Electrical Signals in Neurons: Membrane Potentials

DPT 5035

Attributes of a Nerve Cell Membrane

• Double layer lipid membrane

• Mostly impermeable to ions EXCEPT when ion channels or transporter proteins are present.

Ion Channels

• Proteins that create holes in the membrane when open

• Can be selective for specific ions or classes (i.e. Na channel)

• Allows ions to flow to passively diffuse according to concentration and electrical charge.

Ion Transporters

• Actively transport ions across the membrane against the concentration gradient

• Create ion concentration gradients

• Use energy (ATP)

Resting Membrane Potential

The voltage (electrical charge) difference between the inside of the neuron and the extracellular fluid, when the neuron is at rest (no stimulus)

Neurons have a ____ resting potential.

negative (inside of the cell is more negatively charged than the outside of the cell)

Creation of Resting Potential

• Extracellular fluid is arbitrarily considered to be at zero volts.

• If intracellular ions= extracellular ions (equal ion charges) resting potential would be zero.

• Most cells have different concentrations inside vs. outside, voltage difference across the membrane.

• Most cells have more negative ions inside than positive ions– negative resting potential.

When does membrane potential change?

When ions move across the membrane

Depolarization

• If + ions move in (or - ions move out)–> more positive inside than it was at rest.

• As its potential moves closer to zero–> depolarized.

Hyperpolarization

• If ions move in (or + ions move out)–> more negative inside than it was at rest.

• As its potential moves farther from zero–> hyperpolarized.

Ion Channels and Membrane Potentials

• Openings in the cell membrane formed by proteins

• Change configuration so that they are open, closed, or inactivated.

• Channels are specific for type of ions (ie: Na channels, Cl channels)

Chemically-Gated Channels= Ligand-Gated

• Opened by binding of neurotransmitter

• Can be directly gated– Neurotransmitter opens channel

• Or indirectly gated– Neurotransmitter activated a second messenger that opens the channel.

Voltage Gated Ion Channels

• Opened by voltage hange across the membrane and it changes configuration.

• Voltage gated Na channels at the nodes of Ranvier and Axon Hillock

• Produce Action Potentials

• Voltage gated Ca channels– in the axon terminal, for neurotransmitter release

Mechanically-Gated Channels

• Trigger is touch, pressure, stretch, or vibration

• Used as sensory receptors

Leakage Channels

• The neuron does NOT have a totally impermeable membrane, a little “leaky”

• Movement of ions: Potassium>Sodium

• Generally, sodium (Na+) leaks in and potassium (K+) leaks out

• Causes the interior of the cell to be negatively charged

Na+/K+ Pump/Transporter

• Na/K pump attempts to return these leaking ions back across the cell membrane.

  • Operated by “carrier proteins” in the membrane.

  • At rest: Na+ leaks in and K+ leaks out

  • Pump protein moves Na+ ions back out & carries K+ back in.

  • Takes 10-20% of a neuron’s total energy consumption to run the pump.

Concentration Gradients

Ions are not distributed equally on both sides of the membrane

Ions want to diffuse from areas of high concentration to low

• Sodium & chloride (NaCl) ions are concentrated outside neurons

• Potassium (K+) & protein anions (A-) are high inside

Concentration Gradient for Na+

Inside: 18mM, Outside: 145 mM

Concentration Gradient for K+

Inside: 135mM, Outside: 3mM

Concentration Gradient for Cl-

Inside: 7mM, Outside: 120mM

Concentration Gradient for Ca2+

Inside: 100mM, Outside: 1.2mM

Voltage Gradient

Ions are NOT always free to follow their concentration gradient

• Ions have an electrical charge (positive or negative)

• Attracted or repelled by other charged particles

• Large protein ions are trapped inside the neuron (too big)

• Negative charged proteins attract positively charged small ions & repels negatively charged ions

• This process creates electrochemical force or a voltage gradient.

Ionic Equilibrium Potential

• The neuron membrane potential at which the concentration gradient and the electrochemical gradient will be equally balanced and no net ion flow occurs

• Some ions, both the concentration and electrochemical gradients will act in the same direction (moving the ion into or out of the cell), while for other ions, the two forces will act in opposite directions

Potassium Ionic Equilibrium Potential

-90mV

Chloride Ionic Equilibrium Potential

-90mV

Sodium Ionic Equilibrium Potential

+55mV

Driving Force

• Equal to the difference between the equilibrium potential for the individual ion (Na+, etc.) vs. the present neuron potential.

• Larger the voltage gradient, the greater the tendency for ions to move across the membrane

  • At rest, K+ and Cl- have small driving forces, so they leak little

  • At rest, Na+ has large driving force, so it leaks a lot

Ionic Basis of Resting Potential

• If the neuron membrane was permeable to only one of these ions, the neuron resting potential would be the same as the ion’s equilibrium potential.

• Membrane is really permeable to all three ions

• Normal resting potential (-65mV) lies between the equilibrium potentials for K+ and Cl- (-90mV) and the equilibrium potential for Na+ (+55mV)

• Closer to K+/Cl-

• Can be calculated via Goldman Equation