Electrical Activity & The Action Potential - Video Notes (Ch 6 & 7)

Flashcards cover core concepts of membrane potential, action potentials, ion channels, gating mechanisms, and factors affecting propagation and excitability as presented in the lecture notes.

Membrane potential (Vm)

Voltage difference across the cell membrane; the potential of the cytoplasm relative to the extracellular space.

Resting membrane potential

Vm when the neuron is not firing; typically around -60 mV to -90 mV in neurons, -55 mV in smooth muscle, and -9 mV in erythrocytes.

Nernst potential

Equilibrium membrane potential for a specific ion based on its concentration gradient; ranges roughly from -100 mV (K+) to +100 mV (Ca2+).

Ionic currents sum (ITotal)

If the membrane were permeable only to K+, Na+, and Cl-, the total current would be ITotal = IK + INa + ICl.

Ohm’s law in membranes

Voltage and current are related by V = IR; the membrane behaves as a capacitor with ions moving according to conductances.

Driving force

The force pushing an ion across the membrane; equal to (Vm − Eion). Current equals conductance × driving force.

Patch clamp recording

A technique to study ion channels; voltage-clamp studies examine channels in a controlled membrane potential; patch-clamp can measure single-channel activity.

Channel gating

Opening and closing of ion channels; governed by probabilities that gates are open or closed.

Voltage-dependent gating

Gating controlled by membrane voltage; channels open or close in response to changes in Vm; e.g., Kv channels during depolarization.

Ligand-gated channel

Channel opened by binding of a chemical ligand (e.g., neurotransmitter); mediates local, graded responses.

Action potential (AP)

All-or-none electrical spike; initiated when threshold is reached; involves Na+ and K+ channel dynamics and feedback mechanisms.

Threshold

Membrane potential at which an action potential is elicited; marks the transition from subthreshold to propagating activity.

Propagation of the AP

AP travels along the axon with almost constant amplitude and shape; the delay between stimulus and response increases with distance.

Axon radius and propagation

Larger diameter increases conduction speed because of lower axial resistance and greater membrane area, balancing leak and cytoplasmic volume.

Myelination

Myelin insulation increases conduction speed via saltatory conduction and increases membrane resistance while reducing capacitance.

Length constant (λ)

Distance passive voltage changes travel before decaying; increased by larger radius and higher membrane resistance; determines how far signals spread.

Time constant (τ)

τ = RC; how quickly Vm changes at a site; smaller τ (due to lower capacitance or higher resistance) speeds up propagation.

Absolute refractory period

Period when no new action potential can be fired because Na+ channels are inactivated.

Relative refractory period

Period following the absolute phase when a larger-than-normal stimulus may trigger an AP because Na+ gates recover and K+ gates are still active.

Hodgkin–Huxley model (conductances)

Mathematical model showing Na+ (m^3h) and K+ (n^4) channel conductances govern AP dynamics; Na+ activation (m), Na+ inactivation (h), K+ activation (n).

Na+ channel gating

Activation by depolarization (m gates) and fast inactivation (h gates); open when m^3h are satisfied.

K+ channel gating

Activation by depolarization with four activation gates (n^4) opening to allow K+ efflux; contributes to repolarization.

Leak channels

Non-gated channels that help stabilize resting potential; typically have little to no voltage dependence.

Ion-channel diversity

Many channel types (e.g., Kv, Eag, KCNQ, Slo, Kir) with distinct genes and currents contributing to neuronal and cardiac excitability.

Transmembrane topology of channels

Voltage-gated channels have a pore domain and voltage-sensing domains (notably S4) with hydrophobic transmembrane segments forming the channel.

Channel modulation and accessory subunits

Channel function can be modified by accessory subunits, GPCR signaling, phosphorylation, and lipid interactions (e.g., PIP2).

Patch vs. voltage clamp

Patch clamp can isolate single channels; voltage clamp controls membrane potential to study channel kinetics and ionic currents.