Biological Control Systems and Feedback_V3

Biological Control Systems and Feedback_V3 - BIOLOGICAL...

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Page 1 BIOLOGICAL CONTROL SYSTEMS AND FEEDBACK V3 Based on the work of Dr. David Robinson and modified/adapted for BME 251 by Dr. Cynthia B. Paschal WHY CONTROL SYSTEMS ANALYSIS IN BIOMEDICAL ENGINEERING? 1) The human body is literally crammed with control systems. There are systems to control balance, movement, blood pressure, pH, blood O 2 and CO 2 levels, temperature, hormonal levels, enzyme levels and on and on… Indeed it is difficult to find any part of the body that is not involved in some pathway to control some biological variable. 2) Much of biomedical engineering is focussed on trying to repair or replace broken control systems. 3) At a more immediate and practical level, it is not difficult to come across behavior in the clinic which seems enigmatic at the least unless one knows something about feedback and which can obscure or even misdirect a diagnosis. An example from oculomotor neurology is given here; many other examples could be used, drawn from other branches of medicine, since, as already mentioned, almost every bodily function one looks at is under the control of some feedback system. Example: A patient is examined who is suspected of having myasthenia gravis. A main feature of this disease is a deficiency of acetylcholine (Ach) receptors in the neuromuscular junction. The result is muscle weakness. It occasionally acts on specific muscle groups and this patient has limited eye movements. The normal range is + 50 ° . This patient, exerting maximum effort, cannot look more than 15 ° left or right. The patient has had this problem for many years. An anticholinesterase such as Tensilon is usually a definitive test. It inhibits the removal of Ach from the endplate and restores almost normal muscular strength. When Tensilon is administered (i.v.), this patient begins to make extraordinary eye movements, looking 50 ° to the far left, then right, alternating back and forth about once a second. These movements are evidently uncontrollable. The patient is unable to stop them and look at anything. Tensilon wears off shortly and these movements more or less persist until it does. What caused these remarkable eye movements? Has Tensilon unmasked some brain stem lesion that causes uncontrolled eye movements? (This does occur and is called opsoclonus.) Is this some side affect of the drug? How does one interpret this sign? Anyone familiar with feedback systems would be able to deduce fairly quickly why the above eye movements occurred. The patient does not have additional neurological complications; the eye movements should be totally expected. Indeed, one would worry if these oculomotor responses did not occur. However, someone unfamiliar with feedback might have difficulty in explaining why these eye movements occur. The reader might try before reading on.
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Page 2 THE BLOCK DIAGRAM OR MODEL Since about 1930 when feedback was first formally recognized as a powerful asset in control systems (Mother Nature recognized it millions of years ago), the engineering sciences have developed fairly standard
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Biological Control Systems and Feedback_V3 - BIOLOGICAL...

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