What is negative feedback

I never understood this thing.
No idea what it is, what it mean, when it starts and when it ends.
My teacher tried explaining it to me twice, but I still don't get it.
So can someone please clearly explain it to me ?

Reply 1

CameronWS

15

Here's a GCSE explanation. Are you GCSE/AS/A/?

https://www.youtube.com/watch?v=_Mts354VC7A

Reply 2

Leah.J

OP

13

Original post by CameronWS

Here's a GCSE explanation. Are you GCSE/AS/A/?

https://www.youtube.com/watch?v=_Mts354VC7A

A level , I know I should know this by now and I do , I mean, ofcourse I understand everything that was said in the video but I'm still not sure about the "negative feedback" part
Suppose my body temperature increases, and my body wants to counteract that change, so impulses are sent and stuff happens and your body temperature is lowered, is that what's called negative feedback ? Or is negative feedback what happens after ? = when the temperature is lowered, the body recognizes that and no more impulses are sent to whatever caused the body temp to be lowered

Which one of them is negative feedback ?

Is the heart rate increasing during exercise and example of positive or negative feedback and why ?
I'm thinking -ve because it helps increase the pH of the blood but at the same time Is it positive because the heart rate is already high, it's just made higher

Idk what I'm saying

Reply 3

Joinedup

20

From the point of view of electronics & industrial control systems, negative feedback is a way of maintaining a variable at a set level.

you've got a set point - the target level for your variable
you've got the variable itself
and you've some way of comparing the variable to the set point and producing an 'error' value which is proportional to the difference between the two.
i.e. if the variable is too high the error is positive
if the variable is just right the error is zero
if the variable is too low the error is negative

it's called negative feedback because you multiply the error by a negative number and then feed that back into the system that controls the variable.

so to use the example in the video if the level of thyroxine is too high a signal is sent to reduce the production rate of thyroxine... and if the level of thyroxine is too low a signal is sent to increase the production rate of thyroxine... and that's your negative feedback.

the alternatives would be :
positive feedback - which would increase the level of thyroxine production if the level of thyroxine was too high or decrease the level of thyroxine production if the level of thyroxine was too low... obviously this would be unstable and drive the thyroxine level to either worryingly high or low levels.

No feedback at all which would just imply no effort being made to control the level.

Reply 4

CameronWS

15

Original post by Leah.J

A level , I know I should know this by now and I do , I mean, ofcourse I understand everything that was said in the video but I'm still not sure about the "negative feedback" part
Suppose my body temperature increases, and my body wants to counteract that change, so impulses are sent and stuff happens and your body temperature is lowered, is that what's called negative feedback ? Or is negative feedback what happens after ? = when the temperature is lowered, the body recognizes that and no more impulses are sent to whatever caused the body temp to be lowered

Which one of them is negative feedback ?

Is the heart rate increasing during exercise and example of positive or negative feedback and why ?
I'm thinking -ve because it helps increase the pH of the blood but at the same time Is it positive because the heart rate is already high, it's just made higher

Idk what I'm saying

I don't take Biology unfortunately I could help ya. Bumping this thread though, hopefully you get some help on this.

Reply 5

macpatgh-Sheldon

20

Original post by Leah.J

A level , I know I should know this by now and I do , I mean, ofcourse I understand everything that was said in the video but I'm still not sure about the "negative feedback" part
Suppose my body temperature increases, and my body wants to counteract that change, so impulses are sent and stuff happens and your body temperature is lowered, is that what's called negative feedback ? Or is negative feedback what happens after ? = when the temperature is lowered, the body recognizes that and no more impulses are sent to whatever caused the body temp to be lowered

Which one of them is negative feedback ?

Is the heart rate increasing during exercise and example of positive or negative feedback and why ?
I'm thinking -ve because it helps increase the pH of the blood but at the same time Is it positive because the heart rate is already high, it's just made higher

Idk what I'm saying

Hi Leah (again!),
Let me walk you thru this problem you have.
The explanation by @Joinedup is very good, but because physics is an exact science, and biology is not, it is a bit difficult for you to extrapolate from physics (engineering) to biology.

Let us take a different endocrine example instead of thyroxine, to keep your alertness and interest alive. The pituitary gland secretes a hormone called ACTH (adrenocorticotrophic hormone - you DO NOT NEED to know this name!); this hormone controls the levels of certain adrenal hormones (those secreted in the adrenal cortex [outer layer]) e.g. cortisone (a steroid)). When cortisone levels are approaching their higher normal level (are relatively high), THIS HIGH CORTISOL LEVEL ITSELF results in a reduction in the release of ACTH -----> less ACTH in blood -----> less stimulation of adrenal gland ----> less cortisone secreted ----> blood cortisone levels tend to reduce towards a good normal level - so because the effect of the original high cortisol level is to lead to a REDUCTION (i.e. opposite direction to the original abnormality [from underlined to bold in my formatting], which was an increase, yeah?) therefore NEGATIVE feedback. Because the high cortisol level leads to a reduction of this level i.e. opposite effect to the original change [which was an increase in our example (and, actually it can be vice versa too [the whole thing]), THEREFORE IT IS CALLED NEGATIVE FEEDBACK.

In your heart example, negative feedback occurs as follows:
If a person's blood pressure drops (even slightly e.g. when he/she stands up from a sitting position), it needs to be restored to a normal level. So the drop in blood pressure is detected by baroreceptors (baro = pressure as in barometer used in weather forecast) in the carotid sinus - an area in the carotid artery, which supplies the head and neck. This sends a nerve signal to the hypothalamus in the brain (a region quite close to the medulla in the brainstem), which responds by sending larger numbers of signals to the heart (the sino-atrial node, to be specific [which determines the heart rate in normality i.e. is the pacemaker]) to increase the heart rate, and this tachycardia tends to increase the blood pressure back to normal SO NEGATIVE FEEDBACK [opposite of the original reduction in bp], are you with me?

Actually, you can do a simple experiment by converting yourself to a guinea-pig (sorry - :colone:

); feel your radial pulse while sitting down by placing your forefinger and middle finger on your other wrist on the palm side, but about a cm from your thumb edge (1.8 cm odd if you have massive hands/wrists like mine!) - take a note of the approximate rate per minute by counting the beats you feel over 15 secs then X 4. Now stand up AND YOU WILL IMMEDIATELY NOTICE a speeding up of the rate.

Best,
M

Reply 6

macpatgh-Sheldon

20

Original post by CameronWS

I don't take Biology unfortunately I could help ya. Bumping this thread though, hopefully you get some help on this.