Positive Feedback

What is positive feedback?

Positive feedback is a pathway that, in deviation from homeostasis, amplifies the output and strays away from homeostasis even more.

Let's expand a little on the concepts of homeostasis and feedback loops to fully understand the implications of positive feedback. Our body is constantly trying to maintain equilibrium to ensure it can survive under all conditions. This self-regulating process is called homeostasis. Homeostasis is maintained by feedback loops.

The feedback mechanism is triggered when there is a change in the system and as a result, triggers an output.

Feedback can be either positive or negative. These systems either increase the output and deviate from the normal interstate of the body or reduce the output to bring the body back to homeostasis.

Positive feedback is also known as a self-reinforcing response to external or internal stimuli. Positive feedback enhances a change in the physiological state instead of reversing it. The change in variation is sensed by the receptor, and in response, the effector works to produce the same output and thus, enhances the physiological change. This process keeps occurring in a loop until the original stimulus is removed. Positive feedback processes are less common in the human body and are only required in cases of fast and efficient processes to amplify the output.

Mechanism of Positive Feedback

Positive feedback has 4 steps:

1. Stimulation
2. Reception
3. Processing
4. Further activation of stimulus.

Stimulation

When a stimulus is set off that disrupts homeostasis, it initiates a feedback loop, in this case, positive feedback. The stimulus causes the optimum range of homeostasis to be disturbed and moved from the normal range. A stimulus can be anything that disrupts normal physiological processes. Examples of stimuli include physical injuries, infections, childbirth, or other major physiological changes.

Reception

Reception includes receptors, also called sensory units. The sensory units, in response, receive this stimulus and transmit signals to the control unit, which then processes this data. In the case of humans, the sensory units are the nerves and the control unit, as you guessed, is the brain. An example of reception is when the sensory units in the cervix sense the head of the fetus during childbirth and send signals to the brain to process this stimulus.

Processing

After the control unit receives the data, it processes it and if the stimulus is outside the normal range and is disrupting homeostasis, the control unit will display an output. In human bodies, the control centre is the pituitary gland, which is located near the brain. It may not be the main control unit, but it is responsible for the secretion of plenty of hormones like oxytocin, anti-diuretic hormone (ADH), etc. in response to the stimuli.

Further activation of stimuli

The information sent by the control unit via the sensory units to the location triggers an output in response to the stimulus. This output further increases the stimulus in the same direction. An example of this is blood clotting, which we will discuss shortly.

The activation of the stimulus is done by an effector, which can be any organ or cell. For instance, during childbirth, the effector is the uterus which stimulates uterine contraction in order to push the fetus out of the womb in a process called birth.

These 4 processes are also found in a negative feedback loop. The only difference is, while negative feedback seeks to bring the body back to homeostasis by hindering the process, positive feedback amplifies the output to further disrupt homeostasis. That said, positive feedback does not go on forever, as it is usually stopped by negative feedback once the result for positive feedback has been achieved. This way, positive feedback and negative feedback work hand-in-hand.

Positive feedback examples

Now, let's take a look at some important examples of positive feedback: blood clotting, fruit ripening, and childbirth. Let’s explore how positive feedback aids in these processes.

Blood clotting

Have you ever gotten injured and noticed that after a few days, there’s a brown scab covering the wound? This brown scab is the blood that was clotted to stop the bleeding.

In case we get injured and start losing blood, the damaged blood vessels release chemicals that attract clotting factors called blood platelets. These platelets cling onto the injured site and release chemicals that more platelets.

As more platelets are attracted to the injured site and start clotting blood, this process is amplified to increase the speed of blood clotting. Once the blood clot is big enough to cover the injury and stop the blood loss, this chemical stops getting released and positive feedback stops.

Fruit ripening

Fruits on a plant go through various stages – unripe, to ripe, to overripe – which are triggered by a chemical called ethylene (C₂H₄) and are further stimulated by positive feedback.

As the fruit will begin to ripen, it will release this gas. The nearby fruits on the branch get exposed to this gas and start to ripen, thereby triggering a chain reaction and releasing this gas. As a result, all the fruits ripen at a rapid rate. The fruit industry is known to utilize this feedback loop to accelerate the ripening of fruits by exposing them to ethylene gas.

Menstrual Cycle

Just before ovulation starts in women, a hormone called estrogen is released by the body which travels to the brain. The hypothalamus, in turn, releases gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) from the hypothalamus. LH stimulates the release of estrogen from the ovaries, which in turn, promotes the release of GnRH and LH. As these hormones increase in concentration, they cause ovulation to occur!

Lactation

When an infant suckles on the breasts of the mother, a chemical called prolactin gets released, which increases the production of milk. More suckling leads to the release of more prolactin which promotes more production of milk. When the child is no longer hungry and stops breastfeeding, prolactin stops getting released and as a result, milk production also stops.

Childbirth

This is perhaps the most common example of positive feedback. Positive feedback, after all, helps in labour and childbirth.

During childbirth, when the fetus’s head pushes against the walls of the cervix, the receptors in the uterus send signals to the hypothalamus. The hypothalamus processes the stimulus and releases oxytocin. The release of oxytocin induces labour and muscle contractions in the uterus. This keeps happening in a loop, with oxytocin being released and the cervix contracting until the original stimulus (fetus) gets delivered from the womb.

Positive feedback vs. negative feedback

You now have an understanding of what positive feedback is. But what about negative feedback? Is it the same as positive feedback? What is the difference between the two? Let's discuss the similarities and differences between the two.

Similarities between positive and negative feedback

There are some similarities between positive and negative feedback. These include:

1. Both feedback loops are based on a stimulus mechanism and either increase or decrease the effects of the stimulus.

2. Both of these systems play a crucial role in ensuring survival through homeostasis.

Differences between positive and negative feedback

There are many differences between positive and negative feedback, mainly how they affect the stimulus and whether they move away or towards homeostasis.

Direction of stimulus

Positive feedback maintains the direction of the stimulus and amplifies the output, whereas, negative feedback attempts to reverse the stimulus and decreases the output

Effect on homeostasis

Positive feedback disrupts homeostasis, as increasing the stimulus halts the normal physiological functions of the body. Negative feed, on the other hand, attempts to restore homeostasis by decreasing the stimulus.

Stability

Since positive feedback is known to disrupt the body’s homeostasis and increases the stimulus, it is not stable and may require an external interference to stop its mechanism. Negative feedback, in contrast, is a more stable mechanism as its main purpose is to restore the body’s homeostasis. It is also an independent mechanism and will stop once homeostasis is achieved.

The table below summarizes the key differences between positive and negative feedback.