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Week 1.docx

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University of Toronto St. George
Michelle French

Week 1: Endocrine System Chapter 1: Introduction to Physiology Homeostasis does not mean Equilibrium Stability of the bodys internal environment Intracellular fluid compartment (ICF) = cellular homeostasis The composition of both body compartments are in a dynamic steady state - Materials are constantly moving back and forth between the two comparemtns Steady state equilibrium - Equilibrium implies that the composition of body compareemtns is identical - i.e - ECF and ICF exist in a state of disequilibrium Control Systems and Homeostasis Human body monitors certain functions such as blood pressure and blood glucose concentrations Regulated variables are kept within their acceptable range by physiological mechanisms All control systems have 3 components: 1. An input signal 2. A controller or integrating center 3. An output signal Local Control is Restricted to a Tissue Simplest form of control that is restricted to the tissue or cell involved - Relatively isolated change occurs in tissue - Nearby groups of cells respond by releasing a chemical Example: Reflex Control Uses Long Distance Signaling Changes that are systemic, require more complex control systems to maintain homeostasis - i.e. blood pressure to drive blood flow reflex control = long distance pathways that uses the nervous system, endocrine system or both reflex can be broken down into two parts: 1. response loop 7 steps: Stimulus sensor input signal integrating center output signal target response - in mammals, integrating centers are usually part of the nervous system or endocrine system - output signals may be chemical or electrical refer to page 195 Response Loops Begin with a Stimulus refer to slide printout Feedback Loops Modulate the Response Loop where the response feeds back to influence the input portion of the pathway Negative Feedback Loops are Homeostatic are designed to keep the system at or near a set point how well the integrating center succeeds depends on the sensitivity of the system end result is a regulated variable that oscillates around the set point pathway in which the response opposes or removes the signal is called negative feedback - loops stabilize the regulated variable and aid systems in maintaining homeostasis - can restore the normal state but cannot present the initial disturbances Positive Feedback Loops are Not Homeostatic the response reinforces the stimulus rather than decreasing or removing it - sends the variable even further from its normal value - requires an event from outside the loop to stop the response example: Feedforward Control Allows the Body to Anticipate Change negative feedback loops can stabilize a function and maintain it within a normal range but are unable to prevent the change that triggers the reflex in the first place feedforward control: enables the body to predict that a change is about to occur and start the response loop in anticipation of the change example: salivation reflex Biological Rhythms Result from Changes in a Set point the set points of many variables change from person to person or for the same individual over a period of time - affected by normal biological rhythms, inheritance and conditions to which the person has become accustomed biorhythms are predictable, repeating patterns or cycles of change -i.e. circadian rhythm many hormones have 24 hour cycles; thus important to know when the hormone levels were measured biological rhythms create an anticipatory response to a predictable environmental variable some variability in set points are associated with changing environmental conditions rather than biological rhythms: - acclimatization: - acclimation: Chapter 6: Communication, Integration and Homeostasis Cell to Cell Communication 2 basic types of physiological signals: electrical and chemical 1. Electrical: 2. Chemical: - Responsible for most communication Target cells are those that respond to signals four basic methods of cell to cell communication: 1. local communication: gap junctions contact dependent signals chemicals that diffuse through the extracellular fluid to act on cells close by 2. long distance communication uses a combination of chemical and electrical signals carried by nerve cells and blood vessels Gap Junctions Create Cytoplasmic Bridges simplest form of cell to cell communication is the direct transport of signals formed from connexins in two adj cells - united connexins create a protein channel = connexon large molecules cannot pass through are the only means by which electrical signals can pass directly from cell to cell Contact Dependent Signals Require Cell to Cell Contact requires that surface molecules on one membrane bind to a membrane protein of another cell - occurs in the immune system and during growth Cell Adhesion Molecules (CAMs) act as receptors in cell to cell signaling Paracrine and Autocrine Signals Carry Out Local Communication Paracrine signal is a chemical that acts on cells in the immediate vicinity of the cell that secreted the signal autocrine signal is a signal that acts on the cell that secreted it reach their target cells by diffusing through the interstitial fluid - distance is limiting factor for diffusion - i.e. histamine Long Distance Communication May be Electrical or Chemical most long distance communications take place through the endocrine and nervous systems endocrine system communicates using hormones - chemical signals that are secreted into the blood and distributed through the body by circulation the nervous system uses both chemical and electrical signals
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