Resting Membrane Potential Overview

Resting Membrane Potential Overview diagram

Previouse Notes:

 https://mednotesbyrazaullah.blogspot.com/2026/08/chapter-05-membrane-potentials-and.html

Resting Membrane Potential 

the Resting Membrane Potential (RMP) is the constant electrical voltage difference across a nerve or muscle cell membrane when it is completely at rest, typically sitting at -90{ mV} in large nerve fibers. This internal negativity is mainly generated by potassium ions ({K}^+) diffusing out of the cell through leak channels down their steep concentration gradient, leaving behind un-diffusible negative organic anions like proteins and phosphates. A tiny amount of sodium ({Na}^+) leaks inward, slightly reducing this negativity, but the membrane remains far more permeable to potassium.
{K}^+ ATPase pump actively transfers three {Na}^+ ions out for every two {K}^+ ions brought inside. This continuous net movement of positive charges out of the cell adds an extra -4{0 mV}, cementing the stable -90{ mV} baseline required for generating nerve impulses.

Basic Resting Value:

  • Large nerve resting membrane potential is -90mV.
  • Fiber interior is 90mV more negative than exterior.
  • Sodium-Potassium:
  • Transports sodium outside and potassium inside continuously.
  • An electrogenic pump moves 3 Na^+ out per 2 K^+ in.
  • Leaves a net positive charge deficit inside the cell.
  • Creates negative electrical potential inside the membrane.

Ion Concentrations and Ratios:

  • Na^+ outside: 142 mEq/L.
  • Na^+ inside: 14 mEq/L.
  • Na^+ inside/outside ratio: 0.1.
  • K^+ outside: 4 mEq/L.
  • K^+ inside: 140 mEq/L.
  • K^+ inside/outside ratio: 35.0.

Potassium Leak Channels:

  • Tandem pore domain channels allow continuous potassium leak.
  • Channels permit potassium leakage in resting cells.
  • Channels leak sodium ions only slightly.
  • The membrane is 100 times more permeable to potassium.
  • Origin of Normal Resting Membrane Potential

Contribution of Potassium Diffusion Potential:

  • High internal to external potassium ratio is 35:1.
  • Potassium Nernst potential calculates to -94mV.
  • Calculated as -61mV \times \log(35).
  • Potassium diffusion alone would create -94mV potential.

Contribution of Sodium Diffusion:

  • Minute sodium diffusion occurs through leak channels.
  • Sodium inside/outside ratio of 0.1 equals +61mV.
  • Potassium permeability is 100 times greater than sodium.
  • Goldman equation yields -86mV internal potential.
  • High potassium permeability dominates net diffusion potential.
  • Contribution of Na^+K^+ Pump:
  • Continuous pumping moves  Na^+ out, 2K^+ in.
  • Continuous loss of positive charges adds internal negativity.
  • An electrogenic pump contributes about -4mV additional negativity.

Final Summary of Resting Potential:

  • Combined diffusion potentials equal -86mV.
  • Potassium diffusion determines most of the diffusion potential.
  • An electrogenic pump adds -4mV additional potential.
  • Net resting membrane potential totals -90mV.

Nerve Action Potential Overview

Basic Concept:

  • Action potentials transmit nerve signals along membranes.
  • Rapid membrane potential changes spread along nerve fiber.
  • Begins with sudden change to positive potential.
  • Ends with rapid return to negative potential. 
  • Conducts signals by moving along the entire nerve fiber.

Stages of Action Potential

Resting Stage:

  • Precedes the onset of action potential.
  • Resting membrane potential remains at -90mV
  • The membrane is polarized during this stage.

Depolarization Stage:

  • The membrane suddenly becomes permeable to sodium ions.
  • Massive sodium influx diffuses into the interior axon.
  • -90mV polarized state becomes neutralized.
  • Membrane potential rises rapidly toward a positive direction.
  • Large fibers overshoot beyond zero to positive state.
  • Small fibers and central neurons approach zero level.

Repolarization Stage:

  • Sodium channels close within ten-thousandths of a second.
  • Potassium channels open wider than normal.
  • Rapid potassium efflux re-establishes negative resting potential.
  • Restores normal negative membrane potential level.

Voltage-Gated Channels Role

Channel Types Involved:

  • Voltage-gated sodium channels cause depolarization/repolarization.
  • Voltage-gated potassium channels speed up the repolarization process.
  • Operate alongside Na^+-K^+ pump and leak channels.

Voltage-Gated Sodium Channel States

Gate Structure:

  • The channel contains activation and inactivation gates.
  • Activation gate located near channel exterior.
  • Inactivation gate located near channel interior.
  • Resting State:
  • Present at normal -90mV resting potential.
  • The activation gate remains closed during the resting state.
  • Closed activation gate blocks sodium ion entry.

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