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Phys Lect 6
| Question | Answer |
|---|---|
| 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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