Origin of Resting Membrane Potential
Under resting conditions inside of the cell is electronegative.
Example: Resting membrane potential in a neuron is -90 mV
Factors Responsible For Generation of Resting Membrane Potential
Diffusion of Potassium
Cells contain potassium leak channels. These channels are open even during resting conditions.
Potassium concentration is high inside the cell as compared to outside
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Potassium diffuses out of the cell through potassium leak channels
↓
Exit of positively charged potassium makes inside of the cell electronegative
This is the main factor behind generation of resting membrane potential.
Electrogenic Nature of Na-K ATPase pump
Na-K ATPase pump moves 3 sodium ions out, and 2 potassium ions in during each cycle
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Exit of net 1 positively charged ion
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Makes the inside of the cell electronegative
Provides a small contribution to generation of action potential.
Although the direct contribution is small, the activity of Na-K ATPase pump is very important for establishing resting action potential. This is because the diffusion of Potassium (the main factor as discussed above) is made possible by concentration gradient of potassium, and this gradient in tern is developed by Na-K ATPase pump only.
Notes for Nerds
Potassium
K concentration inside the cell is about 140 mEq/L and outside is 4 mEq/L
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Concentration gradient from inside to outside
↓
K diffuses out
↓
Makes inside of the cell electronegative
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Electronegativity inside the cell attracts positively charged K back into the cell
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K stops moving out when electrical gradient balances concentration gradient
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At this equilibrium, voltage tends to be -94 mV inside the cell
Sodium
Na concentration is high outside and low inside
↓
Concentration gradient from outside to inside
↓
Na enters the cell
↓
shifts voltage to positivity.
However, during resting conditions, Na channels are mostly closed
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Amount of Na entering is small
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Contribution in voltage is small i.e. + 8 mV
Na-K ATPase Pump
Causes net removal of one positive ion from cell as explained above
↓
Makes the cell electronegative
↓
Contribution is -4 mV
Total:
= (Contribution from K) + (Contribution from Na) + (Contribution from NaK ATPase pump)
= (-94) + (+8) + (-4)
= -90 mV