Negative Feedback

9 Key excerpts on "Negative Feedback"

• 

  eBook - ePub

  Simulations in Medicine

  • Irena Roterman-Konieczna, Irena Roterman-Konieczna(Authors)
  • 2015(Publication Date)
  • De Gruyter

    (Publisher)

  Cellular level Passage contains an image Jakub Wach, Marian Bubak, Piotr Nowakowski, Irena Roterman, Leszek Konieczny, and Katarzyna Chtopas

  2  Negative Feedback inhibition – Fundamental biological regulation in cells and organisms

  The functioning of biological systems is governed by the laws of physics and chemistry. Biological processes occur spontaneously and are subjected to thermodynamic regulation.

  Thermodynamics introduces a distinction between open and closed systems. Reversible processes occurring in closed systems tend to a state of equilibrium. This state is achieved when the reaction proceeds at an identical rate in both directions (from substrate to product and from product to substrate) – a situation which can be denoted v 1 = v 2 . In terms of energy transfer this state is inert, with nil spontaneity (ΔG = 0) (Fig. 2.1(a) ). In thermodynamically open systems (including all biological systems) equilibrium is not achieved spontaneously – it can, however, be maintained by continually replenishing the required substrates. This so-called stationary state is characterized by nonzero spontaneity ΔG ≠ 0) (Fig. 2.1 (b) ).

  Maintaining a stationary state requires regulation. In nonsentient systems, such as the interior of a cell, regulation must be automatic and is typically based on Negative Feedback loops, as shown in Fig. 2.2 .

  The Negative Feedback loop may be symbolically represented as a closed circuit in which a detector monitors the state of a controlled process while an effector counteracts detected changes. The function of the effector is dependent on signals received from the detector (see Fig. 2.2 ).

  In cells and organisms detectors are usually referred to as “receptors”. Most receptors are proteins which react with the products of a controlled process. Their genetically-conditioned affinity to a specific product enables them to control its concentration. Such structures can be found inside the cell, as well as in the cellular membrane, with receptors protruding into the environment and capable of registering external signals (Fig. 2.3 (a) , (b) and (c)

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Medical Sciences

  • Jeannette Naish, Denise Syndercombe Court, Jeannette Naish, Denise Syndercombe Court(Authors)
  • 2018(Publication Date)
  • Elsevier

    (Publisher)
• • Negative Feedback to depress the change if the variable level has increased beyond the narrow, set range. This is the commonest mechanism.
• • Positive feedback to affect an increase or acceleration in the output variable that has already been triggered. The result is to push the level beyond the physiological range.
• • Feedforward control to either depress or enhance the level of a variable before the change is needed, i.e. anticipatory (or open loop). Open-loop systems have no way to calibrate against the set point and so always need an accompanying closed-loop Negative Feedback to correct any over- or underanticipation.

Negative Feedback

Negative Feedback mechanisms can either increase or reduce the activity of tissues or organs back to normal, set levels, and the system is sometimes called a Negative Feedback loop ( Fig. 1.1 ) . Numerous examples of Negative Feedback exist in the metabolic processes of all physiological systems.

Fig. 1.1 Negative Feedback loop. An increase in the variable produces an effector response to decrease it and a reduction in the variable leads to an effector response to increase it with the aim of returning to equilibrium.

Homeostatic control of glucose metabolism

An example of a Negative Feedback system is the homeostatic control of blood glucose. Among the tissues of the body, red blood cells and the brain (under normal conditions) can only use glucose to generate the energy needed to drive metabolic processes. Glucose is essential to ensure an adequate supply of energy for the vital functions performed by these and other tissues. Blood glucose concentration (measured as fasting blood glucose) is therefore tightly maintained within a narrow range (3.5–8.0 mmol/L) normally. Here, the sensor is specialised pancreatic cells that receive blood from the portal circulation. The control is the autonomic nervous system, and the effectors are the α (secreting glucagon) and β (secreting insulin) pancreatic cells in the islets of Langerhans. A simplified explanation of glucose homeostasis is shown in Fig. 1.2 .

Fig. 1.2