BSCI450: Membrane Potential Review (Topic 3)

Membrane Potential (Vm)

The electrical potential difference across the plasma membrane caused by unequal ion distribution and selective membrane permeability

Two Requirements for a Membrane Potential

(1) Ion concentration gradients across the membrane

(2) selective membrane permeability, without either one, Vm = 0 mV

Resting Membrane Potential (RMP)

The inside of the cell is negative relative to the outside because K⁺ permeability is much greater than Na⁺ permeability

Major Ion Distribution Across a Mammalian Cell

Na⁺: High outside, low inside

K⁺: High inside, low outside

Ca²⁺: Extremely high outside, extremely low inside

Cl⁻: High outside, low inside

Na⁺ (sodium) ion solute distribution between intracellular (ICF) and extracellular space (ECF)

ICF: 15.0 mM

ECF: 145.0 mM

High outside, low inside

K⁺ (potassium) ion solute distribution between intracellular (ICF) and extracellular space (ECF)

ICF: 140.0 mM

ECF: 4.0 mM

High inside, low outside

Ca²⁺ (calcium) ion solute distribution between intracellular (ICF) and extracellular space (ECF)

ICF: <0.001⁺ mM

ECF: 1.8 mM

Extremely high outside, extremely low inside

Cl⁻ (chloride) ion solute distribution between intracellular (ICF) and extracellular space (ECF)

ICF: 4.0 mM

ECF: 115.0 mM

High outside, low inside

Why is the inside of the cell negative?

K⁺ diffuses out of the cell down its concentration gradient, leaving behind negatively charged proteins and other impermeable anions

Selective permeability (P)

The membrane allows certain ions to cross more easily than others because only specific ion channels are open

Ion Channels

Transmembrane proteins that allow ions to diffuse down their electrochemical gradients

Types of Ion Channel Gating

Ion channels may be opened by voltage, neurotransmitters, or hormones

Which ion dominates the resting membrane potential (RMP) and why?

At rest, PK = 1, while PNa ≈ 0.03–0.05, making K⁺ the largest contributor to Vm

If ion concentrations are equal across the membrane, what does that mean for the concentration gradient? What about the membrane potential (Vm)?

There is no concentration gradient, so Vm = 0 mV

If the membrane is equally permeable to all ions, what does this mean for the Vm and selective permeability?

Selective permeability is lost, so Vm = 0 mV

If Na⁺ were to be the most permeable ion, will the Vm hyperpolarize (negative than the RMP) or depolarize (positive than the RMP)?

Vm would depolarize and become more positive because Na⁺ would diffuse into the cell

Na⁺ has higher rate of diffusion from ECF than ICF → ion will pull in more to the cell since cell is mostly negative

Equilibrium Potential (Eion)

The membrane potential where the concentration gradient and electrical gradient are equal and opposite, resulting in no net ion movement

What is the Nernst Equation used for?

61 log (x out/x in)

Calculates the equilibrium potential (Eion) for a single ion

Which ion’s Vm is closest compared to Ek in a mammalian cell and why?

Vm is normally closest to Ek because K⁺ has the greatest permeability at rest (100)

Relative permiability at rest for ions

K⁺: 100

Na⁺: 1

Cl⁻: 25

Ca²⁺: 0 or near zero

Why is the Vm slightly more positive than Ek in sodium (Na⁺)?

A small amount of Na⁺ leaks into the cell because the membrane is slightly permeable to Na⁺

Permeability (P)

How easily an ion crosses the membrane through open ion channels

Conductance (G or g)

The ability of ions to flow across the membrane; depends on permeability and ion availability

Difference Between Permeability and Conductance

Permeability depends on open channels, while conductance requires ions to be present

What does zero conductance look like physiologically?

Conductance is 0 if no ions are available, even if many channels are open

What if there is a increase in permeability?

Vm shifts closer to that ion's equilibrium potential

What determines permeability?

More open ion channels

Hyperpolarization

Vm becomes more negative than the resting membrane potential

Depolarization

Vm becomes more positive than the resting membrane potential

If GNa⁺ increases, Vm will….

depolarize because Vm moves toward ENa

If GK⁺ decreases, Vm will….

depolarize because less K⁺ leaves the cell

If GNa⁺ decreases, Vm will….

hyperpolarize because Na⁺ entry decreases

If GK⁺ increases, Vm will….

hyperpolarize because Vm moves toward EK

Increasing Extracellular K⁺ ([K⁺]out)

Depolarizes the membrane by reducing the K⁺ concentration gradient

Decreasing Extracellular K⁺ ([K⁺]out)

Hyperpolarizes the membrane by increasing the K⁺ concentration gradient

Increasing Intracellular K⁺ ([K⁺]in)

Hyperpolarizes the membrane by increasing the K⁺ concentration gradient

Increasing Extracellular Na⁺ ([Na⁺]out)

Depolarizes the membrane by increasing Na⁺ influx

Decreasing Extracellular Na⁺ ([Na⁺]out)

Hyperpolarizes the membrane by reducing Na⁺ influx

Goldman-Hodgkin-Katz (GHK) Equation

Calculates Vm using the concentration gradients and relative permeabilities of K⁺, Na⁺, and Cl⁻

Action potential

A rapid, non-degrading electrical signal conducted along excitable cells

Voltage-Gated Na⁺ Channels

Open after sufficient depolarization (+) and initiate the rapid upstroke of the action potential

Voltage-Gated K⁺ Channels

Open after depolarization (+) and repolarize the membrane

Local (Graded) Potential

A small, localized change in membrane potential produced by a stimulus

What are the sources of a graded potential?

Physical stimuli or neurotransmitter release from presynaptic neurons

Channel inactivation

Shortly after opening, an inactivation gate blocks Na⁺ flow even while the membrane remains depolarized

• as sodium channels open and Vm depolarizes → quickly inactivate

Absolute Refractory Period

Na⁺ channels are inactivated (PNa ≈ 0), so another action potential cannot occur

Repolarization

Caused by Na⁺ channel inactivation and opening of voltage-gated K⁺ channels

Afterhyperpolarization (Undershoot)

Vm becomes more negative than resting because K⁺ permeability remains elevated

Excitatory Postsynaptic Potential (EPSP)

A graded depolarization that moves Vm closer to threshold

Inhibitory Postsynaptic Potential (IPSP)

A graded hyperpolarization or any local change that moves Vm farther from threshold

Location of Graded Potentials

Neuronal dendrites

Action Potential Conduction

Occurs through intracellular current flow and sequential opening of voltage-gated ion channels

Conduction Velocity

Increased by specialized neuronal structures and mechanisms that speed action potential propagation