PS%202_solutions

PS%202_solutions - BIOE 322/BIOS 332 Fundamentals of...

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BIOE 322/BIOS 332 Fundamentals of Systems Physiology Spring 2008 Problem Set #2 Solutions 1. 2. A prolonged stronger-than-threshold stimulus produces more action potentials than a prolonged threshold stimulus of the same duration. A very strong stimulus can stimulate action potentials during the relative refractory period, whereas a prolonged threshold stimulus stimulates a low frequency of action potentials. Thus, when a prolonged stronger-than-threshold stimulus is applied, less time elapses between the production of one action potential and the next, resulting in the production of a greater number of action potentials. 3. Lithium ions reducing the permeability of the plasma membrane to Na+ suggests that the Na+ channels in the plasma membrane are tending to remain closed in the presence of lithium. A normal stimulus causes Na+ channels to open, allowing Na+ to diffuse into the cell, thereby resulting in depolarization. The neuron in this case is less sensitive to stimuli because the membrane is less permeable to Na+. 4. Smooth muscle cells contract spontaneously in response to spontaneous depolarizations that produce action potentials. One way action potentials can be produced spontaneously is if membrane permeability to Na+ spontaneously increases. As a result, a few Na+ enter the smooth muscle cells and cause a small depolarizing graded potential. The small depolarization can cause voltage-gated Na+ channels to open, which results in further depolarization, thereby stimulating additional Na+ voltage-gated ion channels to open. This positive-feedback cycle can continue until the plasma membrane is depolarized to its threshold level and an action potential is produced. 5. Organophosphates inhibit acetylcholinesterase, thereby causing an increase in acetylcholine (ACh) in the synaptic cleft, leading to overproduction of action potentials and tetanus of muscles. Curare is the best antidote because it blocks the effect of ACh (by preventing ACh from binding nicotinic receptors – Table + ++ +++
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7.5) and counteracts the organophosphate. Injecting ACh would make the effect of the poison worse. Potassium chloride will make the membrane potential less negative, pushing muscle cell membranes towards depolarization, thereby making them more sensitive to ACh. This will also make the effect of the poison worse.
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