APK 2105C Lecture Notes - Lecture 25: Threshold Voltage, Axon Hillock, Resting Potential

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Chapter 7, Lecture 4
Action potentials (AP)
o Occur in the membranes of excitable cells (neurons and muscle cells) in
response to graded potentials that reach threshold
o Rapid depolarization of the cell which reverses the Vm
More positive inside vs. outside
Massive influx of Na ions
Depolarization is enough of a stimulus to open the voltage gated Na
channels on the axon hillock
o Not decrementalcan be propagated longer distances without a decrease in
No change in amplitude
o Graph at right
Phase 1 = depolarization = brings Na in very
Gets close to -60 but not all the way to -60
because still permeable to K
Phase 2 = repolarization
Not letting any more Na in
Phase 3 = hyperpolarization
Gets back up to resting potential
o Reaching threshold voltage signals
Opening of Na channels
First part of AP
Lots of Na rushes in
Closing of Na channels
Decreasing permeability to Na
During repolarization
o Has to be together with
opening K channels because it
doesn’t actually repolarize, it
just prevents further
Opening of K channels
Increasing permeability to K
During repolarization
o Actually causes repolarization
o Removes some of the positive
Also causes after hyperpolarization
o There is a little too much positive charge released from the
o Keeps the cell from generating an action potential right
after the 1st one
Voltage gated Na channels
o Different protein conformation
Double gated
Activation gate
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