Lecture 3 + 4: Membrane Potential

Ions

Charged particles that are responsible for the electrical signals in neurons.

How is the Resting Membrane Potential created?

The distribution and flow of ions (voltage) across the neuronal membrane at rest (when an action potential isn’t firing)

How is the action potential created?

Changes in the flow of ions when one neuron wants to send a signal to another

Neuronal Cell Membrane

• Lipid Bilayer

  • Outside: Hydrophilic Head - polar

  • Inside: Hydrophobic Tail

Ion Channels

specialized proteins that allow specific ions to pass through; act as pores that can be opened or closed in response to environmental signals; allows the cell to regulate what is passing across its memrane

Importance of ion channels

Ions need to get in and out of the cell because this is crucial for neuronal function and communication

Can ions go through any channel they want?

No. Different ion channels allow diff ions to pass thru; selective; a Na+ channel will only allow Na+ to pass through

Leak Gates*

always open

Gated Channels

channels that can be opened or closed by a stimulus

Ligand-gated channels

open when a chemical messenger (a ligand, such as a neurotransmitter) binds to them

Voltage-Gated Channels*

open and close in response to changes in the cell membrane's electrical potential (voltage) to control the flow of specific ions; in axon

Mechanically-gated

open / close in response to pressure

What ion has a high concentration inside the cell?

Potassium

What ion has a high concentration outside the cell?

Sodium

Sodium-potassium pump

• uses ATP to pump ions against their concentration gradients; actively transports 3 sodiums out the cell and 2 potassiums in.

Why is the cell using ATP?

To create a large store of potential energy in the form of ion concentration gradients

What is membrane potential?

The voltage of the cell; the difference in electrical charge (voltage) between the inside and outside of a cell's membrane.

Voltage

the electrical potential of ions across the membrane

How is voltage expressed?

Inside of the neuron relative to the outside

What are the two requirements for establishing a membrane potential?

1. Ion concentration gradient across the membrane

2. Permeability: selective ion channels in the membrane that will allow the ions to flow across

Resting Membrane potential

• ~-65mV

• potassium ions are constantly trickling out through leak channels due to high intracellular concentration and they’re always open

Potassium Leak Channels

• always open; allow K+ to flow across membranes out of cell

• At rest the cell membrane is impermeable to most ions EXCEPT potassium leak channels

What causes large changes in membrane potential?

SMALL changes in ion concentration and what actual ion is moving

Equilibrium Potential

• voltage that ONE ion wants to bring the cell to if a channel was always open to it

• the theoretical voltage of the cell that would result from that ONE ion moving freely across the membrane and achieving equilibrium

• takes only ONE ion into account

What does membrane potential depend on?

1. ALL ions and their equilibrium potentials

2. Permeability

What are the two basis’ of the equilibrium potential?

Concentration and charge

Why do charges matter for membrane potentials?

Eventually as an ion is leaving/entering a cell, the ionic charge will be attracted to the opposite charge - electrical force

ex: K+ will eventually attract to the inner cell even as it’s exiting because it’s positive and the cell interior is becoming more negative

What is the equilibrium potentinal for an ion?

The voltage where the chemical and electrical gradients are equal and opposite in direction

Nerst Equation

• calculation of the equilibrium potential

• increasing the outside concentration of a positive ion will make the equilibrium potential more positive; increasing inside = more negative

• E_x= 61.5/z [log ( (x+)₀ / (x+)₁ ) ]

• E_x = equilibrium potential

• z = charge (negative or positive

• X⁺₀ = concentration of ion x outside the cell

• X⁺₁ = concentration of ion x inside the cell

If concentration of ion outside and inside is equal….

• Log (1) = 0

• E_x = 0

• No movement of ions (no conc gradient)

If concentration of a positive ion is a little bigger outside than inside…

• Log (10/1) = 1

• + 65mv

If concentration of a positive ion is a LOT bigger outside than inside…

• Log (10000/1) = 4

• + 264mV

If concentration of a positive ion is a little bigger inside than outside…

• Log (1/10) = -1

• EP = -61.5mV

Are total ion concentrations changed?

No; there is a very small discrepancy

Where does the net difference in electrical charge occur?

Inside and outside SURFACES of the membane

Ionic Driving Force Equation

V_m - E_ion

• V_{m =} V that the cell is currently at

• E_ion = V which that particular ion wants to be at

Ionic Driving force

Determines the direction and magnitude of ion movement;

Negative Driving Force

Wants to make the cell more positive (positive ions move in, negative ions move out)

Positive Driving Force

wants to make cell more negative ( positive ions move out, negative ions move in

What conditions would allow V_m = E_ion?

Channels to that ion and ONLY that ion are open

The more permeable the membrane is to an ion…

the closer the membrane potential will be to that ion’s equilibrium potential.

What do we use to calculate membrane potential when there is more than one ion?

The Goldman equation

Goldman Equation

A weighted average of the Nernst potentials for all of the ions in the cell, weighted according to their relative permeaiblities

Why is resting membrane potential not -80mV (EP of K⁺)

Na⁺ also has some leak channels so a little bit is leaking in butit’s more permeable to K⁺