Equilibrium, Resting Membrane, and Action Potentials

Vocabulary terms and definitions related to membrane potentials, ion equilibrium, and the mechanics of action potential propagation and conduction velocity.

Resting membrane potential (RMP)

The potential difference across the membrane when the cell is not generating an action potential, typically valued between $-70 \text{ to } -90\,mV$.

Equilibrium potential

The point at which the movement of an ion into the cell because of electrical potential is balanced by the diffusion of the ion out of the cell due to its concentration gradient (diffusion force = electrical force).

$$E_{K^+}$$

The equilibrium potential for potassium, valued at $-85\,mV$ (or between $-85 \text{ to } -90\,mV$).

$$E_{Na^+}$$

The equilibrium potential for sodium, valued at $+65\,mV$.

$$E_{Ca^{2+}}$$

The equilibrium potential for calcium, valued at $+120\,mV$.

$Na^+/K^+$ ATPase

The pump responsible for maintaining ion concentration gradients that allow diffusion potentials to develop.

Driving force

The difference between the measured membrane potential and the ion’s calculated equilibrium potential, which predicts the direction and strength of ion movement.

Depolarization

A change in the membrane potential that makes it less negative.

Hyperpolarization

A change in the membrane potential that makes it more negative.

Inward current

The flow of positive charge into the cell.

Outward current

The flow of positive charge out of the cell.

Threshold potential

The membrane potential, typically around $-60\,mV$, at which voltage-gated $Na^+$ channels open rapidly to initiate an action potential.

Overshoot

The portion of the action potential where the membrane potential becomes positive.

Undershoot

The portion of the action potential where the membrane potential is more negative than the resting membrane potential (also called hyperpolarization).

Absolute refractory period (ARP)

The period overlapping most of the action potential during which no second action potential can occur because $Na^+$ channels are inactivated.

Relative refractory period (RRP)

The period from the end of the ARP through most of hyperpolarization where a stronger-than-normal stimulus is required to generate an action potential.

All-or-none response

The characteristic of action potentials where a stimulus either reaches threshold and generates a full action potential or does not generate one at all.

Time constant

A property that determines how quickly a membrane depolarizes; a smaller value allows the membrane to reach threshold faster.

Length constant

A property that determines how far a local current spreads before it dies out; a larger value increases conduction velocity.

Membrane resistance

A property that, when high, prevents current from leaking out of the axon, thereby increasing the length constant and conduction velocity.

Membrane capacitance

A property determining how much charge is required before membrane voltage changes; when decreased (e.g., by myelin), the membrane depolarizes faster.

Internal resistance

The resistance to current flow inside the axon; it is decreased by increasing axon diameter, which in turn increases the length constant.

Myelination

The wrapping of a nerve in insulation that increases membrane resistance and decreases membrane capacitance to increase conduction velocity.

Saltatory conduction

The process in myelinated nerves where the action potential appears to "jump" from node to node as the membrane depolarizes at each node.