Biological Sciences
Feedback Mechanisms
Feedback mechanisms are processes in living organisms that maintain homeostasis by regulating internal conditions. They involve the detection of a change in a physiological parameter, a response to counteract the change, and a signal to indicate whether the change has been reversed. These mechanisms are essential for the stability and proper functioning of biological systems.
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11 Key excerpts on "Feedback Mechanisms"
eBook - ePubSimulations in Medicine
Pre-clinical and Clinical Applications
- Irena Roterman-Konieczna(Author)
- 2015(Publication Date)
- De Gruyter(Publisher)
Its constituent elements – receptors and effectors – provide answers to two key questions: “how much?” and “how?” respectively. The receptor addresses the “how much?” aspect since it determines the level of activity or product concentration. For its part, the effector initiates action when triggered by the detector; therefore it embodies the “how?” aspect – as shown in Fig. 2.6. Fig. 2.6: Information conveyed by a negative feedback loop consisting of a receptor and an effector. Since almost all processes occurring in the cell and in the organism are subject to automatic control, we can assume that all cellular and organism related structures belong to regulatory circuits. This supports the conclusion that the negative feedback loop can be treated as a basic structural and functional unit of biological systems. Evidently, the goal of the cell and the organism is to maintain a steady state of chemical reactions and product concentrations. Signals originating beyond the regulatory circuits for a given process and altering the sensitivity of its receptor are called steering signals. They facilitate coordination of various biological processes, which, in turn, ensures targeted action and stabilizes the cell’s environment (see Fig. 2.7). Fig. 2.7: Negative feedback loop with indication of steering modifying the affinity of the receptor for the controlled product. Coordination is effected by ensuring that the product of one process acts upon the receptor of another process, modifying its affinity. These types of relationships aggregate cellular processes into higher-order chains (Fig. 2.8). Signals sent by the organism can also exert a coordinating influence, which is strictly hierarchical in nature, overriding cell-level control. In this way the organism coordinates the function of various cell types, ensuring overall homeostasis. Fig
eBook - PDF- Claire Leader, Ian Peate(Authors)
- 2024(Publication Date)
- Wiley-Blackwell(Publisher)
An example of this might be the regulation of blood glucose, where an increase or decrease in blood TABLE 6.1 Changes that receptors can initiate. Blood pressure Blood glucose levels Oxygen and carbon dioxide levels within tissues and blood Body fluid pH levels Concentration levels of water and electrolytes Feedback Mechanisms 107 glucose levels outside of the homeostatic range sets in motion processes that will either reduce or increase glucose levels so that blood glucose levels remain constant over time. Positive Feedback Mechanisms Positive Feedback Mechanisms play a much smaller role in the body’s homeostatic control and there are only a few of these ‘amplifier’ systems in the body (Waugh and Grant 2018). In positive Feedback Mechanisms, the disturbance is allowed to increase in its direction and resultant loss of homeostasis. While this homeostatic disturbance can be detrimental and lead to ill health, positive feedback is desirable when rapid change is required. An example of this is lactation. A suckling action of the baby at its mother’s breast leads to the production of prolactin, which leads to milk production. The more the baby suckles, the more prolactin and ultimately breast milk is produced. When the demand for breastfeeding stops, prolactin levels decrease and breast milk production stops. The body’s nervous and endocrine systems are the two main systems that are involved in the maintenance of homeostasis. While each of the body’s individual systems works independently and is to a certain degree self-regulating, they are also reliant on one another to maintain homeostasis, as a disturbance in one body system can affect the functioning of another. Therefore, there is a need for systems to interrelate and work together to maintain a constant and balanced, stable internal environment that maintains health. Table 6.2 outlines the functions that individual body systems perform that maintain homeostasis.
- Ewart Carson, Claudio Cobelli, Joseph Bronzino(Authors)
- 2000(Publication Date)
- Academic Press(Publisher)
2.3.1. INHERENT FEEDBACK REGULATION At each level of the hierarchy of chemical reactions in the cell, feedback control mechanisms can be seen. However, in addition to a variety of explicit feedback control mechanisms, some of which are illustrated in the following 18 Ewart Carson, Tom Hennessy, and Abdul Roudsari Figure 2.2. perspective. Environment ~I~ Sensors I [ En~oen~e_s I Neuroendocdnes ] System behaviour] __ _ _ [Structural changes ] [Muscle contraction,secretion ] [ Synth~esis storage~--~ ELy I t t , , I Proteinsynthesis I I t Chemicalreactions[ t dEnzymes] -q Chemicalprocessesin cell The hierarchy of control mechanisms in the human organism from a chemical paragraph, regulation of a feedback nature can be found whenever chemical dynamics are in operation. Consider the case of a simple chemical reaction taking place within the organism, in which it may be assumed that the rate of decrease (loss) of concentration of the metabolite M taking part in that reaction is directly proportional to its concentration. Mathematically, this can be expressed in the form: dCM /dt = -kCM (2.1) where CM is the concentration of metabolite M, and k is the rate constant for the reaction. Expressing (2.1) in the form of a signal flow diagram (Figure 2.3), it can be seen that there is effectively a negative feedback connection. That is, an increase in concentration results in an increase in the negative rate of change of concentration, leading, via the process of integrating that rate of concentration change, to a reduction in the concentration itself. In other words, there is a process that regulates the concentration of that metabolite. This is an example of an inherent regulatory effect exhibited in this chemical reaction, despite no physical feedback link. Control in Physiology and Medicine 19 i dt dCa ,.I r Figure 2.3. The signal flow diagram of a simple, first-orderchemical reaction.
eBook - PDFFeedback Systems
An Introduction for Scientists and Engineers, Second Edition
- Karl Johan Åström, Richard Murray, Richard M. Murray(Authors)
- 2021(Publication Date)
- Princeton University Press(Publisher)
Chapter One Introduction Feedback is a central feature of life. The process of feedback governs how we grow, respond to stress and challenge, and regulate factors such as body temperature, blood pressure, and cholesterol level. The mechanisms operate at every level, from the interaction of proteins in cells to the interaction of organisms in complex ecologies. —M. B. Hoagland and B. Dodson, The Way Life Works , 1995 [119]. In this chapter we provide an introduction to the basic concept of feedback and the related engineering discipline of control . We focus on both historical and current examples, with the intention of providing the context for current tools in feedback and control. 1.1 WHAT IS FEEDBACK? A dynamical system is a system whose behavior changes over time, often in response to external stimulation or forcing. The term feedback refers to a situation in which two (or more) dynamical systems are connected together such that each system influences the other and their dynamics are thus strongly coupled. Simple causal reasoning about a feedback system is difficult because the first system influences the second and the second system influences the first, leading to a circular argument. A consequence of this is that the behavior of feedback systems is often counter-intuitive, and it is therefore necessary to resort to formal methods to understand them. Figure 1.1 illustrates in block diagram form the idea of feedback. We often use the terms open loop and closed loop when referring to such systems. A system is said to be a closed loop system if the systems are interconnected in a cycle, as shown in Figure 1.1a. If we break the interconnection, we refer to the configuration as an open loop system, as shown in Figure 1.1b. Note that since the system is in a feedback loop, the choice of system 1 versus system 2 is somewhat arbitrary. It just depends where you want to start describing how the system works.
- Peter Dallos(Author)
- 2012(Publication Date)
- Academic Press(Publisher)
In biological systems both regulators and servos are common. The first, so-called homeostatic mechanisms, are typified by the various control systems that stabilize blood chemistry. For example, if an animal's environment changes so that he breathes a larger than normal concentration of C 0 2 in the air then the arterial C 0 2 pressure, pC0 2 , increases. The effect of this is to stimulate ventilation, which in turn causes the reduction of arterial pC0 2 and brings this variable back to normal. We are clearly dealing with a feedback regulator whose function is to keep arterial pCQ 2 constant. An 468 7. Feedback Mechanisms example of the biological servomechanism is the accommodation reflex of the eye. As visual targets move toward and away from the eye, the image formed on the retina by the crystalline lens becomes blurred. The blurring of the image activates a control process whose end result is a change in the focal length of the lens and the consequent refocusing of the image on the retina. We can now begin a more formal analysis of feedback systems, which will help us to understand how disturbances are eliminated and how other advantageous properties are gained by the incorporation of feedback. Let us first express the output of the system shown in Fig. 7.1 in terms of the input ( x j , the disturbance (x d ) and the transfer functions G X and G 2 . The following series of relationships hold: X e = X { — X 3 , X l =G l X e , X 2 = X x + X d , and X 3 = G 2 X 2 . After eliminating the internal variables X e ,X i9 and X 2 one obtains the relationship G1G7 G t x > -x = x <üh-2 (7 · 2) In the steady state the gains of the two transfer functions G x and G 2 can be symbolized by k x and k 2 . In other words, in the steady state, x x = k x x Q and x 3 = k 2 x 2 .
eBook - PDF- Bryan H. Derrickson(Author)
- 2019(Publication Date)
- Wiley(Publisher)
If dis- ruption is extreme, regulation of homeostasis may fail. 6. Most often, the nervous and endocrine systems, acting together or separately, regulate homeostasis. The nervous system detects body changes and sends action potentials to counteract the changes. The endocrine system regulates homeostasis by secreting hormones. 7. Feedback systems include three components: (1) Receptors monitor changes in a controlled variable and send input to a control center; (2) the control center determines the set point at which a controlled variable should be maintained, evaluates the input it receives from receptors, and generates output commands when they are needed; and (3) effectors receive output from the control center and produce a response (effect) that alters the controlled variable. 8. If a response reverses the original stimulus, the system is operat- ing by negative feedback. One example of negative feedback is the regulation of blood pressure. If a response enhances the original stimulus, the system is operating by positive feedback. One exam- ple of positive feedback is uterine contractions during the birth of a baby. 9. In addition to feedback systems, homeostasis may also involve feed- forward control. In feedforward control, events occur in anticipation of a change in a controlled variable. An example of feedback control occurs when the sight, smell, or thought of food causes your mouth to salivate and your stomach to produce gastric juice. 10. Disruptions of homeostasis—homeostatic imbalances—can lead to disorders, diseases, and even death. 1.5 Physiology as a Science 1. The term physiology is derived from the physiologoi, a group of ancient Greek philosophers who speculated about the existence and purpose of all things in nature. 2. Over time, the scope of physiology began to focus on how living things in nature function. Hippocrates, Aristotle, Erasistratus, and Galen (all Greek) were among the first to study body function.
eBook - PDF- Bernard L. Horecker, Earl R. Stadtman(Authors)
- 2014(Publication Date)
- Academic Press(Publisher)
Reviews covering alternative approaches to these problems have recently been presented (28, S3). 63 64 m i c h a e l α. s a v a g e a u L Introduction The existence of control systems in living organisms has been appre-ciated for some time [e.g., Cannon (12)]. Nevertheless, the early ideas in this area remained somewhat metaphorical, for it was not until recent years that the underlying molecular mechanisms were uncovered. In the 1950's specific control systems involving feedback inhibition (83, 95) and repression {Ji8) were discovered in biosynthetic pathways of micro-organisms. These two systems of regulation, often operating in conjunc-tion, have been found in nearly every biosynthetic pathway that has since been examined {47, 74, la, 35, 82, 84), Feedback inhibition is the best understood of the two processes. During the past 15 years there have been great advances in our understanding of the molecular mechanisms by which this type of control system is realized (41, 2, 84, 37). Understanding of the intact control system is less developed. For ex-ample, feedback inhibition serves to regulate the supply of the end product and conserve energy by allowing only the necessary complement of intermediate metabolites to be produced. Although ideas such as these have been apparent since the initial discoveries in this area, there have been few attempts to establish criteria by which the effectiveness of different systems in performing these functions can be evaluated and compared. This is true for biochemical systems in general. There are numerous chemical and physical techniques for characterizing the com-ponent parts of biochemical systems (37), but there are relatively few quantitative measures for those specific properties and behavior patterns, such as regulation, that emerge only at the level of organized networks of enzyme-catalyzed reactions (68).
eBook - PDF- Srinivas Aluru(Author)
- 2005(Publication Date)
- Chapman and Hall/CRC(Publisher)
basic idea of feedback is simply to use the current state of a system to make decisions about the course of action for its future. Such a scheme is called “closed loop”, while a scheme where no information on the state of the system is used to influence its future operation is called “open loop”. An important form of feedback is negative feedback which we illus-trate through a common example: Heat Regulation. A simplified block diagram scheme of temperature regulation in a house is shown in Figure 28.4. In this scheme, it is desired to keep the temperature of a house (plant or process) at a certain reference temperature (set point). For this purpose, temperature is measured (sensor) and its deviation from the desired temperature is assessed (error signal). The error signal is then fed to the thermostat (controller) that devises that appropriate control action. The output of the controller is used to operate the heat fuel valve (actuator), therefore generating appropriate actuation signal (fuel to furnace). Errors or deviations from temperature setpoint are hence corrected through the action of this negative feedback. 28.6 Feedback Loops and Their Dynamic Role in Gene Reg-ulatory Networks Much like technological systems, gene regulatory networks need to operate robustly. Hence, it does not come much as a surprise that regulatory feedback loops are ubiquitously used in gene networks to tackle this robustness demand. In addition to robustness, feedback influences many other dynamical properties in these networks. We review some of these features in the following sections. 28.6.1 Feedback and Steady State Behavior Often times, the feedback structure in a system dictates its steady state behavior. Here, we focus on the role of feedback in creating monostability, multistability and periodic behavior.
eBook - PDFFoundations of Mathematical Biology
Supercellular Systems
- Robert J. Rosen(Author)
- 2013(Publication Date)
- Academic Press(Publisher)
Such a deviation would typically be caused by disturbances or pathogenic factors. Homeostasis thus produces a continual readjustment of vital physiological values as a result of uninterrupted monitoring by means of sensing elements. Occasionally, it also involves resetting the physiological value, as exemplified in the change of thermoregulatory homeostasis from the normal warm-blooded mam-malian condition to the torporific condition of hibernation. Homeostatic mechanisms and physiological control systems in general can be described in terms of the typical feedback system configuration of Fig. 10. Receptors are readily recognized as elements which provide con-tinuous information about the actual state of the system and, therefore, constitute the feedback portion of the system. The comparator can usually be viewed as a center of nervous integration receiving signals from the receptors, from the integrative centers of other control systems (Fig. 1) and also from higher nervous centers. Its function may be reasonably inferred as one of comparison between a desired state of the system determined by the activity supplied by the higher centers and the actual state of the systems as described by the receptor signals. However, this idea is not necessary in practice, although it is useful conceptually. Thus, in detail the condition of zero error may in fact merely be the condition of equilibrium for the system.
eBook - PDFPhysiology and Anatomy for Nurses and Healthcare Practitioners
A Homeostatic Approach, Third Edition
- John Clancy, Andrew McVicar(Authors)
- 2017(Publication Date)
- Taylor & Francis(Publisher)
The authors share the view that an increase in the white blood cell count, as occurs in response to an infection, and the increase in certain hormones (e.g. adren-aline, noradrenaline, cortisol), as occurs in response to stress, are further examples of homeostatic adaptive process rather than homeostatic imbalances. Receptors and control centres The initial disturbance in a physiological parameter is detected by receptors, sometimes referred to as monitors or error detec-tors. The function of these receptors is to relay information about the deviation to homeostatic control centres (analysers or interpreters). These centres interpret the disturbance as being above or below the homeostatic range, and determine the magnitude of the deviation. As a result, they stimulate appropriate responses via effectors that bring about the correc-tion of the disturbance in order to restore homeostasis. Once the parameter has been normalized, the response will cease (Figure 1.8a, b). Homeostatic controls Occasionally, only one homeostatic control mechanism is nec-essary to redress the balance. For example, when the distur-bance of blood glucose concentration exceeds its homeostatic range (hyperglycaemia; ‘hyper-’ = over or above, ‘glyc-’ = glu-cose, ‘-aemia’ = blood) the hormone insulin is released, which promotes glucose removal from blood. More frequently, a number of controls are involved. For example, blood pressure is controlled by a number of neural and hormonal mechanisms (see Figure 12.27, p.346). Another example is when blood acidity exceeds its homeostatic range (a condition referred to as an acidosis), three controls act to reduce the acidity values within the normal range (Clancy and McVicar, 2007a): • Buffers : these chemicals act to neutralize the excess acidity (see Equations 5–9 in Chapter 6, p.130–2).
eBook - PDF- Carlo Cosentino, Declan Bates(Authors)
- 2011(Publication Date)
- CRC Press(Publisher)
4 Negative feedback systems 4.1 Introduction Negative feedback is a powerful mechanism for changing and controlling the dynamics of a system. Through the expert use of this type of feedback, control engineers are able to manipulate the dynamics of a huge variety of different systems, so that they behave in a way that is desirable and efficient from the point of view of the user, [1, 2, 3]. In biological systems, evolutionary pres-sures have led to the use of negative feedback for a wide variety of purposes, including homeostasis, chemotaxis, adaptation and signal transduction. As shown in Fig. 4.1, the principle of negative feedback is extremely simple: a feedback loop is closed around a system G and the measured output of the system y is compared to its desired value r . The resulting error signal e is acted on by a controller K , which generates an input signal u for the system which causes its output to move towards its desired value. Note that, depend-ing on the type of system, and the level of control required, the controller K could be as simple as a unity gain or as complex as a high-order nonlinear dynamical system. Consider, for example, a simple first-order system G ( s ) K G + -Σ r(t) u(t) y(t) e(t) FIGURE 4.1: Negative feedback control scheme. which has a time constant of 3 seconds and a system gain of 10: Y ( s ) = G ( s ) U ( s ); G ( s ) = 10 3 s + 1 (4.1) 115 116 Feedback Control in Systems Biology The response of this system to a step input U ( s ) = 1 /s is shown in Fig. 4.2, and as expected, the system takes 3 seconds to reach 63% of its final value. Suppose the response of the system is now required to be much faster than 0 5 10 15 0 5 10 Step response of G(s) Amplitude 0 0.5 1 1.5 2 2.5 3 0 0.5 1 Closed-loop step response Amplitude Time (s) FIGURE 4.2: Step responses of G ( s ) = 10 / (3 s +1) with and without feedback control.
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