Membrane Potentials and Action Potentials

Intracellular Fluid (ICF) K+ concentration

High (30 times more potassium inside the cell than outside).

Intracellular Fluid (ICF) Na+ concentration

Low.

Extracellular Fluid (ECF) Na+ concentration

High (15 times more sodium outside the cell than inside).

Extracellular Fluid (ECF) K+ concentration

Low.

Primary anions inside the ICF

Large, nonpenetrating, negatively-charged organic proteins.

Primary anions in the ECF

Chloride ions (Cl-).

How ions diffuse through the cell membrane

Through gated channels and leak channels, not directly through lipids.

Why K+ initially leaks out faster than Na+ leaks in

Greater concentration gradient and more leak channels for K+.

Charge distribution of a resting cell membrane

Negative on the inside surface and positive on the outside surface.

Electrochemical gradient

The combined forces of concentration and electrical gradients driving ion movement.

Role of the Na+/K+ pump in resting membrane potential

Maintains chemical concentration gradients by pumping ions at their leakage rates.

Average resting membrane potential in neurons

-70 mV.

Meaning of the negative sign in -70 mV resting potential

The inside of the cell membrane is negative relative to the outside.

Depolarization

A reduction in membrane potential, moving closer to zero or becoming positive.

Hyperpolarization

An increase in membrane potential, moving below -70 mV.

Repolarization

The membrane potential returning back toward -70 mV after depolarization.

Graded potential

A local current flow that decreases in intensity over distance.

Why a graded potential is called 'graded'

Its magnitude varies directly with the intensity of the stimulus.

Threshold potential in a neuron

The voltage level (-55 mV) needed to open voltage-gated Na+ channels.

Action potential (AP)

A large depolarization that propagates along the entire membrane without losing strength.

Ion movement causing AP depolarization

Na+ rapidly rushing into the cell through voltage-gated Na+ channels.

Ion movement causing AP repolarization

K+ rushing out of the cell through voltage-gated K+ channels.

Absolute refractory period

Time during which no second action potential can be generated.

Relative refractory period

Time when a second AP can occur only if the stimulus is exceptionally strong.

All-or-none property of APs

An AP occurs completely if threshold is met, or not at all.

Frequency coding

Stimulus strength is signaled to the brain by AP frequency, not magnitude.

Saltatory conduction

AP propagation in myelinated axons, jumping from node to node.

Hypokalemia effect on resting potential

Causes hyperpolarization (making the membrane more negative and harder to depolarize).

Hyperkalemia effect on APs

High ECF K+ makes it difficult to quickly repolarize the cell.

Hypocalcemia mechanism on excitability

Low Ca2+ opens gated Na+ channels without stimulus, making depolarization easier.

Hyponatremia effect on cells

Harder to depolarize and causes brain cells to swell osmotically.