Neurobiology: Driving Forces, Membrane Potentials, and Action Potential Biophysics

Comprehensive vocabulary flashcards covering electrochemical driving forces, resting membrane potentials, the GHK equation, ion channel structural domains, and action potential biophysics.

Driving Force

The net electrochemical difference acting on an ion across the cell membrane, calculated as $V_m - E_{\text{ion}}$. Its magnitude and sign dictate the rate and direction of passive ion flux.

Reversal Potential

An alternative term for an ion's equilibrium potential ($E_{\text{ion}}$), so named because the driving force and resulting ionic current reverse direction as the membrane potential ($V_m$) transitions above or below this voltage value.

Inward Current

The net movement of positive charge into the cell (or negative charge exiting the cell), which by electrophysiological graphing convention is plotted in the downward direction and causes depolarization of $V_m$.

Outward Current

The net movement of positive charge out of the cell (or negative charge entering the cell), which by electrophysiological graphing convention is plotted in the upward direction and causes hyperpolarization of $V_m$.

Ohm's Law for Ionic Current

The biophysical equation $I_{\text{ion}} = (V_m - E_{\text{ion}}) \times g_{\text{ion}}$, stating that the current generated by a specific ion is the product of its electrochemical driving force and its specific membrane conductance.

Goldman-Hodgkin-Katz (GHK) Equation

A mathematical formulation used to calculate the resting membrane potential ($V_m$) by factoring in the concentration gradients and relative membrane permeabilities ($p_{\text{ion}}$) of multiple ions: $V_m = \frac{RT}{F} \ln\left(\frac{p_K [K^+]_o + p_{\text{Na}} [Na^+]_o + p_{\text{Cl}} [Cl^-]_i}{p_K [K^+]_i + p_{\text{Na}} [Na^+]_i + p_{\text{Cl}} [Cl^-]_o}\right)$.

Resting Membrane Potential ($V_{\text{rest}}$)

The baseline electrical potential difference across a neuronal membrane when inactive, typically around $-70\,\text{mV}$ in neurons, heavily influenced by baseline potassium permeability.

Potassium ($K^+$) Leak Channel

A non-gated transmembrane protein channel that remains constitutively open at rest, providing baseline potassium permeability ($p_K = 1$) and anchoring the resting potential close to $E_K$.

Relative Permeability at Rest

The comparative baseline permeability hierarchy of neuronal membranes at rest, ordered as $p_K > p_{\text{Cl}} > p_{\text{Na}} > p_{\text{Ca}}$, with typical normalized ratios of approximately $1$ for $K^+$, $0.4$ to $0.45$ for $\text{Cl}^-$, and $0.05$ for $\text{Na}^+$.

KCC2

A neuron-specific potassium-chloride cotransporter (exporter) that extrudes $\text{Cl}^-$ from the cytoplasm; following spinal cord injury (SCI), its down-regulation contributes to elevated intracellular chloride and shifts $E_{\text{Cl}}$ from $-60\,\text{mV}$ toward $-18\,\text{mV}$.

NKCC1

A sodium-potassium-chloride cotransporter (importer) that transports $\text{Cl}^-$ into the neuronal cytoplasm; its expression increases relative to baseline following spinal cord injury (SCI).

Patch-Clamp Electrophysiology

An experimental recording configuration utilizing a glass micropipette that forms a high-resistance giga-seal with a cell membrane patch to measure unitary currents through individual ion channels.

Threshold Potential ($V_{\text{threshold}}$)

The critical membrane potential to which a neuron must depolarize to initiate an all-or-nothing action potential, characterized by the point where inward sodium currents exceed outward potassium currents.

Action Potential

A rapid, transient, all-or-nothing electrical depolarization and repolarization cycle actively propagated along an axon to transmit signals without decremental decay over long distances.

Voltage-Sensor Domain (S4 Segment)

The fourth transmembrane segment of voltage-gated sodium and potassium channels containing repeated positively charged amino acids that physically displace within the membrane field upon depolarization to open the gate.

Selectivity Filter

The narrowest portion of an ion channel's pore, formed by pore loops connecting transmembrane segments S5 and S6, that biochemically discriminates between ion species based on charge and hydration diameter.

Ball-and-Chain Inactivation

A mechanism of rapid channel closing in which a tethered cytoplasmic peptide ball (mapped to amino acids $6\text{--}46$ in Shaker potassium channels) swings into the internal vestibule to occlude the pore while the activation gate remains open.

Afterhyperpolarization (AHP)

The final phase of an action potential during which the membrane potential dips below baseline $V_{\text{rest}}$ due to delayed closure of voltage-gated potassium channels, pulling $V_m$ closer to $E_K$ ($-90\,\text{mV}$).

Mauthner Neuron

A specialized reticulospinal neuron in zebrafish that drives the escape reflex; it integrates graded, subthreshold acoustic signals in its lateral dendrite while generating an all-or-nothing thresholded binary response at the soma.

Myelin Sheath

A thick, multi-lamellar lipid wrapper generated by oligodendrocytes around axons that reduces membrane capacitance and reduces leak conductance to essentially zero, enabling long-distance passive electrotonic spread.

Nodes of Ranvier

Periodic unmyelinated gaps along an axon spaced $200\,\mu\text{m}$ to $2\,\text{mm}$ apart with exceptionally high densities of voltage-gated sodium channels, serving as sites of action potential regeneration.

Saltatory Conduction

The rapid and energy-efficient mode of action potential propagation in myelinated axons, characterized by passive local current flow beneath internodes and active regeneration jumping from node to node.

Multiple Sclerosis (MS)

A demyelinating autoimmune disease in which the immune system degrades the myelin sheath along axons, introducing high leak conductance, slowing or preventing saltatory conduction, and producing neurological impairments.