APK 2105C Lecture Notes - Lecture 25: Threshold Voltage, Axon Hillock, Resting Potential
5 Jun 2018
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Chapter 7, Lecture 4
Neurons
• 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 decremental—can be propagated longer distances without a decrease in
strength
▪ No change in amplitude
o Graph at right
▪ Phase 1 = depolarization = brings Na in very
quickly
• 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
depolarization
▪ Opening of K channels
• Increasing permeability to K
• During repolarization
o Actually causes repolarization
o Removes some of the positive
charge
• Also causes after hyperpolarization
o There is a little too much positive charge released from the
cell
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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