Negative and Positive Feedback Loops

We regulate our homeostasis through two major types of feedback mecahnisms; negative and positive feedback loops. There are key differences between these two types of mechanisms but they still both revolove around one central premise: dealing with stimuli. There are three main parts to a feedback loop: stimulus, control center, effectors. Both negative and positive feedback have these but the effectors and control centers operate differently.

A stimulus is something that disrupts our physiology or state of balance (homeostasis). Depending on what the stimulus is we have built in mechanisms to handle it. One of those mechanisms is negative feedback. Negative feedback is the most commonly used feedback loop in the body. Negative feedback is used to negate or counteract a stimulus without completely eradicating it. Let's use blood pressure as an example of this. If our blood pressure rises pressure receptors called baroreceptors (baro = pressure) will be stimulated. Baroreceptors are found in the arch of the aorta and the carotid sinuses of the carotid arteries. They will then generate action potentials (nervous impulses) to the medulla oblongota (cardioaccelratory center & vasomotor centers to be specific). The medulla oblongota would be the control center in this situation. Then the control centers will stimulate two effects: dilation of peripheral blood vessels to lower something called peripheral resistance, and lower our heart rate which will lower cardiac output (CO). So the peripheral vessels and the heart are the effector organs in this situation. Once our blood pressure goes back down to normal then the barorecptors will become inactivated and the medulla oblongata will stop its actions on the effector organs. So as you can see here we only cary out a change to negate a change in blood pressure, not eliminate it. 

Positive feedback has the same mechanisms but they are used differently. One key difference is that positive feedback loops are used to eradicate a stimulus. Another key difference is that responses through positive feedback become more amplified over time. So once a stimulus activates a positive feedback loop it is not turned off until the stimulus is completely removed. Postivie feedback loops/mechanisms tend to be more intense than negative feedback and can be life threatening if they get out of control. Let's use a fever as an example here. A fever is caused by an invading organism (stimulus) such as a virus (pathogen). There are certain cells in our blood and tissues that will begin to attack these pathogens and secrete chemicals called pyrogens to the brain (pyro- fire). These pyrogens will travel to the hypothalamus (control center), which is here body temperature is regulated. These pyrogens trick the hypothalamus into thinking our core body temperature should be higher than 98.6º F. So we will get cold at first and start to shiver because our brain thinks our body temperature should be higher. Then we will feel very hot once we reach this new temperature. The brain will maintain the fever and high body temperature until the virus is completely eradicated. So as long as the virus is present, the cells will continue to secrete pyrogens and will also increase the rate at which it does this. If our body temperature climbs too high then this can be threatening to our life and well being.

If you understand feedback loops then you will understand diseases. If we are unable to physiologically compensate for a stimulus on our own we will need medical intervention. Think of the blood pressure example from above. If we are unable to get our blood pressure under control on our own we will need help. If we don't get blood pressure under control harm will come our vessels.