Membrane Potentials and Action Potentials

A set of vocabulary flashcards defining key terms related to membrane potentials, action potential phases, and ionic movements as discussed in the HBG122 lecture.

Membrane potential

The electrical energy difference between the inside and outside of the cell, measured in millivolts ($mV$).

Action potential (AP)

A very rapid change in membrane potential that occurs when a nerve cell membrane is adequately stimulated, used by neurons to send signals over long distances.

Threshold stimulus

The minimum strength of stimulus required to evoke an action potential.

Depolarization phase

The phase where an excitatory stimulus causes voltage-gated $Na^+$ channels to open, leading to $Na^+$ influx and the membrane potential reaching as high as $+35\,mV$.

Repolarization phase

The phase where voltage-gated potassium channels open and there is a large outward movement of $K^+$ ions, causing a reversal of the membrane potential to negative inside.

Hyperpolarization

A temporary undershoot of the electrical gradient where the inside of the neuron becomes even more negative than the usual resting potential due to large outward $K^+$ current.

Sodium-potassium pumps

Mechanisms that pump sodium ions out and potassium ions in to prevent long-term loss of ion gradients and re-establish the resting potential.

Resting potential

The baseline electrical potential across the membrane, recorded at $-70\,mV$.

Slow channels

Voltage-gated $Na^+$ and $Ca^{++}$ channels in cardiac and smooth muscle that are activated after some time during depolarization.

All or none law

The principle stating that the tissue either responds maximally to threshold stimuli or does not respond at all.

Absolute refractory period

The period from the beginning of depolarization to one third of repolarization during which a second stimulus cannot evoke a response because all $Na^+$ channels are already activated.

Relative refractory period

The period in the later part of repolarization when $Na^+$ channels can be activated by a stronger than normal stimulus.

Electrotonic Potentials

Short-lived, localized changes in membrane potential where current flows decrease in magnitude with distance.

Firing level

The specific potential of $-55\,mV$ (or reaching $7$–$15\,mV$ of depolarization) at which an action potential occurs.