BM4041 Lecture 2 2024 PDF

Homeostasis: The Foundation of Physiology Body systems maintain homeostasis by regulating a dynamic steady state in the internal environment Information flow between cells, tissues, and organs is an essential feature of homeostasis and allows for the integration of physiological processes...

Homeostasis: The Foundation of Physiology Body systems maintain homeostasis by regulating a dynamic steady state in the internal environment Information flow between cells, tissues, and organs is an essential feature of homeostasis and allows for the integration of physiological processes Firing Homeostasis as a Master Regulator of Integrative Homeostatic Network that Maintains the Stability of Neural Circuits and Safeguards from Neurodegeneration. Adapted from Frere S and Slutsk I. Neuron,97, Jan 2018.  L2. Chapter 1. Characteristics of Homeostatic Control Systems Learning Outcomes variable. 5. Recognize that error signals vary in 1. Understand the concepts of steady state magnitude- the smaller the error signal and set point in homeostatic systems. that can drive the control system, the 2. Understand that homeostasis is more closely the regulated variable can achieved by balancing inputs and be controlled. outputs. 6. Understand that the set points for 3. Understand that negative feedback regulated variables can be “reset” under mechanisms operate to negate (undo) different physiological conditions, such change i.e., to bring a changed variable as sleep/wakefulness cycles. back toward its original setpoint; contrast 7. Understand the adaptive value of feed negative feedback with positive forward regulation in conjunction with feedback, which exacerbates change negative feedback for homeostatic e.g., parturition. control. 4. Understand the importance of an error 8. Complete Thermoregulation study signal for continued homeostatic activity & read Clinical Case 1 responses to an ongoing event that (Chapter 1 Vander’s Physiology) caused the initial change in the regulated  Homeostasis Most physiological variables such as blood pressure, body temperature, and blood gases, are maintained within a predictable range. Physiological variables can change dramatically over a 24-hour period, but the body is still in overall balance. Homeostasis refers to physiological variables in a state of dynamic constancy; it is not a static process. When homeostasis is maintained, we refer to physiology; when it is not, we refer to pathophysiology.  Body systems - contribution to homeostasis: circulatory - transports materials (e.g., nutrients, gases) digestive - breaks dietary food into small nutrient molecules. respiratory - obtains oxygen and eliminates carbon dioxide urinary - removes and eliminates wastes from the plasma skeletal - provides support and protection for soft tissues muscular - moves the bones integumentary - serves as an outer protective barrier immune - defends against foreign invaders nervous - controls and coordinates activities rapidly endocrine - regulates activities that require duration reproductive - perpetuation of the species Many homeostatic systems regulate the balance between addition and removal of a chemical substance from the body concentration of nutrient molecules – Gases (oxygen & carbon dioxide) – Waste products – Water – Salts & other electrolytes pH Temperature Volume (fluids) Pressure (blood) Controlled exchange of materials occurs between compartments and across cellular membranes. Homeostasis at a cell level organ level or whole body level Successful compensation – Homeostasis reestablished – Health/Wellness Failure to compensate – Pathophysiology Illness Disease/Death Homeostasis Figure 1.4 Changes in Blood Glucose Concentration During a Typical 24-Hour Period Blood glucose levels increase after eating, and then levels return to their set point via homeostasis. This is an example of dynamic constancy. Levels change over short periods of time, but remain relatively constant over long periods of time. Figure 1.5 A Homeostatic Control System Maintains Body Temperature When Room Temperature Decreases Interpret the arrows in this textbook's flow charts as “leads to” or “causes.” (For example, decreased room temperature causes increased heat loss from the body, which leads to a decrease in body temperature, etc.)  Homeostatic Setpoint It is not always possible for homeostatic Homeostatic control systems cannot systems to maintain every variable within a maintain complete constancy of any given narrow normal range in response to an feature of the internal environment. environmental challenge. There is a Therefore, any regulated variable will have a hierarchy of importance, so that certain more or less narrow range of normal values variables may be altered markedly to depending on the external environmental maintain others within their normal range. conditions. Oscillation around the setpoint The set point of some variables regulated by homeostatic control systems can be reset – that is, physiologically raised or lowered. ❖ Homeostatic range -oscillation around setpoint ❖ Change in setpoint ✓ Acclimatization ✓ Biorhythms  Adaptation and Biological Rhythms Acclimatization The term adaptation denotes a Many body functions are characteristic that favors survival associated with rhythmical in specific environments. changes. The most common type is the circadian rhythm, which cycles approximately Acclimatization refers to the once every 24 hours. improved functioning of an already existing homeostatic system; in some cases this is due Waking and sleeping, body to prolonged exposure to an temperature, hormone environmental change. concentrations in the blood, the excretion of ions into the urine, and many other functions undergo circadian variation.  Control Pathways Maintain homeostasis Local–paracrine Long-distance–reflex control Nervous Endocrine Cytokines Comparison of local and reflex control  Feedback Loops or systems are a common mechanism to control physiological processes Negative: are homeostatic systems which bring about responses that move a variable opposite to the direction of its original change ✓ Response slows stimulation ✓ Return to optimal range Positive: systems which enhances the production of the product or accelerates a process ✓ stimulation drives more stimulation Feedback Loops Negative and positive feedback  Negative Feedback “Active product” controls the sequence of chemical reactions by inhibiting the rate-limiting enzyme, “Enzyme A.” Positive Feedback A positive feedback loop  A reflex is a specific, involuntary, unpremeditated, “built-in” response to a particular stimulus The pathway mediating a reflex is known as the reflex arc. A reflex arc has several components: stimulus, receptor, afferent (incoming) pathway, integrating center, efferent (outgoing) pathway, and effector. A stimulus is defined as a detectable change in the internal or external environment, and a receptor detects the environmental change. The signal travels between the receptor and the integrating center along the afferent pathway. The information going from the integrating center to the effector travels along the efferent pathway. An integrating center often receives signals from many receptors, some of which may respond to quite different types of stimuli. Thus, the output of an integrating center reflects the net effect of the total afferent input; that is, it represents an integration of numerous bits of information.  Reflex Control Steps of a reflex Stimulus Sensory receptor Afferent path Integration center Efferent path Effector- target cell/tissue Response (feedback loop) Reflex for Minimizing the Decrease in Body Temperature that Occurs on Exposure to a Reduced External Environmental Temperature  Categories of Chemical Messengers A given chemical messenger can fit into more than one category. For example, the steroid hormone cortisol affects the very cells in which it is made, the nearby cells that produce other hormones, and many distant targets, including muscles and liver.  Types of Receptors: Membrane, CNS & Peripheral Multiple meanings of the word receptor Complexity and Modulation of Control Systems Specificity Nature of signal Speed Duration Stimulation intensity Control pathways Review of Control Pathways Some basic patterns of nervous, endocrine, and neuroendocrine control pathways  Key Concepts to know ❑ Stability of an internal environmental variable is achieved by balancing inputs and outputs. It is not the absolute magnitudes of the inputs and outputs that matter but the balance between them ❑ Most physiological functions are controlled by multiple regulatory systems ❑ Integration of systems uses local, endocrine and nervous communications ❑ Signals travel via diffusion, gap junctions, axons, and blood to target cells ❑ Receptor types and functions: binding, transduction, amplification, activation, cell responses ❑ Models of homeostasis ❑ Reflex control pathways, types, feedback and their regulation