4.4 Negative Feedback Loop

Negative Feedback and Homeostasis

Negative feedback is critical in regulating the body’s homeostasis to promote optimal health and well-being. These inhibitory loops allow the body to self-regulate by counteracting any deviation from normal conditions. The process begins with an increase in output from a body system, leading to higher levels of specific proteins or hormones. This increase subsequently triggers the homeostatic response system to inhibit or reverse the production of those proteins or hormones, preventing their levels from rising further. The body thus reduces the number of these proteins or hormones, maintaining a balance within the normal physiologic limits (Chakravarty et al., 2023; Molnar & Gair, n.d.). An example of negative feedback within the body is depicted in Figure 4.6.

Figure 4.6 A negative feedback loop regulates body temperature. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Homeostatic Response System

The homeostatic response system is a biological structure that is in a constant state of flux because of external and internal factors that push the body’s systems away from equilibrium. The homeostatic response system is vital for maintaining a stable internal environment. When the body experiences a stimulus, such as an elevated body temperature, a neuron (nerve cell) within the brain acts as a receptor or sensor, relaying the message to the control center, which may be the hypothalamus, pituitary gland, or nervous system. The control center then activates effectors, whose role is to counteract the stimulus, thereby restoring homeostasis (see Figure 4.6) (Libretti & Puckett, 2023). In this way, the homeostatic response system enables the body to adjust to changing conditions and maintain stability.

Positive Feedback

In positive feedback, the body responds to a stimulus by amplifying or enhancing it rather than inhibiting it. This mechanism leads to a continuous increase in the stimulus, also known as a runaway effect, creating a self-reinforcing cycle. Positive feedback loops are commonly found in processes that require rapid and decisive responses, such as:

• Blood clotting
  • When an injury occurs, platelets in the blood stick to the damaged area and release chemical signals that attract more platelets. This recruitment of platelets leads to the formation of a blood clot, which further activates more platelets and reinforces the clotting process.
• Lactation
  • During lactation, the mechanical stimulation of nerve endings in the breast signals the posterior pituitary gland to release oxytocin, which stimulates the contraction of myoepithelial cells within the breast. As breast milk is released, the sucking action of the infant continues, and this further stimulates the nerve endings in the breast, creating a positive feedback loop.
• Uterine contraction during childbirth
  • During childbirth, the pressure of the newborn against the cervix and the tissue of the pelvic floor causes contraction of the uterus, which then results in the release of oxytocin, which stimulates stronger contractions. As contractions intensify, more oxytocin is released, resulting in positive feedback until the infant is delivered (Libretti & Puckett, 2023).

Nonetheless, when not properly regulated, positive feedback loops may give rise to instability and potentially adverse consequences.