NMS Physiology Lecture 2 - Resting Membrane Potential

Components of the neuron plasma membrane

Phospholipid bilayer containing protein pumps and channels

Role of the membrane pumps and channels in the plasma membrane

Set the resting membrane potential, allows and controls the movement of ions into and out of the cell

Protein pumps

Work via active transport use an energy source to pump ions across the membrane

Ion channels

Do not use an energy source and are not active transport. They allow ions to flow down an electro chemical gradient.

Passive channels in the neuron plasma membrane

Leak channels - always open and selective for a given ion

Gated channels and the neuron plasma membrane

Require a stimulus to open the channel and allow ions to flow through examples include ligand gated (synaptic transmission) and voltage gated (nerve impulse) channels.

Resting Membrane Potential (RMP)

Heme of the electrical potential difference between the intracellular and extracellular environment. The neuron RMP is around 70 mV.

What creates the resting membrane potential?

Charge separation between the inside and outside of the cell

Most important ions involved in setting the RMP

Sodium and potassium ions. Chloride and organic and ions play a minor role.

Pumps/channels involved in setting the RMP

The sodium potassium pump, sodium leak channels, and potassium leak channels

How Na/K pump works

1. 3 Intracellular sodium binds to its binding site and ATP binds to its intracellular binding site.
2. ATP hydrolysis changes the confirmation shape of the pump opening up the sodium binding sites to the extracellular environment and releasing 3 sodium ions.
3. Two potassium ions, attach to binding sites on the extra cellular side of the pump.
4. ADP leaves the binding site, changing the confirmation shape of the pump, allowing potassium ions to move inside of the cell.

What gradients exist within a cell

Chemical and Electrical gradients

What creates the chemical gradient of a cell

Na/K pump
- Extracellular Na+> Intracellular Na+
- Extracellular K+ < Intracellular K+

What causes the electrical gradient of a cell

The electrogenic nature of the Na/K pump
- More cations leave the cell than enter the cell (3 Na+ out, 2 K+ in), leaving the cell more negative than the outside

What creates the RMP

Unequal distribution of ions due to the Na/K pump

How is equilibrium attemped to be reached in a cell

Leak channels

Leak channels

Permenently open, selective channels that allow for the passive flow of ions down electrical/chemical gradients

Which way does chemical force push K+ ions

Out of the cell via leak channels
- Na/K pump moves 2K+ in every cycle = intracellular buildup

Which way does electrical force push K+ ions

Into the cell via leak channels
- Inside of cell is negatively charged, more cations would make the cell neutral

Equilibrium potential for K+

-90mV

What happens at the equilibrium potential

The voltage when electrical and chemical forces equal and opposite, negating each other. Calculated via the Nernst Equation

At cell resting potential (-70mV), what does K+ want to do

Move out of the cell, moving the potential to -90mV

Which way does chemical force push Na+ ions

Into the cell via leak channels
- Na/K pump moves 3 Na+ ions out of the cell every cycle

Which way does electrical force push Na+ ions

Into the cell via leak channels
- Inside of cell is negatively charged, more cations would make the cell neutral

Equilibrium potential for Na+

+55mV

At cell resting potential (-70mV), what does Na+ want to do

Enter the cell, moving the potential to +55mV

How is the resting membrane potential of -70mV achieved?

The more permeable the ion, the greater the ability force resting membrane potential to the ion's equilibrium potential. K+ is more permeable, as there is more K+ leak channels than Na+ leak channels

Why do different cells have different resting membrane potentials?

Different cells have different ratios of K+ leak channels to Na+ leak channels

At resting membrane potential, is there still ion movement?

Yes, but ionic fluxes are balanced ensuring charge separation and the resting membrane potential remains constant.

Movement of ions at resting membrane potential

Na/K pump is still active, moving 3Na+ out of the cell and 2K+ into the cell.
- K+ leak channels move potassium ions back out of the cell
- Na+ leak channels move sodium ions back into the cell