Veterinary Physiology-III

Feed back control (or) Servo mechanism

BY FEED BACK MECHANISM (or) SERVO MECHANISM

• It is an automatic means of correcting the hormone output of a gland to a desired level by level-sensing cells in the hypothalamus.
  • The feedback system operates through three pathways
    • Ultra short loop: Between hypothalamic releasing and inhibitory factors; e.g. reciprocal stimulation / inhibition between GHRH and somatostatin
    • Short loop: Between hypothalamic factors and pituitary hormones; e.g. TRH, TSH
    • Long loop: Between hypothalamic factors and peripheral endocrine glands; e.g. GnRH and gonadal hormones
• The feedback control system operates in two mechanisms

Positive Feedback System

• Increase in the level of target organ hormone increases secretion of tropic hormone.
• The hypothalamus controls secretion of tropic hormones from anterior pituitary through the secretion of peptide releasing hormones.Increase in the concentration of target organ hormone stimulates hypothalamus.In the hypothalamus, the releasing hormone output increases to effect increased production of tropic hormones by anterior pituitary and subsequently the secretions of the hormones by the target organ.
• E.g.,preovulatory release of LH, during the late stages of ovarian follicular development causes increased estrogen production which acts on hypothalamus and stimulates LHRH release and increases pulsatile rate of LH secretion from adenohypophysis to effect ovulation.
• NegativeFeedback System It is the most common feedback system.
• An increased target organ hormone concentration above certain physiological limit, causes a shut-down of releasing hormone production in the hypothalamus and also tropic hormone secretion from the anterior pituitary, thus decreases the production of hormone by the target organ.
• E.g.,ACTH secretion stimulates cortisol secretion, while increased blood cortisol concentration inhibits ACTH secretion from adenohypophysis and CRH release from the hypothalamus to bring cortisol level to normal.

Negative feedback mechansim