positive feedback

Examples of positive feedback in the following topics:

• Control of Hormone Secretion

  • The endocrine system relies on feedback systems to regulate hormone production and secretion.
  • Most endocrine glands are under negative feedback control that acts to maintain homoeostasis, i.e., prevent deviation from an ideal value.
  • A key example of a negative feedback system is the regulation of the thyroid hormone thyroxine, which regulates numerous key metabolic processes.
  • Positive feedback mechanisms control self-perpetuating events, that is, they encourage deviation from the mean.
  • Positive feedback systems are much less common although they do exist.

• Homeostatic Control

  • Positive and negative feedback are more complicated mechanisms that enable these three basic components to maintain homeostasis for more complex physiological processes.
  • Positive feedback is a mechanism in which an output is enhanced in order to maintain homeostasis.
  • Positive feedback mechanisms are designed to accelerate or enhance the output created by a stimulus that has already been activated.
  • Positive feedback mechanisms are designed to push levels out of normal ranges.
  • A positive feedback example is blood platelet accumulation and aggregation, which in turn causes blood clotting in response to an injury of the blood vessels.

• Disease as Homeostatic Imbalance

  • If positive and negative feedback loops are affected or altered, homeostatic imbalance and resultant complications can occur.
  • Anything that prevents positive or negative feedback from working correctly could lead to disease if the mechanisms of disruption become strong enough.
  • As an organism ages, weakening of feedback loops gradually results in an unstable internal environment.
  • Heart failure is the result of negative feedback mechanisms that become overwhelmed, allowing destructive positive feedback mechanisms to compensate for the failed feedback mechanisms.
  • Negative feedback between insulin and glucagon levels controls blood sugar homeostasis.

• Platelet Plug Formation

  • Platelet plug formation is considered a positive feedback process because ADP and VWF levels are successively increased as more and more platelets activate to form the plug.
  • The chemical structure of ADP, a molecule that causes platelet activation and is involved in the positive feedback component of platelet activation.

• Gastric Phase

  • As dietary protein is digested, it breaks down into smaller peptides and amino acids, which directly stimulate the G cells to secrete even more gastrin: a positive feedback loop that accelerates protein digestion.
  • Below pH of two, stomach acid inhibits the parietal cells and G cells: a negative feedback loop that winds down the gastric phase as the need for pepsin and HCl declines.

• Hormonal Methods

  • Progesterone negative feedback decreases the frequency of gonadotropin-releasing hormone (GnRH) released by the hypothalamus, which decreases the release of follicle-stimulating hormone (FSH), and greatly decreases the release of luteinizing hormone (LH) by the anterior pituitary.
  • Progesterone negative feedback and the lack of estrogen positive feedback on LH release, prevent a mid-cycle LH surge.
  • Estrogen negative feedback on the anterior pituitary greatly decreases the release of FSH, which inhibits follicular development and helps prevent ovulation.

• Muscle Tone

  • To maintain tone, spindles also operate a feedback loop by directly triggering motor neurons linked to their associated muscles.
  • Tone is maintained through autonomous feedback from the muscle fibers, neurons, and associated tissues.
  • Muscle fibers in relaxed and contracted positions.

• Chemoreceptor Regulation of Breathing

  • Chemoreceptor regulation of breathing is a form of negative feedback.
  • The action potential is sent along nerve pathways to parts of the brain, which are the integrating centers for this type of feedback.
  • Negative feedback responses have three main components: the sensor, the integrating sensor, and the effector.
  • There are several other examples in which chemoreceptor feedback applies.
  • In cases where oxygen intake is too low, feedback increases ventilation to increase oxygen intake.

• Development of the Endocrine System

  • The endocrine system regulates its hormones through negative feedback, except in very specific cases like childbirth.
  • It is positioned on the neck just below the Larynx and has two lobes with one on either side of the trachea.
  • Normally, thyroid hormones act via a negative feedback loop on the pituitary to decrease stimulation of the thyroid.
  • In goiter, the feedback loop cannot be in operation - hence continual stimulation of the thyroid and the inevitable protuberance on the neck.

• Abnormal Contractions of Skeletal Muscle

  • True hypertonic spasm is caused by malfunctioning feedback nerves, is much more serious, and is permanent unless treated.