Human Physio Notes Week 3 - IA Adrian Bahn BIPN 100 Dr...

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IA: Adrian Bahn BIPN 100 – Dr. Fortes 01/20/16 Last week we covered the classic experiments by Hodgkin and Huxley and the determination of the voltage-gated sodium and potassium channels’ roles during the propagation of an action potential. The functions of these channels are voltage- dependent and time-dependent. Through keeping the membrane potential at a certain voltage, the current through the squid axon was measured to see what effect different membrane potentials had on ion flow. In addition, through sodium and potassium-gate blocking chemicals ( tetradotoxin “TTX” and tetra ethyl ammonium “TEA”, respectively), it was further confirmed that the inward current was due to sodium entering the cell, and the outward current was due to potassium leaving the cell. We’ve discussed before the general mechanism of an action potential: when the membrane potential depolarizes above the threshold potential, there is a positive feedback loop where the voltage-gated sodium channels open and sodium rushes in, rapidly depolarizing the membrane. Then the inactivation gates close the sodium channel and the slow potassium channels open, repolarizing and actually hyperpolarizing the membrane slightly in a negative feedback loop, until the potassium inactivation gates close the channels. Eventually, the sodium channels reset and another action potential can be transmitted. Now, a positive feedback loop can get out of control and there must be an independent mechanism (something other than voltage or current) to stop it. In this case, that mechanism is the inactivation gate physically blocking the channel to stop the flow of ions . This is something known as the ball-and-chain model of the inactivation gate. After a period of time from the initial depolarization, a protein that looks like a ball-and-chain swings shut and plugs the ion channels. Another dangerous thing regarding action potentials is that as the ions flow into the axon, they do not travel unidirectionally down the axon. The positive charge spreads throughout the axon, which can potentially depolarize the membrane that is further upstream (already conducted an action potential). In other words, the influx of sodium could create an infinitely-looping action potential, if it were not for one characteristic of action potentials. The refractory period is the period during and after an action potential when the membrane cannot be stimulated to produce another action potential . There are two components: the absolute refractory period and the relative refractory. The absolute refractory period is the period when the action potential is rising above threshold and repolarizing. During this time the sodium channels are open (during the rising action potential), inactivated (around the peak of the action potential), or closed (after the peak of the action potential).
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