take-home - BME 417 Take-Home Exam 1 1. Features: i. At...

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1 BME 417 Take-Home Exam 1 1. Features: i. At -5mv and below (in magnitude), current doesn’t go above 0. ii. At -115mv and above (in magnitude), current doesn’t go above 0. iii. For range -10mv to -100 mv, current first shoot above zero and soon fall back to below zero. iv. The negative current at the end of all plots is higher (in magnitude) when the voltage clamp is higher (in magnitude). v. The shoot above zero is reaches a maximum at around -60mv. Explanations: i. At -5mv, there’s no significant change in m – the proportion of sodium channels that are open. The depolarization is below the threshold of the sodium channels, and therefore there’s only a very small current through the sodium channels. The current is so small that it is unable to offset even the leakage current of the potassium channels. The slight increase in n and the decrease in h only add to the domination of the potassium current. The sodium current is positive (depolarization), and the potassium current is negative (repolarization/hyperpolarization). At -5m the potassium leakage current is stronger than the tiny depolarization current from the sodium channels, as a result the current never go above zero. It implies that no depolarization occurs on the membrane and no action potential is expected at -5mv. ii. At -115mv, the despite the sharp rise in m, there’s no sodium current because the equilibrium potential of sodium ion happens to be -115mv. At this potential sodium ions will not move and there will be no sodium current. Naturally, potassium current will dominate and no depolarizing current can take place. The same for voltage below -115mv, sodium current would have reversed direction and add on to the repolarizing current. No polarizing current will be observed. iii. For range -10mv and -100mv. The clamp voltage is above the threshold of the sodium channels, therefore the increase in m. The clamp voltage is also below the equilibrium potential of sodium ions, so the ions move in the depolarizing direction. The sudden opening of sodium channels pushes the sodium conductance above that of the potassium, producing the shoot of current above zero. However, the deactivation gates of the sodium channels soon close, and the decreasing h catches up to bring down the sodium conductance. The rest of the plot is then dominated by the potassium current which is a repolarizing current in the negative direction.
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2 iv. As mentioned in the above section, the tail end of each plot is dominated by potassium current. n – the proportion of potassium channel that is open is higher when the clamp voltage is lower. (When the depolarization is higher.) Therefore, the potassium current increases with lower clamp voltage. v. As mentioned in the above section, the shoot above zero is due to sodium current.
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This note was uploaded on 04/30/2008 for the course BIOMEDE 417 taught by Professor Cain during the Winter '07 term at University of Michigan.

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take-home - BME 417 Take-Home Exam 1 1. Features: i. At...

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