Lecture 4: Equilibrium Potential I

Vocabulary practice flashcards covering ion channel properties, driving forces, diffusion laws, drift equations, and equilibrium potential from Lecture 4.

Tetrodotoxin (TTX)

A toxin derived from the puffer fish (Fugu) that selectively blocks voltage-gated $Na^+$ channels.

Tetraethylammonium (TEA)

A substance that selectively blocks voltage-gated $K^+$ channels.

Rectifying Channel

An ion channel that allows ions to flow more easily in one direction across the membrane than the other.

Channel Conductance ($G$)

The electrical conductance of an ion channel that influences the rate of ion flow (current, $I$) and depends on channel permeability ($P$), the amount of ion around the pore, and membrane voltage ($V_M$).

Permeability ($P$)

A property that determines how rapidly a solute can be transported through a membrane, expressed in units of velocity ($\text{cm/sec}$).

Solute Flux

The amount of solute that crosses a particular surface area in a unit of time, calculated by multiplying permeability by the solute concentration.

Voltage ($V$)

The electrical "pressure" that moves electrons or ions through a circuit, measured in volts ($\text{V}$).

Current ($I$)

The number of electrons or ions moving per unit time, measured in amperes ($\text{A}$), where $1\,\text{A} = 1\,\text{coulomb/sec}$.

Coulomb ($C$)

A unit of electric charge equal to $6.25 \times 10^{18}$ charges.

Faraday ($F$)

A constant representing one mole of charges, equal to $96,500\,\text{coulombs}\,(C)$.

Negative Current (Inward Current)

An ionic current corresponding to either a positively charged ion entering the cell (e.g., $Na^+$ influx) or a negatively charged ion exiting the cell.

Positive Current (Outward Current)

An ionic current corresponding to either a negatively charged ion entering the cell or a positively charged ion exiting the cell (e.g., $K^+$ efflux).

Fick's Law of Diffusion

A law stating that diffusion flux takes place down a concentration gradient and is directly proportional to the magnitude of the gradient: $J_{diff} = -D \frac{\partial [C]}{\partial x}$.

Diffusion Coefficient ($D$)

The proportionality constant in Fick's Law of Diffusion, expressed in units of $\text{cm}^2/\text{s}$.

Ohm's Law for Drift

The mathematical relationship describing ion flow in an electric field: $J_{drift} = -\mu z [C] \frac{\partial V}{\partial x}$, showing that positively charged ions flow down the electric potential gradient.

Mobility ($\mu$)

A parameter in Ohm's Law for Drift indicating the ease with which an ion moves under an applied electric field, expressed in units of $\text{cm}^2/(\text{V}\,\text{s})$.

Nernst-Planck Equation

An electrodiffusion relationship describing total ion flux as the sum of drift flux and diffusion flux: $J = J_{drift} + J_{diff} = -\mu z [C] \frac{\partial V}{\partial x} - D \frac{\partial [C]}{\partial x}$.

Equilibrium Potential ($E_i$)

The membrane potential at which the chemical gradient and electrical field are equal and opposite in magnitude, resulting in zero net cross-membrane current and reversing current direction.

Nernst Equation

The equation defining the membrane potential where net cross-membrane current is zero: $E_i = \frac{RT}{zF} \ln\left(\frac{[C_i]_{out}}{[C_i]_{in}}\right)$.

Simplified Nernst Equation at $20^\circ\text{C}$

The base-10 linear form of the Nernst equation at $20^\circ\text{C}$, expressed as $E_i = \frac{58\,\text{mV}}{z} \log_{10}\left(\frac{[C_i]_{out}}{[C_i]_{in}}\right)$, showing a slope of $58\,\text{mV}$ per 10-fold concentration difference when $z = 1$.

Simplified Nernst Equation at $37^\circ\text{C}$

The physiological form of the Nernst equation evaluated at body temperature ($37^\circ\text{C}$): $E_i = \frac{62\,\text{mV}}{z} \log_{10}\left(\frac{[C_i]_{out}}{[C_i]_{in}}\right)$.