Negative and Positive feedback mechanisms

What is Negative Feedback Mechanisms ?

In human body systems controlled by negative feedback, the effector response decreases or negates the effect of

the original stimulus, maintaining or restoring homeostasis (thus the term negative feedback). 

1) Control of body temperature is similar to the non-physiological example of a domestic central heating system.

2) The thermostat (temperature detector) is sensitive to changes in room temperature (variable factor).

3) The thermostat is connected to the boiler control unit (control centre), which controls the boiler

(effector). 

4) The thermostat constantly compares the information from the detector with the preset temperature and, when necessary, adjustments are made to alter the room temperature. 

5) When the

thermostat detects the room temperature is low, it switches the boiler on. The result is output of heat

by the boiler, warming the room. 

6) When the preset temperature is reached, the system is reversed. The

thermostat detects the higher room temperature and turns the boiler off.

 7) Heat production from the

boiler stops and the room slowly cools as heat is lost. 

This series of events is a negative feedback mechanism and it enables continuous self-regulation, or control, of a variable factor within a narrow

range.

Body temperature is a physiological variable controlled by negative feedback , which prevents problems due to it becoming too high or too low.

 When body temperature falls below the

preset level, this is detected by specialised temperature sensitive nerve endings in the hypothalamus

of the brain, which form the control centre. 

This centre then activates mechanisms that raise body temperature (effectors). These include:

stimulation of skeletal muscles causing shivering narrowing of the blood vessels in the skin reducing the blood flow to, and heat loss from, the

peripheries behavioural changes, e.g. we put on more clothes or curl up.

When body temperature rises within the normal range again, the temperature sensitive nerve

endings are no longer stimulated, and their signals to the hypothalamus stop.

 Therefore, shivering stops and blood flow to the peripheries returns to normal.

Most of the homeostatic controls in the body use negative feedback mechanisms to prevent

sudden and serious changes in the internal environment.

What is Positive Feedback Mechanisms ?

There are only a few of these cascade or amplifier systems in the body. In positive feedback mechanisms, 

the stimulus progressively increases the response, so that as long as the stimulus is continued the response is progressively amplified. 

Examples include blood clotting and uterine contractions during labour.

During labour, contractions of the uterus are stimulated by the hormone oxytocin. These force the

baby’s head into the cervix of the uterus, stimulating stretch receptors there. In response to this, more

oxytocin is released, further strengthening the contractions and maintaining labour. After the baby is

born the stimulus (stretching of the cervix) is no longer present so the release of oxytocin stops .

This mechanism indicates as positive feedback mechanism.