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BME201_2010_HWK1b_soln - NAME 125:201 INTRODUCTION TO...

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NAME:_______________________________ 1 125:201 INTRODUCTION TO BIOMEDICAL ENGINEERING, Fall 2010 Bioelectrical Phenomena Instructor: Nada Boustany HOMEWORK #1- Due Thursday Nov. 18 SOLUTIONS Problem 1 - Indicate if the following statements are True or False. 1- An action potential is generated by changing the bulk concentrations of sodium inside and outside the cell. False 2- A cell membrane at rest with the following ionic concentrations has a resting potential ∆Φ rest = -85 mV. False inside outside Na + 150 mM 150 mM K + 5 mM 5 mM Cl - 155 mM 155 mM 3- The sino-atrial node is composed of cells that can rhythmically self-generate their own action potential. True 4- An action potential is primarily due to the movement of K + (potassium) through the K + leak channels. False Problem 2: What do you expect will happen to the membrane potential of neurons during a stroke? Please explain your answer. The concentration of Na+ will go up inside the cell, and the concentration of K+ will decrease inside the cell such that the concentration gradient Na+(in)/Na+(out) will go up, while K+(in)/K+(out) will go down. Without
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NAME:_______________________________ 2 the presence of fixed charges inside the cell, the concentrations of Na+ and K+ will eventually be equal inside and outside of the cell. However, if you take into account the fact the there is always a concentration of fixed negative charges inside the cell due to charged large macro- molecules trapped inside the cell (e.g. proteins), then the concentrations of Na+ and K+ will eventually try to reach a Donnan equilibrium (we will talk about this in see Lecture V). As we will see, this situation is ultimately unsustainable as the osmotic pressure across the cell will cause the cell to swell and burst. Since the concentration gradients will be decreased across the cell membrane, the potential will get closer to zero. Assuming that the cell starts with a membrane potential close to –70mV, this implies that the membrane potential increases and becomes less negative. Since the potential increases compared to the resting potential, the affected neurons become depolarized . If the membrane at the axonal end is depolarized to levels similar to an action potential depolarization, then the voltage gated calcium channels will open and induce vesicle fusion with the membrane and release of neurotransmitters in the synapse. Typically an action potential results in raising the cell membrane potential to between 0 and +40mV. Thus if the initial change in membrane potential becomes greater than the threshold level needed to initiate an action potential, this action potential can propagate to the axonal end, and induce the release of synaptic vesicles. This caused further uncontrolled depolarization of neurons in the vicinity of the stroke region, which can cause profound neuronal injury and cell death.
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BME201_2010_HWK1b_soln - NAME 125:201 INTRODUCTION TO...

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