Week 4 Neurobiology

Selective permeability (resting neuron)

At rest, the membrane is more permeable to K⁺ than Na⁺ because more K⁺ channels are open, so gK >> gNa.

Conductance (g)

A measure of how easily ions flow through open channels; depends on the number of open channels.

Permeability (P)

How easily an ion crosses the membrane; higher permeability means greater influence on membrane potential.

Resting membrane potential (Vrest)

A stable membrane voltage where net ion flux is zero and inward and outward currents balance.

Electrochemical equilibrium

A condition where electrical and chemical forces on an ion are balanced, producing no net ion movement.

Electrochemical gradient

The combined effect of an ion’s concentration gradient and electrical gradient.

Driving force

The net force acting on an ion that determines the direction and magnitude of ion movement.

Ohm’s law (neurons)

I = g × V, where current equals conductance times driving force.

Nernst equation

A formula used to calculate the equilibrium potential (Eion) for a single ion species.

Equilibrium potential (Eion)

The membrane potential at which there is no net movement of a specific ion.

Goldman (GHK) equation

A formula that calculates membrane potential when multiple ions contribute based on their permeabilities.

What the Goldman equation calculates

The resting membrane potential when K⁺, Na⁺, and Cl⁻ are all in electrochemical equilibrium.

Weighted average concept (Goldman)

The membrane potential reflects a permeability-weighted average of each ion’s equilibrium potential.

Squid neuron resting permeabilities

PK ≈ 1.0, PNa ≈ 0.04, PCl ≈ 0.45.

Why potassium dominates Vrest

K⁺ has the highest permeability at rest, so Vm is closest to EK.

Effect of increasing extracellular K⁺

Makes the membrane potential less negative (depolarizes).

Effect of increasing extracellular Na⁺

Produces only a small depolarization at rest due to low Na⁺ permeability.

Why changing K⁺ affects Vrest more than Na⁺

Because resting conductance to K⁺ is much greater than to Na⁺.

Effect of increasing PK

Moves membrane potential closer to EK (hyperpolarizes).

Effect of increasing PNa

Moves membrane potential closer to ENa (depolarizes).

Effect of increasing PCl

Moves membrane potential toward ECl depending on Vm relative to ECl.

Why chloride is flipped in Goldman equation

Because Cl⁻ is negatively charged, its concentration terms are reversed.

Temperature constant (Goldman, squid)

58 mV is used at 20°C.

Temperature constant (Goldman, mammal)

61.5 mV is used at 37°C.

Ion flux

Movement of ions across the membrane that generates electrical current.

Zero net current condition

Occurs when total inward and outward ion currents balance, stabilizing membrane potential.

Why no ion flux at Vrest

Each permeable ion is at electrochemical equilibrium, so currents cancel out.

Depolarization

A decrease in membrane potential magnitude; the inside becomes less negative.

Hyperpolarization

An increase in membrane potential magnitude; the inside becomes more negative.

Threshold potential

The membrane voltage at which an action potential is triggered.

Action potential

A brief, all-or-none electrical signal caused by rapid changes in ion permeability.

Rising phase of action potential

Caused by rapid Na⁺ influx through voltage-gated sodium channels.

Overshoot phase

The portion of the action potential where the inside of the cell becomes positive.

Falling phase of action potential

Caused by K⁺ efflux through voltage-gated potassium channels.

Undershoot (afterhyperpolarization)

A period where membrane potential is more negative than resting due to continued K⁺ efflux.

Voltage-gated sodium channels

Channels that open in response to depolarization and allow Na⁺ influx.

Voltage-gated potassium channels

Channels that open after depolarization and allow K⁺ efflux.

Refractory period

A time after an action potential when generating another action potential is difficult or impossible.

Absolute refractory period

A period when no action potential can be generated regardless of stimulus strength.

Relative refractory period

A period when a stronger-than-normal stimulus is required to trigger an action potential.

Why sodium does not affect Vrest much

Few sodium channels are open at rest, so Na⁺ conductance is low.

Goldman equation exam takeaway

Changes in permeability matter more than concentration changes for ions with high conductance.

At rest, sodium conductance is greater than potassium conductance.

FALSE — potassium conductance is much greater than sodium at rest.

The resting membrane potential is closest to the equilibrium potential of potassium.

TRUE — because K⁺ permeability dominates at rest.

Increasing extracellular potassium depolarizes the neuron.

TRUE — Vm becomes less negative.

Increasing extracellular sodium has a large effect on resting membrane potential.

FALSE — sodium permeability is low at rest.

The Goldman equation calculates membrane potential for a single ion.

FALSE — it accounts for multiple ions.

The Nernst equation calculates membrane potential when multiple ions are permeable.

FALSE — it applies to one ion only.

Goldman equation uses ion permeability as weighting factors.

TRUE — ions with higher permeability contribute more to Vm.

If potassium permeability increases, membrane potential moves toward EK.

TRUE — Vm follows the dominant ion.

If sodium permeability increases, membrane potential becomes more negative.

FALSE — it becomes less negative (depolarizes).

Chloride concentrations are reversed in the Goldman equation because chloride is negatively charged.

TRUE.

If all ion permeabilities were equal, potassium would still dominate Vm.

FALSE — no single ion would dominate.

At resting membrane potential, there is no net ion current.

TRUE — inward and outward currents balance.

No net ion current means ions are not moving.

FALSE — ions may move, but net current is zero.

Resting membrane potential means the neuron is inactive.

FALSE — it means electrically stable, not inactive.

Changing permeability has a greater effect on Vm than changing concentration for low-permeability ions.

TRUE.

Doubling extracellular potassium affects Vm more than doubling extracellular sodium.

TRUE.

Goldman equation depends on temperature.

TRUE — the constant changes with temperature.

The Goldman constant is 58 mV for mammalian neurons.

FALSE — 58 mV is used at 20°C (squid).

The Goldman constant is 61.5 mV at human body temperature.

TRUE.

If potassium channels close at rest, Vm moves away from EK.

TRUE — potassium influence decreases.

If sodium channels open at rest, Vm depolarizes.

TRUE.

If conductance is zero, current must be zero.

TRUE — I = g × V.

Resting membrane potential requires ATP to maintain.

TRUE — pumps maintain ion gradients.

Ion pumps directly generate action potentials.

FALSE — channels generate action potentials.

The resting membrane potential is a weighted average of ion equilibrium potentials.

TRUE.

Goldman equation predicts action potential shape.

FALSE — it predicts membrane potential, not spikes.

Increasing chloride permeability always hyperpolarizes the neuron.

FALSE — effect depends on ECl relative to Vm.

At rest, potassium efflux is the primary determinant of Vm.

TRUE.

Sodium influx is responsible for the rising phase of the action potential.

TRUE.

Potassium efflux causes repolarization of the action potential.

TRUE.

If gK >> gNa, changing sodium concentration has little effect on Vm.

TRUE.

Membrane voltage-dependent permeability

Change in ion permeability of the membrane that depends on membrane potential

Membrane potential (Vₘ)

Electrical potential difference across the cell membrane

Goldman equation

Equation that calculates membrane potential using relative ion conductances and equilibrium potentials

Conductance (g)

Measure of how easily ions flow through an open ion channel

Permeability

Ability of an ion to cross the membrane through channels

Equilibrium potential (Eion)

Membrane potential at which there is no net movement of a specific ion

Driving force (Vₘ − Eion)

Difference between membrane potential and equilibrium potential that determines direction and magnitude of ion flow

Ionic current (Iion)

Movement of charged ions across the membrane

I = g × (Vₘ − Eion)

Equation describing ionic current as the product of conductance and driving force

Voltage clamp technique

Method that holds membrane potential constant while measuring membrane current

Command voltage (Vcommand)

Voltage set by the experimenter in a voltage clamp experiment

Feedback loop (voltage clamp)

System that injects current to keep Vₘ equal to Vcommand

I/V curve

Graph showing relationship between membrane current and membrane voltage

Squid giant axon

Large axon used in experiments because electrodes can be easily inserted

Capacitive current

Transient current caused by redistribution of charge across the membrane

Voltage-gated ion channels

Channels that open or close in response to changes in membrane potential

Early inward current

Rapid inward current caused by sodium ion influx

Delayed outward current

Slower outward current caused by potassium ion efflux

Sodium conductance (gNa)

Conductance of voltage-gated sodium channels

Potassium conductance (gK)

Conductance of voltage-gated potassium channels

Sodium equilibrium potential (ENa)

Equilibrium potential for sodium ions

Potassium equilibrium potential (EK)

Equilibrium potential for potassium ions