explain the feedback mechanism to regulate the action of hormones with the help of suitable example
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Neurons in the hypothalamus secrete thyroid releasing hormone (TRH), which stimulates cells in the anterior pituitary to secrete thyroid-stimulating hormone (TSH).TSH binds to receptors on epithelial cells in the thyroid gland, stimulating synthesis and secretion of thyroid hormones, which affect probably all cells in the body.When blood concentrations of thyroid hormones increase above a certain threshold, TRH-secreting neurons in the hypothalamus are inhibited and stop secreting TRH. This is an example of "negative feedback".
Inhibition of TRH secretion leads to shut-off of TSH secretion, which leads to shut-off of thyroid hormone secretion. As thyroid hormone levels decay below the threshold, negative feedback is relieved, TRH secretion starts again, leading to TSH secretion.
Another type of feedback is seen in endocrine systems that regulate concentrations of blood components such as glucose. Drink a glass of milk or eat a candy bar and the following (simplified) series of events will occur:
Glucose from the ingested lactose or sucrose is absorbed in the intestine and the level of glucose in blood rises.Elevation of blood glucose concentration stimulates endocrine cells in the pancreas to release insulin.Insulin has the major effect of facilitating entry of glucose into many cells of the body - as a result, blood glucose levels fall.When the level of blood glucose falls sufficiently, the stimulus for insulin release disappears and insulin is no longer secreted.
Hormone Profiles: Concentrations Over Time:
One important consequence of the feedback controls that govern hormone concentrations and the fact that hormones have a limited lifespan or halflife is that most hormones are secreted in "pulses". The following graph depicts concentrations of luteinizing hormone in the blood of a female dog over a period of 8 hours, with samples collected every 15 minutes:
The pulsatile nature of luteinizing hormone secretion in this animal is evident. Luteinizing hormone is secreted from the anterior pituitary and critically involved in reproductive function; the frequency and amplitude of pulses are quite different at different stages of the reproductive cycle.
With reference to clinical endocrinology, examination of the graph should also demonstrate the caution necessary in interpreting endocrine data based on isolated samples.
A pulsatile pattern of secretion is seen for virtually all hormones, with variations in pulse characteristics that reflect specific physiologic states. In addition to the short-term pulses discussed here, longer-term temporal oscillations or endocrine rhythms are also commonly observed and undoubtedly important in both normal and pathologic states.
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Inhibition of TRH secretion leads to shut-off of TSH secretion, which leads to shut-off of thyroid hormone secretion. As thyroid hormone levels decay below the threshold, negative feedback is relieved, TRH secretion starts again, leading to TSH secretion.
Another type of feedback is seen in endocrine systems that regulate concentrations of blood components such as glucose. Drink a glass of milk or eat a candy bar and the following (simplified) series of events will occur:
Glucose from the ingested lactose or sucrose is absorbed in the intestine and the level of glucose in blood rises.Elevation of blood glucose concentration stimulates endocrine cells in the pancreas to release insulin.Insulin has the major effect of facilitating entry of glucose into many cells of the body - as a result, blood glucose levels fall.When the level of blood glucose falls sufficiently, the stimulus for insulin release disappears and insulin is no longer secreted.
Hormone Profiles: Concentrations Over Time:
One important consequence of the feedback controls that govern hormone concentrations and the fact that hormones have a limited lifespan or halflife is that most hormones are secreted in "pulses". The following graph depicts concentrations of luteinizing hormone in the blood of a female dog over a period of 8 hours, with samples collected every 15 minutes:
The pulsatile nature of luteinizing hormone secretion in this animal is evident. Luteinizing hormone is secreted from the anterior pituitary and critically involved in reproductive function; the frequency and amplitude of pulses are quite different at different stages of the reproductive cycle.
With reference to clinical endocrinology, examination of the graph should also demonstrate the caution necessary in interpreting endocrine data based on isolated samples.
A pulsatile pattern of secretion is seen for virtually all hormones, with variations in pulse characteristics that reflect specific physiologic states. In addition to the short-term pulses discussed here, longer-term temporal oscillations or endocrine rhythms are also commonly observed and undoubtedly important in both normal and pathologic states.
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