Feedback Mechanisms

Feedback mechanism definition

Our bodies work tirelessly to keep us alive. Every day, our bodies have to adapt to different environmental cues, from waking up in the morning, to digesting a meal, moving around, focusing at school, and then going back to sleep. But at each step, our body adapts to the environment so that we are not too hungry, not too full, not too hot or cold, and not too tired.

For the most part, our bodies respond to all these different cues by activating feedback mechanisms to maintain a relatively stable internal state. This tendency for our body to return to a steady state following an environmental change is called homeostasis.

Homeostasis

Let's take a look at the definition of homeostasis.

There are several physiological processes that need to remain in homeostasis, including body temperature, blood pH, and blood glucose concentration. In fact, the body has tight regulation of these processes and even slight variations from the homeostatic set point can be fatal.

The homeostatic set point of body temperature, for example, is 97.7–99.5 °F (approximately 37 °C) and fluctuations to the set point by 4 °F (3 °C) can be fatal. Therefore, it is important for the body to have mechanisms in place to tightly maintain homeostasis!

There are three components involved in homeostasis:

• Receptor - receives the stimulus (ex. change in temperature sensed by the skin) and sends input to the control center.
• Control center - processes the signal received from the receptor and sends instructions (output command) to the effector. The control center also sets the range of values to be maintained.
• Effector - carries out instructions from the control center by producing a response or effect that changes the controlled condition.

Feedback mechanisms

The mechanisms by which homeostasis is maintained are called feedback mechanisms. Feedback mechanisms describe the way that an organism, cell, or even enzyme maintains homeostasis following an environmental disruption.

Figure 2. This diagram shows the feedback mechanism to control body temperature.

In living systems, feedback mechanisms are often described as self-regulating loops because they can first sense that a change has occurred, respond to the change, and then stop the response once homeostasis has been reached. Then the loop is "reset" so that it can respond to a different stimulus.

Types of feedback mechanisms in biology

There are two main types of feedback mechanisms in biology: positive feedback and negative feedback. In both cases, feedback mechanisms begin when there is an external stimulus that leads to changes away from the set point, triggering the correct feedback loop.

Negative feedback is a control mechanism that reduces or reverses a change in the external environment.

Positive feedback is a control mechanism that senses a change and triggers mechanisms to amplify that change instead of return to homeostasis.

But, how do these feedback mechanisms work? We will talk about them more in depth in the next section!

Positive feedback and negative feedback mechanisms in biology

Now that you know a little bit about what positive feedback and negative feedback mechanisms are, and how they are connected to homeostasis, let's go ahead and explore each of them in more detail!

Negative feedback

Let's start with negative feedback (also called feedback inhibition).

After a stimulus induces a change in homeostasis, the living system will work to return the body back to the homeostatic set point. The general steps of negative feedback are stimulation, reception, processing, and response.

Step 1: Stimulation - During stimulation, an external stimulation causes a deviation from the set point.

Step 2: Reception - During reception, sensory receptors in our bodies will detect that an external stimulus has occurred.

Step 3: Processing - During processing, the sensory receptors will send signals to our brain (regulatory center) that an external stimulus is sensed, to that it can be interpreted.

Step 4: Response - During the response, the brain will coordinate the activity of many components of the body to counteract the initial stimulation, and send the instructions to the effector that will then carry out the response.

Blood pressure is regulated through negative feedback. First, baroreceptors (pressure sensitive receptors) detect higher blood pressure and send nerve impulses to the control center in the brain for interpretation. Then, a response is sent to heart and blood vessels telling the arterial walls to relax, causing blood pressure to decrease.

Figure 3. Negative feedback mechanism to control blood pressure.

Positive feedback

Now, let's focus on positive feedback.

Similar to negative feedback, the four steps of positive feedback mechanisms include stimulation, reception, processing, and response. However, instead of bringing changes back, it strengthens/amplifies it!

As an example, let's take a look at the feedback mechanism that occurs during normal childbirth. Increasing of contractions causes the baby to be forced into the cervix. The cervix stretches, and stretch-sensitive receptors send nerve impulses to the control center in the brain. The control center interprets the input signal and releases oxytocin, causing the muscles in the uterus wall to increase contraction. This amplification/increase in contraction leads to an increase in the stretching of the cervix, allowing the baby to pass through. Once the baby is born, the stretching of cervix decreasing, causing the positive feedback loop to stop.

Figure 4. Positive feedback loop in normal childbirth.

Feedback mechanism examples in biology

By now, you have probably noticed that feedback mechanisms are essential for our bodies to function effectively. So, let's look at other examples involving negative and positive feedback mechanisms in biology.

Being able to regulate blood sugar is critical for survival. When regulation through negative feedback does not function correctly, chronic disorders such as type 2 diabetes might arise.

In a typical person, when blood sugar levels rise after eating a meal, the pancreas responds by producing and then releasing the hormone insulin into the blood. When insulin binds to receptors located on the liver, it triggers the liver to take up glucose, thereby lowering blood glucose concentrations.

However, in a person with type 2 diabetes, although insulin secretion is normal, they have insulin resistance. Type 2 diabetes is characterized by the poor regulation of blood sugar due to insulin resistance (insulin receptors are desensitized to insulin and no longer respond to insulin). Insulin resistant leads to hyperglycemia, or elevated glucose!

Figure 5. This diagram shows the negative feedback mechanism that occurs in a typical person when blood glucose levels increases or decreases.

Now, let's look at an example involving positive feedback and lactation in mammals. Lactation is the process of synthesizing and secreting milk from mammary glands located in the mother's breasts. Lactation generally serves as the primary source of nutrients for a newborn following birth.

In humans, when a newborn begins to nurse from the nipples of the mother, it triggers production of the hormone prolactin in the mother. The release of prolactin (from the anterior pituitary) and oxytocin (from the posterior pituitary) further stimulates lactation so that the mother produces sufficient amounts of milk for the baby. When the baby is weaned, prolactin levels drop back to levels before breastfeeding, thus ending the cycle.

Figure 6. Positive feedback and lactation in mammals.

Difference between positive and negative feedback loops in homeostasis

A summary of the differences between positive feedback mechanisms and negative feedback mechanisms and their effects on homeostasis is listed in table 1 below.

Table 1: The differences between positive and negative feedback mechanisms