01:146:295 Lecture Notes - Lecture 5: Voltage-Gated Potassium Channel, Resting Potential, Axon Hillock

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5: The Action Potential
Chapter 4, Bear
I. Action Potential
A. Resting Potential: Low Na+
permeability, High K+ permeability
B. Rising Phase and Overshoot: Na+
channels open→ very positive, close
to MP for Na
C. Falling Phase: No Na+ channels are
open and leak and voltage-gated K+
channels are open
D. Undershoot: voltage goes below
resting potential; takes time before
Na+ reactivate
II. Flipping the membrane potential by changing the relative ionic permeability of the
membrane (g = conductance, number of open channels; permeability)
A. Rising Phase
1. Na+ channels open; membrane more permeable to Na+ than K+
2. Na+ coming in overpowers any K+ leaving
3. gNa >> gK
B. Falling Phase:
1. Na+ channels deactivate at the same time
2. Leak K+ channels are open; Voltage-gated K+ channels are also opening
3. Diffusion forces push K+ out of the cell → voltage becomes negative
4. gK >> gNa
C. Rest:
1. More K+ channels are open, Na+ channels are closed
2. gK >> gNa
III. Long-distance communications along axons
A. AP is constantly being generated/same magnitude no matter where it is on axon
B. Voltage across membrane in axon hillock needs to reach threshold
1. EPSP in dendrites → travel through cell body → sometimes achieve
threshold
2. Threshold = -40 mV
C. “All-or-none” Action Potential
1. Action potentials are generated when axonal membrane is depolarized
beyond threshold
2. Injected current must reach threshold to achieve an action potential; high
intensity stimulushigh frequency action potential
D. Voltage-Gated Sodium Channels
1. A Long polypeptide chain that makes loops, that can be categorized into
4 domains (I, II, III, IV), in and out of the membrane
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