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What is an action pontential?
- a sudden temporary depolarizing deviation from the normal resting potential.
- Allows cells to transmit informtaion through nervous system.
Typical action potentials characteristics
- Each successive one looks the sameHave a threshold--once cell is depolarized to this level, AP is generatedTypical threshold is ~ -55mV
- All or nothing
- In crease in stimulus will not change amp of AP
- have refractory period, absolute refractory, relative refractory, self-generating
- Propagate with decrement
Action potential in relation to membrane potential
Em = TKEK + TNaENa
- -transient change in emebrane potential
- -Equilibrum potential stays the same
- -Tranferences change--so conductances have changed
Membrane potential for EK
Membrane potential for ENa
What happens in the AP:
- sudden and temporary increase in the Na+ conductance
- Increase Na+ conducdance= increase Na+ Tranference (decreases K+ tranference)
- Em moves closer to ENa+ so the cell depolarizes (less negative)
- Em becomes positive and aproaches ENa+ at overshoot
- At peak of AP, the sodium conductance(gNa) is at its peak
- Sodium conductance spontaneously decreases at inactivation of the sodium channels
- Decrease in sodium transference and membrane potential begins to return towards resting value
- Increase in K+ conductance--increases K+ tranference and brings membrance potential closer to resting value
- So many K+ channels are open, K+ Transference is large enough to make the membrane potential even more negative than normal resting potential: hyperpolarization (undershoot!)
Relationship between AP and ion gates
Why does the Refractory period need more voltage to produce an AP?
- a. there are not as many Na+ channels
- available as normal to trigger another AP because many are inactivated.
- b. Also, many more K+ channels are open,
- so it is more difficult to depolarize the membrane to threshold.