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Department
Human Kinetics
Course
HK 2810
Professor
c
Semester
Fall

Description
Feedback Homeostasis is extremely important in physiology. It is defined as the maintenance of nearly constant conditions in the internal environment. In helping with maintaining homeostasis feedback is a useful mechanism in the body which sends a response to an organ or system to adjust a change which may have occurred and bring the level of the target back to normal. Negative feedback is a control mechanism in which is initiated if some factor becomes excessive or deficient, and it will perform a series of changes which will return the altered factor back to normal, in order to maintain homeostasis. Positive feedback is not as common in physiological processes, however when it occurs it is known as a vicious cycle. Once positive feedback is initiated an event will be stimulated which will increasingly repeat and repeat till a final result has been achieved, and the body can return back to homeostatic conditions. Feedback: • Lung has a feedback system working through 3 types of neurons. “A” neurons are inspirators which turn on breathing, these stimulate the interneurons “B” which stimulate “C” which in turn inhibit “A” • In the regulation of CO2 concentration, a high concentration of CO2 in the extracellular fluid will increase pulmonary ventilation through chemoreceptors which feedback to A to increase ventilation • This in turn will decrease the extracellular fluid CO2, due to the lungs expiring greater concentrations of CO2, ∴ maintaining homeostasis • With an increase in respiration there are stretch receptors, which detect ↑V and are able to feedback T information to B to stimulate C in order to inhibit A • Whether it be an ↑CO2(breath hold) or ↑O2 (↑tidal volume) there are chemoreceptors(mechanisms) which send a response to the respiration loop to change actions in order to maintain homeostasis within the body Negative Feedback: Hypothalamus - GnRH Anterior Pituitary LH/FSH Gonads Testosterone/estrogen • In hormone control, negative feedback is commonly used, because without it homeostatic conditions would be lost immediately, because there needs to be some mechanism which is capable of controlling the amount produced • Whenever the secretion of testosterone/estrogen becomes to great, it can inhibit the secretion of GnRH from the hypothalamus, therefore reducing testosterone/estrogen levels back to normal. • This relates to negative feedback because an increase in either testosterone or estrogen will alter homeostatic conditions therefore to escape that, those hormones can themselves downregulate their production. Positive Feedback • In child birth +ve feedback plays a valuable role • When the baby “drops” or is lowered into the uterus there is cervical stretch • On the cervix are stretch receptors which causes the hypothalamus to secrete oxytocin, which is another means for ↑contractility of the uterine o On the uterine wall there is an ↑for oxytocin • This ↑contractility pushes the baby even more against the cervix and the cycle begins again. • This is a good example of positive feedback because as soon as the baby drops in the uterus the loop begins producing stronger and stronger contractions until the baby is born, and homeostatic conditions can be reestablished. Negative Feedback Acts like a physiological off-switch that will either shut down or down regulate the system or process as soon as there may be an alteration in homeostatic state. Hypothalamus - GnRH Anterior Pituitary LH/FSH Gonads Testosterone/estrogen • In hormone control, negative feedback is commonly used, because without it homeostatic conditions would be lost immediately, because there needs to be some mechanism which is capable of controlling the amount produced • Whenever the secretion of testosterone/estrogen becomes to great, it can inhibit the secretion of GnRH from the hypothalamus, therefore reducing testosterone/estrogen levels back to normal. • This relates to negative feedback because an increase in either testosterone or estrogen will alter homeostatic conditions therefore to escape that, those hormones can themselves down regulate their production. 2.Control of GFR by Kidney • If the flow through the renal tubule is high due to a high GFR, ChemoRc in the macula densa sense the high [NaCl] that has resulted from decreased equilibration time • The macula densa is situated adjacent to both renal afferent and efferent arterioles • The macula densa response to ↑NaCl by: o A)↓renin ∴ ↓AII which will vasodilate EFF>AFF ∴↓P .↓GFR G o B)paracrine effects to ↑Aff resistance ∴↓P ∴G↓GFR • This process is called Tubuloglomular feedback and has the function to down regulate GFR to maintain favorable equilibration time to keep the system in homeostasis 3.Control of Gastric Hcl secretion in intestinal phase • This digestion pahse is triggered by the presence of food in the upper small intestine • ChemoRc in the duodenum sense ↓pH and activate S-cells to release secretin. • Secretin inhibits G-cells and parietal cells to ultimately ↓Hcl secretion into the stomach • In the steps prior to this gastric secretion of Hcl was important for food breakdown. Once there is sufficient amount of Hcl and it is no longer needed it can then utilize the above mechanism to down regulate the process to maintain a homeostatic environment Positive feedback Positive feedback is a physiological theme, which is used in order to enhance or intensify the initial stress, result or response. Through processes of positive feedback directly or indirectly homeostatic conditions may be reestablished. AP Generation: • An important use of +ve feedback is for the generation of nerve signals • When the membrane of a nerve fiber is stimulated, leakage of Na+ is initiated through Na+ channels into the nerve fiber • The Na+ entering the fiber then changes the membrane potential positively which causes depolarization causing an AP to occur, which then enhances the opening of even more Na+ channels, this depolarization creates a vicious cycle till all Na+ channels have been opened • Due to the explosion of Na+ leakage into the inside of the nerve fiber, this is the creation of a nerve AP • This AP in turn can then excite the next nerve fiber to enhance the length of the signal. Child Birth: • In child birth +ve feedback plays a valuable role • When the baby “drops” or is lowered into the uterus there is cervical stretch • On the cervix are stretch receptors which causes the hypothalamus to secrete oxytocin, which is another means for ↑contractility of the uterine o On the uterine wall there is an ↑for oxytocin • This ↑contractility pushes the baby even more against the cervix and the cycle begins again. • This is a good example of positive feedback because as soon as the baby drops in the uterus the loop begins producing stronger and stronger contractions until the baby is born, and homeostatic conditions are reachieved. Control of Acid Secretion: • In the stomach without positive feedback homeostatic conditions would never be maintained • Peptide amino acids stimulate G cells which eventually stimulate parietal cells to secrete HCl in order to ↓pH • This ↓pH through sensory neurons can feedback to the enteric plexus neurons in order to stimulate the production of chief cells in order to produce pepsin which is the enzyme used for the breakdown of proteins into peptides and amino acids, which will then enhance this system in order to ↓pH to a desired level, in order to maintain homeostatic conditions • ∴peptide amino acids can intensify the production of G cells which will eventually produce more peptide amino acids Local vs. Central Control The brain and spinal cord(CNS) monitor the interaction of our many bodily systems centrally ensuring our overall survival. However they don’t have the capacity to maintain/monitor everything at all times due to too much traffic. Therefore we have local tissue control systems present in a variety of systems which work both in cooperation and competition with the CNS, in order to help regulate the body and help with maintaining proper bodily functions Reproductive System • Example 1 – Local and Central Control working in competition A)Local Control • Thecal Cells and Granulosa cells release estrogen from the ovary to the follicular cells in females • The increase in local estrogen stimulates Thecal and Granulosa cells to release more estrogen • This positive feedback loop results in high concentrations of local estrogen • The increase in estrogen in the follicular cells ↑the local Rm population for LH and FSH on the thecal cells and granulose cells ∴ allowing for an ↑secretion of estrogen and therefore a high concentration of local estrogen B) Central Control • Centrally, estrogen negatively feeds back to the anterior pituitary and hypothalamus to decrease secretions of LH and FSH • LH and FSH stimulate Granulosa cells and Thecal cells to release Estrogen • This central control is attempting to regulate estrogen at an average value, and therefore the decrease in LH and FSH is an attempt to decrease the amount of estrogen released by the Thecal and Granulosa cells. • Conclusion, local and central control of estrogen are competing systems. Centrally, the bodies attempts to maintain an average concentration of estrogen by decreasing LH and FSH production, thereby reducing the stimulus for Thecal Cells and Granulosa Cells to release estrogen. Locally however, the presence of estrogen positively feeds back to Granulosa and Thecal cells to release more estrogen, thereby increasing local concentrations of estrogen. Example 2 – Local and Central Control working in cooperation to establish internal environment - GI Tract A)Local Control • The GI system contains its own nervous system called the Enteric NS ∴ locally can perform everyday events involved with the gastrointestinal system such as controlling GI movements and secretions • The enteric nervous system contains mechanoRc which detect a stretch in the stomach due to food which locally will send a short reflex to the enteric NS to release parietal cells to secrete HCL to lower pH • Mechano receptors also stimulate the production of G cells which produce gastrin in order to increase intestinal motility of smooth muscle, gastrin also ultimately produces parietal cells to secrete Hcl • Receptor cells(chemo,mechano,osmo) at lumen trigger short reflexes that end up in either myentric or submucosla plexuses. These represent the primary gut control in most secretions and segmental and propulsive smooth muscle contraction B)Central Control • The same mechanoRc mentioned locally also create a central reflex to the CNS which will also stimulate the production of G cells and the stimulation of parietal cells to have the exact same outcome of the local control due to the enteric nervous system • The big difference between local and central control is that central control has a longer reflex therefore information traveling towards the CNS will take longer, that is why the GI tract contains its own local nervous system so actions such as secretion of HCl in order to ↓pH can occur at a much quicker time scale • The same sensors mentioned above send ascending information called a long reflex to pre- vertebral ganglia, spinal cord + brain. If necessary these area may display extrinsic neural control via: a)SNS-mostly inhibitory b)PNS-mostly stimulatory • In this case local control works mostly independently of central control, in most cases the enteric NS is enough to autoregulate but central control may intervene if necessary Example 3 - Kidney Regulation of Blood Volume where Local control is working against central control in order to maintain a relatively stable osmolarity while regulating blood volume A)Central Control: • Hormonal regulation of ADH-VP,ALDO,ANP • All induced by stimulation of centralized sensors • They may also display control over each other
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