Basic concepts of endocrine system

Basic concepts of endocrine system

1. Coordination of body functions

   • Neurotransmitters
     • Released by axon terminals of neurons into synaptic junctions
     • Act locally to control nerve cell functions
   • Endocrines
     • Secreted by glands/ specialized cells into the circulating blood
     • Influence the function of cells at other location in the body
   • Neuroendocrines
     • Secreted by neurons into the circulation
     • Influence the function of cells at other location in the body
       • E.g. hypothalamic hormones
   • Paracrines
     • Secreted by cells --> extracellular fluid (ECF)
     • Affect neighbouring cells (i.e. other than those that produced the hormone )
       • E.g. Action of sex steroid on the ovary
   • Autocrines
     • Secreted by cells --> ECF
     • Affect the same cells that produced them (i.e. site of production)
       • E.g. Release of insulin from pancreatic B-cells can inhibit its release from the same cells
   • Cytokines
     • Are peptides secreted by cells --> ECF
     • Can function as autocrines, paracrines/ endocrines
       • E.g. interleukins

2. Body's 2nd largest communication system

3. Serves to REGULATE, INTEGRATE, & COORDINATE diff. physiological processes by means of hormones

4. Hormonal regulations: 4 areas

   • 1. Digestion, utilization & storage of nutrients
     • E.g. Gut hormones, insulin
   • 2. Growth & development
     • E.g. Growth hormones, FSH, LH
   • 3. Water & electrolyte balance
     • E.g. PTH, aldosterone, ADH
   • 4. Reproductive functions
     • E.g. Estrogen, testerone

5. Hormones

   • Mode of action
     • Def.
       • Chemical messengers secreted by endocrine organs
       • Released in small quantities into blood & upon delivery by the circulation to target cell
       • Act to regulate reactions that elicit a typical response
         • Endocrine action
   • Chemical structures
     • 3 categories
       • Amines & amino acids
         • Derived from single aa (Tyrosine)
           • E.g Norepinephrine & epinephrine
         • Derived from 2 iodinated tyrosine aa residues
           • E.g. Thyroid hormones
       • Peptides (<20 aa) & Proteins (>20 aa)
         • Small
           • E.g. thyrotropin-releasing hormone (3 aa)
         • Large & Complex
           • E.g. Growth hormone & FSH (approx. 200 aa)
       • Steroids
         • Derived from cholesterol
           • E.g. Testosterone, oestrogen & progesterone
   • Synthesis
     • Protein & peptide hormones
       • Synthesized in rough ER
       • Usually synthesized as precursor hormone --> modification
         • E.g. Insulin contains an extra aa when synthesized
       • Packed & stored in granules/ vesicles by Golgi & secreted when required
       • May exist as free hormone & circulates unbound in blood/ complexed with specific binding proteins
     • Steroid hormones
       • Synthesized in smooth ER/ mitochondria
       • Certain steroids --> precursors for production of other hormones
         • E.g. In adrenal cortex: progesterone --> aldosterone, cortisol/ androgens
       • Carried by specific carrier proteins
   • Metabolism & excretion