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Flashcards in Feedback mechanisms Deck (10):

What are the stages of homeostasis control?

-the optimum point
-a receptor
-a coordinator
-an effector
-a feedback mechanism


What is the optimum point?

-the optimum point, or desired level, at which the system operates


What is a receptor?

-a receptors, which sects the stimulus of any deviation from the set point (norm)


What is a coordinator?

-a coordinator, which coordinates information from various sources


What is an effector?

-an effector, which brings about the corrective measures needs to return the system to the optimum point (norm)


What is a feedback mechanism?

-a feedback mechanism, by which a receptor detects a stimulus created by the change to the system and the effect or brings about the appropriate response


What is the relation between feedback and effectors?

-let us now look in more detail at the last stage in the lit - the feedback mechanism
-when an effect or has correlated any deviation and returned the system to the optimum point, it is important that this information is fed back to the receptor
-if the information is not red back, the receptor will continue to stimulate the effector leading to an over-correction and causing a deviation in the opposite direction
-there are two types of feedback -negative feedback and positive feedback


What is negative feedback?

-negative feedback occurs when the stimulus causes the corrective measures to be turned off
-in doing so this tends to return the system to its original optimum level and prevents any overshoot
-there are separate negative feedback mechanisms to regulate departures from the norm in each direction
-an example is in the control of blood glucose
-if there is a fall in the concentration of glucose in the blood this stimulates is detected by receptors on the cell-surface membrane of the alpha cells coordinator in the pancreas
-these alpha cells secrete the hormone glucagon
-glucagon causes liver cells (effectors) to convert glycogen to glucose which is released into blood raising the blood glucose concentration
-as this blood with a raised glucose concentration circulates back to the pancreas there is reduced stimulation of alpha cells which therefore secrete less glucagon
-so the secretion of glucagon leads to a reduction in its own secretion (=negative feedback)
-in the same way if the blood glucose concentration rises, rather than falls, insulin will be produced from the beta cells in the pancreas
-insulin increases the uptake of glucose by cells and its conversion to glycogen and fat
-the fall in blood glucose concentration that results reduced insulin production once blood glucose concentrations return to their optimum (=negative)
-having separate negative feedback mechanisms that control departures from the norm in either direction gives a greater degree of homeostatic control
-this is because there are positive actions in both directions
-for example if glucagon raised the blood sugar concentration above the optimum, it would take some time for it to fall again if the only way of lowering it was through metabolic activity
-however, by having a second hormone, insulin, that lowers blood sugar concentration far more rapidly


What is positive feedback?

-positive feedback occurs when the feedback causes the corrective measures to remain turned on
-in doing so it causes the system to deviate even more from the original (normal) level
-examples are less common, but one occurs in neurones when a stimulus causes a small influx of sodium ions
-this influx increases the permeability of the neurone to sodium ions so more items enter, causing a further increase in permeability and even more rapid entry causing a further increase in permeability and even more rapid entry of ions
-this results in a very rapid build-up of an action potential that allows an equally rapid response to a stimulus
-positive feedback occurs more often when there is a breakdown of control systems
-in certain diseases, for example typhoid fever, there is a breakdown of temperature regulation resulting in a rise in body temperature leading to hyperthermia
-in the same way when the body gets too cold (hypothermia) the temperature control systems ten dot breakdown, leading to positive feedback resulting in the body temperature dropping even lower


What is negative feedback in temperature control?

-if the temperature of the blood increases, thermo receptors in a region of the brain called the hypothalamus send more nerve impulses to the heat loss centre, which is also in the hypothalamus
-this in turn sends impulses to the skin (effect or organ)
-vasodilator, sweating and lowering of body hairs all lead to a reduction in blood temperature
-if the fact that blood temperature has returned to normal is not fed back to the hypothalamus it will continue to stimulate the skin to lose body heat
-blood temperature will then fall below normal and may continue to do so causing hypothermia and the death of the organism
-what happens in practice is that the cooler blood returning from the skin passes through the hypothalamus
-as a result thermoreceptors send fewer impulses to the heat loss centre
-this in turn stops sending impulse3s to the skin and so vasodilation, sweating, etc. cease, and blood temperature remains at its normal level rather than continuing to fall
-the blood, having been cooled to its normal temperature, has been resulted in turning off the effect or (the skin) that was correcting the rise in temperature
-this is therefore negative feedback