In this lesson, you'll study positive and negative feedback in the sympathetic and parasympathetic nervous systems. You'll also learn why we have a fight or flight instinct and how it works.
Types of Feedback
Imagine a game of soccer between two teams. One team is losing, and the other one is winning. There is some time left in the game to tie the game up or even win it. The captain of the losing team knows that something must be done in order to make one last push to try and score a goal. He can use a bunch of signals - through words, hand motions or even a certain gaze - to sort of rouse the troops.
As his teammates see that the leader of the team still has belief in all of them and their chances of winning the game, it gives his teammates a confidence boost. This confidence boost will improve the team's performance. Likewise, this positive increase in performance will be recognized by the captain, who, in turn, will be enthused by his team's improved outlook on the game, and this will make him even more likely to keep pushing his team to do better.
On the other hand, the captain may use some stimulating words to rouse his teammates, who will in turn react negatively and try to get him to be quiet instead. As you can tell, these two scenarios are all conducted through feedback mechanisms: the hand signals, words and reactions of the captain and his teammates.
In your body, very similar events occur between body systems, locations and hormones. All of these feedback mechanisms serve to keep your body's internal mechanisms running smoothly. If a feedback mechanism were to go into overdrive, or be impaired, the team (akin to your body) would either never try to tie the game up or would fail due to the exhaustion of trying too hard.
When a child suckles, the brain of the mother releases more prolactin
When a mother gives birth to a child, she will undergo a process by which milk is secreted from the mammary glands. We call this process lactation. This process is regulated by quite a few hormones. One of the hormones responsible for the production of milk is called prolactin. As the newborn child suckles on the mother's nipple, milk is released from the mammary glands and, therefore, must be replenished.
In order to replenish the released milk, more milk must be produced. The suckling reflex of the child actually sends signals to the brain to release more prolactin in order to produce more milk. The more the child suckles, the more the nerves signal for the release of more prolactin. This positive output allows for the child to suckle more milk in the future, which, in turn, causes more prolactin to be released yet again, so the child can continue to suckle milk.
Hence, the process in which X produces Y, which, in turn, stimulates more of X to be produced, is termed positive feedback. One process feeds off of and enhances the other. This is another way of looking at positive feedback. In our example, the suckling of an infant depends on prolactin, and this suckling enhances prolactin's production so the baby has something to feed on.
On the flip side, there are examples of when a process's outputs reduce the processes ability for further output - something we call negative feedback. For example, if your blood pressure were to increase due to increased sympathetic nervous system activity, causing vasoconstriction, receptors in certain parts of your great vessels would send signals up to your brain to tell you to shut off your sympathetic nervous system. Once the sympathetic nervous system's effect on your vasculature has been turned off, the blood vessels will be able to dilate, or expand, in order to decrease the blood pressure.
High blood pressure caused by the sympathetic nervous system leads to a negative feedback loop
Therefore, a scenario whereby the original output (increased sympathetic nervous system activity) causes an output that must be stopped (the high blood pressure), there exists a mechanism by which the output (high blood pressure) triggers a feedback to shut off the original process that caused the problem in the first place.
The final output may inhibit or excite intermediary steps, causing positive or negative feedback
One thing you must keep in mind is that in both negative and positive feedback loops, there are intermediary steps that may actually inhibit or excite a certain pathway. But, the end result is either positive or negative feedback, regardless of any excitatory or inhibitory intermediary steps.
Another thing you should also know is that inhibition or stimulation of a positive or negative feedback loop does not have to occur by the final output, influencing what started the entire process in the first place. The final output, or intermediary steps of a process, may inhibit or excite any one or more intermediary steps in a process and, therefore, cause positive or negative feedback.
Fight or Flight
Let's tie what we've learned to a real world example I hope no one has to experience. If you are walking around the woods and see a massive bear coming towards you, it's highly unlikely you'll want to cuddle with it. That grizzly may look cute, but to the bear, you're just a tasty snack. The bear is almost certain to actually stress you out and leave you with a choice of either running away or trying to fight off the bear. The concept of our body's physiological reaction to a stressor, which causes us to fight it or run away from it, is called the fight or flight response. One of the most critical things that occurs during a fight or flight response is a massive increase in your heart rate.
Under normal resting conditions, your sympathetic nervous system's actions on your heart are not as significant as your parasympathetic nervous system's effects. When everything is calm and cool in your life, the parasympathetic nervous system exerts more control over things like the heart rate. In very simple terms, the parasympathetic nervous system lowers your heart rate.
However, when placed under stressful conditions, your sympathetic nervous system actually begins to gain the upper hand over the parasympathetic nervous system's control of the heart. In this case, the sympathetic nervous system stimulates the heart to beat faster, much faster. All of this happens to help you increase blood flow to either fight the bear or run away from it. It's your choice in the end, but at least your body prepares you for either scenario through the fight or flight response.
Let's recap all of our important points. The process in which X produces more of Y, which in turn stimulates more of X to be produced, is termed positive feedback. Conversely, when a process's outputs reduce the processes ability for further output, we term this negative feedback. Finally, our body's physiological reaction to a stressor, which causes us to fight it or run away from it, is called the fight or flight response.
Just remember, positive and negative feedback loops can have one or multiple outputs that may influence one or more steps in the entire process in order to increase or decrease the output of a final or intermediary step. Basically, anything can happen along the way.
Following this video, you should be able to:
- Explain positive feedback, negative feedback and intermediary steps
- Differentiate between the functions of the sympathetic and parasympathetic nervous systems