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Phys Lect 6

What 4 things contribute to the AP threshold? 1.Voltage-gated Na+ channels (must be opened to allow Na+ influx). 2.High density of voltage gated Na+ channels (allow for massive influx and rapid depolarization). 3.Overcoming the resting K+ efflux.. 4.Positive feedback of Na+ channels to open neighb
What voltage is the Threshold? -50mV
What voltage opens the Na+ Channels? -50mv. Therefore at the threshold the membrane becomes more permeable to Na+ and depolarization occurs with the Na+ influx
What type of feed back system is used by Na+ channels? Positive Feedback. **Open channels activate other neighboring channels.
How is the resting K+ efflux over come? There needs to be enough open Na+ channels to cancel out the K+ efflux and initiate an AP.
What causes HYPERcalcemia? Hyperparathyroidism (inc PTH in the BL)
HYPERcalcemia's effect on excitability Decreased excitability (muscle weakness): The threshold is raised causing a greater membrane potential needed to generate an AP.
What causes HYPOcalcemia? Hypoparathyroidism (dec PTH in the BL). **Also Chronic renal failure
HYPOcalcemia's effect on excitability Increased excitability (Muscle twitching): The threshold is lowered causing more easily generated APs (lower Em required).
Why does Em not reach Ena during the AP depolarization? Because the current of Na+ ions decreased due to decreased driving force.
Why does the driving force of Na+ decrease just before the peak of the AP? Because close to the peak of the AP, the difference between Ena and Em is much less than at the initiation of the AP. This cuases the Na+ ion current to decrease.
Why is the Na+ current so breif during an AP? at -50mV two gates are open on both the outside and inside of the Na+ channel. The inner, INACTIVATION Gate swing shut at a much slower rate than the outer ACTIVATION gate. therefore Na+ can rush in but only briefly. **Causes the refractory period
At resting membrane potential, which Na+ channel gate is open? closed? Open: Inactivation. Closed: Activation
During depolarizatoin, which Na+ channel gate is open? closed? Open: Both. Closed: Neither
At the Peak of the AP, which Na+ channel gate is open? closed? Open: Activation. Closed: Inactivation.
During repolarization, which Na+ channel gate is open? closed? Open: Neither. Closed: Both.
Is the Inactivation Na+ channel gate voltage-dependent? YES, it just closes more slowly. The inactivation gate swings shut with depolarization and opens with repolarization
What does it mean to say that the Na+ channel is INACTIVATED? The Inactivation gate is CLOSED, the Activation gate can be opened or closed (it doesn't matter)
How do Inactivation gates affect the relative and absolute refractory periods? Absolute: Na+ channels are INACTIVATED (meaning the the inactivation gate is closed and there is no way an AP can be generated). Relative: Some of the inactivation gates have began to re-open so the Na+ channel can now be activated with a larger stimul
A drug prolongs the recovery of Na+ channels from inactivation. What effect would you expect this to have on the absolute refractory period? PROLONGED Absolute refractory period. **b/c those Na+ channels can not be recruited since the Inactivation gate hasn't re-opened.
How does Hyperkalemia affect Na+ Channel inactivation? why? it will reduce excitability by cuasing the inactivation gate to close before the activation gate can open. This is because less K+ will leave the cell and the Em will become more positive.
What are the two types of K+ channels 1.Inward Rectifier channels. 2.Voltage-gated channels.
What are Inward Rectifier K+ channels responsible for? Affecting the resting membrane potential (open at negative potentials). They have NOO impact on the repolarization of the membrane.
What are Voltage-gated K+ channels responsible for? Allows repolarization of the Em. Opens at depolarized potentials during the AP.
Why does the K+ current occur after the Na+ current? becuase the voltage gated K+ channels do not open until the Em is depolarized which is caused by the Na+ current. **They also open with a delay and much slower than Na+ channels.
Are the Inward Rectifier channels open or closed at the Peak of the AP? CLOSED
What 2 things determine the DURATION of the AP? 1.Magnitude of K+ currents. 2.Increased Ca2+ influx through Ca2+ voltage-gated channels.
How does a Smaller K+ Current impact the AP? A larger K+ current? Smaller K+ current = longer AP duration (Because it is taking the Em longer to reach it's resting Em of -70mV). Larger K+ current = shorter AP duration (becuase the Em reaches -70mV much quicker).
What are two ways to Prolong the duration of an AP? 1.Increase depolarizing agents (Na+ and Ca2+ currents). 2.Decrease repolarizing agents (K+ currents)
Created by: WeeG



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