Positive and Negative Feedback in the Sympathetic and Parasympathetic Nervous System

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  • 0:06 Types of Feedback
  • 1:44 Positive Feedback
  • 3:15 Negative Feedback
  • 4:22 Intermediary Steps
  • 5:15 Fight or Flight
  • 6:57 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

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.

Positive Feedback

When a child suckles, the brain of the mother releases more prolactin
Lactation Feedback

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.

Negative Feedback

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
Negative Feedback

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.

Intermediary Steps in Positive and Negative Feedback

The final output may inhibit or excite intermediary steps, causing positive or negative feedback
Intermediary Steps

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