The Resting Membrane Potential

Neurons

• specialized for the reception, conduction, and transmission of electrochemical signals

• convert electrical signals to chemical signals and then back to electrical signals

• The electrical to chemical to electrical process is the foundation of synaptic transmission

• polarized

Synaptic Transmission Steps

1) Presynaptic neurons send an electrical signal from the axon hillock to the axon terminal

2) At the synapse the electrical signal gets converted into chemical signal

3) The postsynaptic neuron converts the chemical signal into an electrical signal

What is electricity

• Electrical energy is gleaned from the flow of charged particles

• Electricity flows from high to low “potential” or charge

• Potential is expressed in volts

Neuronal Signaling: The Resting Potential

• Electrical signals are a reflection of the movement of ions (charged particles) into and out of the cell

• Electrical signals: receptor, synaptic, and action potentials

Measuring Resting Membrane Potential

• A microelectrode inserted through the membrane measures the electrical potential inside the axon relative to the outside

• Resting Membrane Potential of a Neuron is about -70mV

Batteries

• A device in which charges are separated by a barrier so that there is a difference in charge across the barrier

• Differences in Charge = Voltage

• Flow of Charge = Current

Stimulate axon

triggers change in potential — like light bulb turning on

Things Needed to Generate an Electrical Signal

• charge carrier

• charge separator

• pathways for charge to travel (to produce current)

Membrane Potential

1. Charge Carrier = charged particles (ions)

2. Membrane charge = separator

3. Channels = pathways

Ions

charged particles in solution (positive or negative)

Plasma Membrane

• surrounds neuron (and all cells)

• a lipid bilayer

• composed of phospholipids and is impermeable to charged molecules

• a barrier to ions (charged particles just bounce right off)

• semipermeable because of ion channels

Hydrophobic Lipid Membrane

will not allow hydrated ions into the neuron

Ion Distribution

• not permeable or impermeable

• some ions (A- and K+) have higher concentration on the intracellular side of the membrane

• some ions (Cl- and Na+) have higher concentration on the extracellular side of the membrane

Ion channels

• proteins embedded in the plasma membrane

• provide a path for ions (charged particles) to flow through the membrane

• can be selective for different ions

Leak Channels

• are always open

• set the resting membrane potential

Gated Channels

• require a trigger to open/close

• voltage-gated channels

• ligand-gated channels

Membrane Potential and Current

• Membrane Potential = separation of charges across the membrane

• Current = movement of ions across the membrane through ion channels

• Currents cause the membrane potential to become positive or negative (depending on which ions move)

• Charged proteins cannot cross the membrane, causing a negative charge on the inside of the cell

Hyperpolarization

neural potential is (or is becoming) more negative than resting membrane potential

Depolarization

neural potential is (or is becoming) more positive than resting membrane potential

When a neuron is at rest, there is no net movement of ions

• No net flux of ions across the membrane, even though ions are moving

• Ions enter and leave the neurons at the same rate

• Achieved by the balance of diffusive force and electrical force

Why are neurons polarized?

• Differential distribution of ions across the membrane

• The concentration of Na+ and Cl- is greater outside the cell

• The concentration of K+ and protein (A-) is greater inside the cell

Four Ions contributing to the Resting Membrane Potential

• Na+

• K+

• Cl-

• A- (proteins)

Two sets of forces work in opposition to contribute to the membrane potential

1. Homogenizing Forces: forces promoting equal distribution of ions across the membrane (concentration gradients and electrostatic pressure)

2. Opposing Forces: Differential permeability and Na+/K+ pump

Diffusion

when there are different concentrations of ions on either side of a biological membrane, ions will move from an area of high concentration to an area of low concentration (ex: ink distributing in water)

Electrostatic Pressure

• The force exerted by the attraction of oppositely charged ions or by the repulsion of similarly charged ions (opposites attract)

• Promotes the even distribution of ions

Accumulation of charge is dispersed by:

• repulsion of like charges

• attraction of opposite charges

Differential Permeability

• opposing forces

• K+ and Cl- pass readily through the resting membrane through leak channels

• the membrane is only very slightly permeable to sodium because there are very few channels open for sodium

Sodium-Potassium Pump

• moves sodium and potassium ions across the cell membrane against their concentration gradients

• pumps 3 Na+ ions out as it pumps 2 K+ ions in

• propose is to maintain Na+ and K+ concentration gradients

• uses ATP

• also known as the Na-K ATPase

• the pump is electrogenic, causing a net transfer of one positive ion

• the pump affects the resting membrane potential

Resting Membrane Potential

• under normal conditions, the neuronal membrane is at resting potential (-70mV)

• the distribution of the various ions across the membrane is what matins the cell at its normal resting potential (-70mV)

• relatively large potassium leak conductance

• very low sodium conductance

• high chloride concentration outside the cell

• negatively charged proteins inside the cells

Concentration Gradient

• diffusive force pushing potassium OUT

• IF diffusive force is greater, there will be net outward potassium movement

Electrical Gradient

• electrical forces pushing potassium IN

• IF electrical force is greater, there will be net inward potassium movement

Concentration/ Electrical Gradient

• there is no net flux of K+ when these 2 forces are eqaul and opposite

• the membrane potential where there is no net flux is called the Equilibriums potential

Equilibriums Potential

• The Vm that provides a force that is equal and opposite to the diffusive force

• There is no net flux of ions across the membrane

• Each ion has its own equilibrium potential that depends on the concentration gradient

• Ions move in the direction that brings the membrane potential closer to its equilibrium potential

Why is the resting potential in a neuron typically around -70mV instead of being equal to the equilibrium potential of K+ (-90mV)?

The resting cell membrane has some permeability to Na+

When the neuron is at its resting membrane potential:

no ion is at equilirbium

At rest potassium is moving ___ and sodium is moving ____.

out (positive driving force); in (negative driving force)

The more permeable an ion:

the more influence it will have on the resting membrane potential

Achieving Equilbrium

1. Intracellular: Excess of K+; presence of non-gated K+ channels.

2. K+ “diffuses” out of cell »»»» down its concentration gradient

3. A- (protein) left behind: Inside becomes increasingly negative and outside positive

4. K+ attracted by intracellular A- (protein); repelled by extracellular positive

5. Equilibrium: net movement of charges is constant (dog/fleas)

Generation of the Resting Potential

1. Selective permeability of membrane to K+ (allows K+ to move out and keeps other ions from moving in)

2. Diffusion along concentration gradients (Acts to move K+ out)

3. Electrostatic Forces (opposes diffusion gradient)

4. Sodium-Potassium Pump

What would happen to the typical neuronal membrane resting potential if the extracellular concentration of K+ was increased?

The resting potential would become more positive

Hyperkalemia

leads to a LESS NEGATIVE (more positive) resting membrane potential

Testing the Model

• Radioactive Labeling

• Change ion concentration in extracellular fluid

Radioactive labeling

• Label ions (Na+ and K+)

• Measures degree of permeability

• Showed Na+ permeability only about 5% that of K+

Change ion concentration in the extracellular fluid

• increase Na+: no effect on resting potential

• increase K+: more positive inside (depolarize membrane)