13-09-2017, 10:26 AM
A feedback mechanism (also called a feedback system) is a cycle of events in which the state of a specific aspect of the body condition (called a "controlled condition"), , is continuously monitored and adjusted as appropriate to maintain the value of that controlled condition within a safe interval for the body to continue to function successfully, as opposed to maintaining the damage, due to overheating.
The correct concentration (ie, within an acceptable range) of hormones should be maintained because the hormones have potent effects on the body. Feedback systems are an ideal means of controlling hormone levels because they involve constant monitoring and adjustments to keep hormone levels stable. This is particularly important in the case of hormone levels because:
• Hormones can affect the target organs in low concentrations, so even a small amount can sometimes be too much.
• The amount of time during which hormones remain active is limited so more hormones should be secreted when needed to replace those that are inactivated.
Control of the regeneration of hormonal production
Feedback circuits are at the root of most of the control mechanisms in physiology, and are particularly prominent in the endocrine system. Positive feedback cases certainly happen, but negative feedback is much more common.
Negative feedback is seen when the output of a track inhibits inputs to the track. The heating system in your home is a simple negative feedback circuit. When the furnace produces enough heat to raise the temperature above the thermostat setpoint, the thermostat activates and closes the oven (heat is being fed negatively on the heat source). When the temperature falls below the set point, the negative feedback is gone and the oven is switched on again.
Feedback loops are widely used to regulate the secretion of hormones in the hypothalamic-pituitary axis. An important example of a negative feedback loop is observed in the control of thyroid hormone secretion. Thyroid hormones Thyroxine and triiodothyronine ("T4 and T3") are synthesized and secreted by the thyroid glands and affect metabolism throughout the body. The basic control mechanisms in this system (illustrated on the right) are:
• Hypothalamus neurons secrete thyroid releasing hormone (TRH), which stimulates the cells of the anterior pituitary to secrete thyroid stimulating hormone (TSH).
• TSH binds to receptors on the epithelial cells of the thyroid gland, stimulating the synthesis and secretion of thyroid hormones, which most likely affect all cells in the body.
• When blood levels of thyroid hormones increase above a certain threshold, neurons that secrete TRH into the hypothalamus are inhibited and fail to secrete TRH. This is an example of "negative feedback".
Inhibition of TRH secretion leads to disruption of TSH secretion, which leads to disruption of thyroid hormone secretion. As thyroid hormone levels fall below the threshold, negative feedback is relieved, TRH secretion begins again, resulting in TSH secretion.
Another type of feedback is seen in endocrine systems that regulate concentrations of blood components such as glucose. Drinking a glass of milk or eating a candy bar and the following (simplified) series of events will occur:
• Lactose or ingested sucrose glucose is absorbed from the intestine and blood glucose levels increase.
• Elevated blood glucose concentration stimulates the endocrine cells of the pancreas to release insulin.
• Insulin has the main effect of facilitating the entry of glucose into many cells in the body - as a result, blood glucose levels fall.
• When the level of glucose in the blood falls sufficiently, the stimulus for insulin release disappears and the insulin is no longer secreted.
Numerous other examples of specific endocrine feedback circuits are presented in the sections on specific hormones or endocrine organs.
The correct concentration (ie, within an acceptable range) of hormones should be maintained because the hormones have potent effects on the body. Feedback systems are an ideal means of controlling hormone levels because they involve constant monitoring and adjustments to keep hormone levels stable. This is particularly important in the case of hormone levels because:
• Hormones can affect the target organs in low concentrations, so even a small amount can sometimes be too much.
• The amount of time during which hormones remain active is limited so more hormones should be secreted when needed to replace those that are inactivated.
Control of the regeneration of hormonal production
Feedback circuits are at the root of most of the control mechanisms in physiology, and are particularly prominent in the endocrine system. Positive feedback cases certainly happen, but negative feedback is much more common.
Negative feedback is seen when the output of a track inhibits inputs to the track. The heating system in your home is a simple negative feedback circuit. When the furnace produces enough heat to raise the temperature above the thermostat setpoint, the thermostat activates and closes the oven (heat is being fed negatively on the heat source). When the temperature falls below the set point, the negative feedback is gone and the oven is switched on again.
Feedback loops are widely used to regulate the secretion of hormones in the hypothalamic-pituitary axis. An important example of a negative feedback loop is observed in the control of thyroid hormone secretion. Thyroid hormones Thyroxine and triiodothyronine ("T4 and T3") are synthesized and secreted by the thyroid glands and affect metabolism throughout the body. The basic control mechanisms in this system (illustrated on the right) are:
• Hypothalamus neurons secrete thyroid releasing hormone (TRH), which stimulates the cells of the anterior pituitary to secrete thyroid stimulating hormone (TSH).
• TSH binds to receptors on the epithelial cells of the thyroid gland, stimulating the synthesis and secretion of thyroid hormones, which most likely affect all cells in the body.
• When blood levels of thyroid hormones increase above a certain threshold, neurons that secrete TRH into the hypothalamus are inhibited and fail to secrete TRH. This is an example of "negative feedback".
Inhibition of TRH secretion leads to disruption of TSH secretion, which leads to disruption of thyroid hormone secretion. As thyroid hormone levels fall below the threshold, negative feedback is relieved, TRH secretion begins again, resulting in TSH secretion.
Another type of feedback is seen in endocrine systems that regulate concentrations of blood components such as glucose. Drinking a glass of milk or eating a candy bar and the following (simplified) series of events will occur:
• Lactose or ingested sucrose glucose is absorbed from the intestine and blood glucose levels increase.
• Elevated blood glucose concentration stimulates the endocrine cells of the pancreas to release insulin.
• Insulin has the main effect of facilitating the entry of glucose into many cells in the body - as a result, blood glucose levels fall.
• When the level of glucose in the blood falls sufficiently, the stimulus for insulin release disappears and the insulin is no longer secreted.
Numerous other examples of specific endocrine feedback circuits are presented in the sections on specific hormones or endocrine organs.