hormones bind to specific receptor molecules located on the plasma membrane. 
  • The hormone receptor complex causes the release of an enzyme adenylate cyclase from the receptor site. 
  • This enzyme forms cyclic AMP from ATP of the cell. 
  • cAMP activates enzymatic actions. 
  • The hormone acts as the first messenger and cAMP is the second messenger. 
  • Other kind of second messengers are Ca++, cGMP and IP3 (Inositol triphosphate).
B. Mode of action through intracellular receptors :

  • Steroid and thyroid hormones are lipid soluble and easily pass through plasma membrane of target cell into the cytoplasm.
  • In the cytoplasm, they bind to specific intra cellular receptor proteins forming a hormone receptor complex that enters the nucleus. 
  • In the nucleus, the hormone receptor complex binds to a specific regulatory site of DNA.
  • Activated genes transcribes mRNA which directs protein synthesis and enzymes in the cytoplasm. 
  • Action of lipid soluble hormones is slower but long lasting.

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