Electrophysiology and Action Potentials

Comprehensive vocabulary flashcards covering membrane biophysics, Nernst and GHK equations, driving forces, and the ionic basis of action potentials based on Week 6 lecture notes.

Equilibrium Potential ($V_{eq}$)

The membrane potential at which electrical forces across the cell membrane balance the chemical concentration gradient, resulting in zero net flux of that specific ion ($\text{net flux} = 0$).

Nernst Equation

An equation ($V_{eq} = \frac{RT}{zF} \ln \frac{[X]_o}{[X]_i}$, simplified to $V_{eq} = \frac{0.0615}{z} \log \frac{[X]_o}{[X]_i}$ at human body temperature of $310\,\text{K}$ or $37^\circ\text{C}$) used to calculate an ion's equilibrium potential based on its charge and concentration gradient.

Driving Force (DF)

The net electrochemical push or pull acting on an ion, calculated as $\text{DF} = E_m - E_X$, which determines the direction and magnitude of ion movement across the membrane.

Negative Driving Force

The condition where $E_m < E_X$, indicating the cell interior is more negative than the ion's equilibrium potential, driving cations (+$) into the cell and anions (-$$) out of the cell.

Positive Driving Force

The condition where $E_m > E_X$, indicating the cell interior is more positive than the ion's equilibrium potential, driving cations (+$) out of the cell and anions (-$$) into the cell.

Membrane Potential ($E_m$)

The actual electrical voltage measured across the cell membrane at any given moment (e.g., $-80\,\text{mV}$ at rest, and up to $+20\,\text{mV}$ at the peak of an action potential).

Ionic Current ($I_X$)

The actual flow of charged ions moving across the cell membrane, calculated by Ohm's law formulation $I_X = g_X(E_m - E_X)$, which requires both open ion channels ($g_X > 0$) and a non-zero driving force.

Conductance ($g_X$)

A measure of how easily ion $X$ can pass through the cell membrane, determined by channel availability and the number of open ion channels.

Goldman-Hodgkin-Katz (GHK) Equation

An equation ($E_m = \frac{RT}{F} \ln \frac{P_K [K^+]_o + P_{Na} [Na^+]_o + P_{Cl} [Cl^-]_i}{P_K [K^+]_i + P_{Na} [Na^+]_i + P_{Cl} [Cl^-]_o}$) that calculates overall resting membrane potential ($E_m$) based on ion concentration gradients and relative membrane permeabilities ($P = \frac{D}{\delta}$).

Chord Conductance

A measure of how easily an ion flows across a cell membrane at any given voltage, defined geometrically as the slope of the straight line (chord) connecting an operating point on an $I\text{–}V$ curve to the ion's reversal potential.

Chord Conductance Equation

The relationship $E_m = \frac{g_K}{g_T} E_K + \frac{g_{Na}}{g_T} E_{Na} + \frac{g_{Cl}}{g_T} E_{Cl}$, expressing membrane potential as a weighted sum of individual equilibrium potentials according to each ion's fraction of total conductance ($g_T$).

Slope Conductance ($g$)

The instantaneous slope of the current-voltage ($I\text{–}V$) curve at any given membrane potential, defined as $g = \frac{dI}{dE_m}$.

Reversal Potential

The membrane potential at which net current is zero and changes direction (where the $I\text{–}V$ curve crosses the x-axis).

Inward Current

A negative electrical current characterized by positive charges ($$+$) entering the cell.

Outward Current

A positive electrical current characterized by positive charges ($$+$) exiting the cell.

Action Potential

A rapid depolarization followed by repolarization of the membrane potential occurring in electrically excitable cells, such as nerves and muscles.

Depolarization

A change in membrane potential where $E_m$ becomes less negative, caused by an inward current of positive charges entering the cell.

Repolarization

The return of the membrane potential toward the resting state following depolarization, driven by an outward current of positive charges leaving the cell.

Hyperpolarization

A state in which the membrane potential ($E_m$) becomes more negative than the normal resting membrane potential.

Threshold Potential

The critical membrane voltage level (approximately $-60\,\text{mV}$ in nerves) at which an applied stimulus successfully triggers an action potential $50\%$ of the time.

Overshoot

The peak phase of an action potential during which the interior of the cell briefly becomes positive ($E_m > 0$).

Undershoot

Also known as after-hyperpolarization, the phase immediately following repolarization where $E_m$ briefly becomes more negative than the normal resting potential.

Latency

The time delay between the application of an electrical stimulus and the initial resulting change in membrane potential.

All-or-None Response

The principle that once an electrical stimulus crosses the threshold potential, an action potential triggers automatically and reaches the same peak amplitude regardless of stimulus strength.

Propagation

The regenerative process by which an action potential travels like a wave down an axon or muscle fiber without losing strength, moving in one direction due to the refractory period.

Absolute Refractory Period

A $1\text{–}2\,\text{ms}$ cooldown period following excitation during which it is impossible to elicit a second action potential because voltage-gated $Na^+$ channels are inactivated.

Relative Refractory Period

A $2\text{–}4\,\text{ms}$ phase following excitation during which a second action potential can be elicited, but requires a stronger stimulus than that needed for a resting cell.

Activation Gate ($Na^+$ Channel)

A voltage-dependent gate on the voltage-gated $Na^+$ channel that opens quickly in response to membrane depolarization.

Inactivation Gate ($Na^+$ Channel)

A time-dependent gate on the voltage-gated $Na^+$ channel that automatically snaps shut a brief moment after depolarization, terminating $Na^+$ influx.

Tetrodotoxin (TTX)

A puffer fish toxin that selectively binds to and blocks voltage-gated $Na^+$ channels, abolishing $Na^+$ conductance and preventing action potentials from firing.

Tetraethylammonium (TEA)

A pharmacological blocker that blocks voltage-gated $K^+$ channels, preventing the outward $K^+$ current and halting repolarization.

Unitary Current ($i$)

The microscopic electrical current carried by a single open ion channel under physiological conditions, contributing to total current via $I_i = N P_o i$.

Ensemble Current

The macroscopic current produced by summing the individual currents of an entire population of stochastic ion channels.

Patch Clamping

An experimental technique used to record single-channel currents by isolating a small patch of membrane, applying voltage clamps, and utilizing channel blockers to distinguish ion currents.