Bio 2 Schaums Chapter 15

Negative feedback is a regulatory mechanism in which a

'stimulus' causes an opposite 'output' in order to maintain an ideal level of whatever is being regulated.

Effectors

- muscle or glands that respond to deviation from the set point. If the parameter is outside this range an impulse will be sent to effectors that bring them back to the set point. In the case of negative feedback, this would cause the parameter to move further from the set point.

Set point

- the normal range that an environmental parameter is to be controlled to.

Homeostasis is also termed a dynamic equilibrium because

1.) the body maintains an internal balance, within fluctuating limits 2.) in Homeostasis parts of the system change but the overall system stays constant

Example homeostasis:

For enzymes between chemicals within a cell to work the PH needs to be finely regulated. Enzymes are proteins that are in folded arrangements to give them a particular shape and if the PH changes, that can alter the nature of the enzymes and if the enzymes can become denatured they will stop performing there biochemical function. That means they will not catalyze the reaction from A to B. That means we will not have the biochemistry and thus means we will not have the physiology which equals no life. PH must be balanced.

Negative Feedback definition

When a change away from the ideal triggers a reaction designed to bring the conditions back to normal

What is the difference between negative feedback loop VERSUS a positive feedback loop?

With negative feedback, the output reduces the original effect of the stimulus. In a positive feedback system, the output enhances the original stimulus. Just remember that positive feedback mechanisms ENHANCE the ORIGINAL STIMULUS and negative feedback mechanisms INHIBIT it.

Examples of NEGATIVE feedback pathways:

Your body has its own internal controller for maintaining its temperature, pH, hormone levels, blood sugar, and other internal variable levels at homeostasis, the optimal internal state at which your body operates best. This controller of homeostasis in most animals is the hypothalamus, without which, organisms would have great difficulty functioning normally. There are many negative feedback pathways in biological systems, including: •Temperature regulation •Blood pressure regulation •Blood sugar regulation

4) Name and describe the FUNCTION of the three parts of a negative or positive feedback loop. A feedback loop helps maintain homeostasis of the internal environment and includes three parts:

1. RECEPTORS - detect certain environmental parameters within the body including temperature, pH, and glucose concentrations. Information from the receptors is transmitted to the regulatory portion that has a set point. 2.) SET POINT - the normal range that an environmental parameter is to be controlled to. EFFECTORS- muscle or glands that respond to deviation from the set point. If the parameter is outside this range an impulse will be sent to effectors that BRING THEM BACK to the set point. In the case of NEGATIVE FEEDBACK, this would cause the parameter to move further from the set point.

Compare and contrast positive and negative feedback.

Answer Both positive and negative feedback are important homeostatic mechanisms and both use a set point and have receptors and effectors. A negative feedback loop is distinguished from a positive feedback loop in the way the set point works. When a deviation occurs away from the set point in a negative feedback loop, sensory receptors detect the change and effectors are stimulated to bring the condition back to that determined by the set point. Conversely, in a positive feedback loop, a deviation from the set point detected by receptors will stimulate effectors that move the condition controlled by the positive feedback loop further from the set point. Of the two types of feedback loops, negative feedback is the most common.

Receptors

- detect certain environmental parameters within the body including temperature, pH, and glucose concentrations. Information from the receptors is transmitted to the regulatory portion that has a set point.

To maintain homeostasis, communication within the body is essential. The image provided is an example of how a homeostatic control system works. Here is a brief explanation: 1.Stimulus- produces a change to a variable (the factor being regulated). 2.Receptor- detects the change. The receptor monitors the environment and responds to change (stimuli).

3.Input- information travels along the (afferent) pathway to the control center. The control center determines the appropriate response and course of action. 4.Output- information sent from the control center travels down the (efferent) pathway to the effector. 5.Response- a response from the effector balances out the original stimulus to maintain homeostasis.

Let's look at a specific example, namely blood glucose. 1. Have a regulated variable -- glucose level in blood. 2. Need a sensor (or receptor) -- to measure levels of "regulated variable" (glucose). Here, sensor is in pancreas. 3. Need effector(s) -- to control levels of regulated variable (glucose) -- usually have one or more effectors that respond in opposing ways. In this case, effectors for uptake of glucose are liver, adipose tissue, and skeletal muscle; effector for release of glucose is liver. 4. Have a set point -- the level the regulated variable (blood glucose) should be. Set point is also sometimes used to mean the level at which corrections (to raise or lower the value) kick in. 5. Signaling -- need some signal system to connect the sensor(s) and the effector(s). Can be nervous &/or hormonal. In this case, primary (but not only) signal is hormonal & primary hormones (signals) are insulin & glucagon.

6. Negative Feedback -- the system responds to negate deviations from the set point. Important features: a. Works to stabilize blood glucose levels b. System is self-correcting -- Deviations in either direction (if blood glucose is either too high or too low) are corrected back to standard. c. There are two opposing actions by effectors, not just one. (1). If [G] gets too high, effectors take G up from blood. (top half of seesaw diagram) (2). If blood [G] gets too low, effector releases G to blood. (bottom half of seesaw diagram) d. NEGATIVE FEEDBACK is NOT always inhibition. In this case, an increase in glucose uptake is used to help lower high blood sugar levels. The deviation from the set point was fixed by ACCELERATING not inhibiting, a process. In negative feedback, deviations from the set point can be corrected either by speeding up a process (such as glucose uptake) or slowing down a process (such as glycogen breakdown to glucose). e. How is this different from positive feedback? In positive feedback, the system responds to increase deviations from the set point -- a small deviation triggers a bigger one, which triggers a bigger one and so on. f. Terminology: In physiology, negative feedback means the system is self correcting as in b & d above. It doesn't matter whether the corrections are achieved by inhibition (turning off the heater) or acceleration (turning on the air conditioner). In biochemistry, negative feedback usually means inhibition of an earlier step. 7. Value of regulated variable does not remain exactly constant, but stays within narrow limits.

Positive feedback loop example A good example of a positive feedback system is child birth. 1.) During labor, a hormone called oxytocin is released that intensifies and speeds up contractions. 2.) The increase in contractions causes more oxytocin to be released and the cycle goes on until the baby is born. 3.) The birth ends the release of oxytocin and ends the positive feedback mechanism.

Another good example of a positive feedback mechanism is blood clotting. Once a vessel is damaged, platelets start to cling to the injured site and release chemicals that attract more platelets. The platelets continue to pile up and release chemicals until a clot is formed. Just remember that positive feedback mechanisms enhance the original stimulus and negative feedback mechanisms inhibit it.

homeostasis: Whenever a body experiences a change, whether internally or externally, the

change is detected and an appropriate response occurs. Dynamic equilibrium