فسلجة ٩

• Endocrine system


  A system of ductless glands that secrete hormones directly into the circulatory system to be carried long distances to other target organs that regulate vital body and organ functions
• Major systems that regulate physiological activities


  • Nervous system
  • Endocrine system
• Neurotransmitters


  Chemical messengers released by axon terminals of neurons into the synaptic junctions to control nerve cell functions
• Endocrine hormones


  Chemical messengers released by glands or specialized cells into the circulating blood to influence the function of target cells at another location in the body
• Neuroendocrine hormones


  Chemical messengers secreted by neurons into the circulating blood to influence the function of target cells at another location in the body
• Paracrine


  Chemical messengers secreted by cells into the extracellular fluid to affect neighboring target cells of a different type
• Autocrines


  Chemical messengers secreted by cells into the extracellular fluid to affect the function of the same cells that produced them
• Cytokines


  Peptides secreted by cells into the extracellular fluid that can function as autocrines, paracrines, or endocrine hormones
• Endocrine glands


  Glands that synthesize and release classical hormones directly into the blood without any duct, so they are called ductless glands
• Hormones


  Chemical signal molecules released by a cell or special endocrine gland in one part of the body that send out messages affecting cells in other parts of the body, transported by the circulatory system to target distant organs to regulate physiology of the body, maintaining homeostasis and regulating reproduction and development

  
• Nature of hormones


  • Secreted from their source directly into the blood
  • Blood carries the hormone to the target cells which respond to it
  • Regulate the physiological processes
  • Produced in very small quantities and are biologically very active; usually measured in micrograms or milligrams per day
  • Excess and deficiency, both, cause serious disorders
• Classification of hormones based on chemical nature


  • Proteins and polypeptides
  • Steroids
  • Derivatives of the amino acid tyrosine

  
• Hormone secretors

  • Exclusively endocrine: Pituitary, thyroid, parathyroid, thymus and the adrenal glands
  • Partially endocrine: The pancreas, gastric and duodenal epithelium, the gonads (testis in males and ovary in females) and placenta in females
• Difference between endocrine and exocrine glands


  Endocrine glands secrete hormones, are ductless, and pour secretion directly into the blood to control long-term target organ functions

  Exocrine glands produce enzymes, have ducts to carry substances outside the body or into body cavities, and control short-term activity
• Hormonal action


  Hormone combines with receptors present on the target cells to form a hormone-receptor complex

  Hormone-receptor complex induces various changes or reactions in the target cells
• Hormone receptors


  Large proteins present in the target cells that are specific for one single hormone
• Location of hormone receptors


  • In or on the cell membrane (for protein, peptide, and catecholamine hormones)

  Cytoplasm (for steroid hormones)

  Nucleus (for thyroid hormones)
• Synthesis and storage of hormones


  Protein and peptide hormones are synthesized on the rough endoplasmic reticulum as larger preprohormones, then cleaved to form smaller prohormones in the endoplasmic reticulum

  Prohormones are transferred to the Golgi apparatus for packaging into secretory vesicles

  Enzymes in the vesicles cleave the prohormones to produce smaller, biologically active hormones

  Hormones are stored in secretory vesicles until needed

  Secretion occurs when the secretory vesicles fuse with the cell membrane and the contents are extruded into the interstitial fluid or directly into the blood stream by exocytosis
• Mechanism of hormonal function


  Hormone-receptor complex acts by:

  1. Altering the permeability of the cell membrane

  2. Activating intracellular enzymes

  3. Acting on genes
• Some hormones, such as norepinephrine and epinephrine, are secreted within seconds after the gland is stimulated and may develop full action within few seconds to minutes
• The actions of other hormones, such as thyroxin or growth hormone, may require months for full effect
• Radioimmunoassay


  Method used to measure extremely minute quantities of hormones in the blood
• Enzyme-linked immunosorbent assays (ELIZA)


  More recently developed method for accurate measurements of hormones in the blood