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Gating controls when an ion pathway is available; selectivity controls which ions can use it. An open ion channel permits passive movement down an electrochemical gradient. Opening does not itself specify current direction or guarantee a net current at the reversal potential. At a simple level, a population current can be written $I=N{P}_o\gamma (V-E_{\mathrm{rev}})$, separating channel number, open probability, single-channel conductance and driving force.

In a typical voltage-gated potassium channel, four subunits each contain six transmembrane helices. The S1–S4 region forms a voltage-sensing domain; S4 carries regularly spaced positive charges. A change in membrane potential alters the relative free energies of voltage-sensor conformations. Sensor rearrangement is coupled to opening of the S5–S6 pore region, including its intracellular activation gate. The charges do not have to traverse the entire membrane as free ions: protein conformational changes redistribute them through the electric field. A simple two-state approximation giveswhere $z_g$ is an effective gating charge and the sign convention makes depolarization favour opening for $z_g>0$. Real channels usually have multiple closed states and more complex kinetics.

voltage-gated sodium channels have four homologous domains in one polypeptide, with related voltage sensors and a central pore. Activation is followed by inactivation, which is a distinct process from the activation gate closing when the membrane repolarizes. Calcium channels share a related architecture. Voltage clamp can separate activation and inactivation kinetics, while gating-current measurements and voltage-sensor mutations test coupling between charge movement and pore opening. A mutation that abolishes current must be checked for expression and membrane delivery before concluding that it identifies the gate.

For potassium selectivity, the narrow selectivity filter contains backbone carbonyl oxygens arranged to coordinate largely dehydrated potassium ions. Binding and permeation balance the energetic cost of shedding water against favourable coordination within the filter. Sodium is smaller, but does not receive the same stabilization in the potassium-selective environment, so selectivity is not explained by a rigid hole that rejects all larger ions and admits all smaller ones. The KcsA structure established a concrete molecular basis for this principle.^[ref] KcsA itself is not the canonical S4 voltage-gated channel: its pore illustrates a conserved selectivity mechanism, not the voltage sensor just described. Voltage-gated sodium channels use a different filter chemistry; eukaryotic calcium-channel filters favour divalent calcium through an acidic coordination environment.

A ligand-gated ion channel instead couples ligand occupancy to changes in pore-opening probability. The nicotinic acetylcholine receptor is a pentamer with extracellular ligand-binding interfaces and a pore lined by M2 helices. Acetylcholine binding favours an opening-associated conformational change transmitted through the extracellular-to-transmembrane interface. Its pore conducts cations, including sodium and potassium, rather than selectively conducting only the transmitter or only sodium. Negatively charged regions and pore geometry favour cation over anion permeation. The ligand-binding site and the ion-selective pathway are spatially distinct, so the chemical identity of the bound transmitter does not define the permeant ion.

Rapid agonist exposure followed by freezing of Torpedo receptor membranes captured an opening-associated conformation, experimentally connecting ligand application to a receptor structural state.^[ref]

Other ligand-gated channels illustrate the independence of these properties: GABA-A receptors primarily support anion conductance, while glutamate-receptor channels support cation conductance with subtype-dependent calcium permeability. A sustained agonist can produce channel desensitization, in which an occupied receptor becomes nonconducting; occupancy is therefore not a permanently open instruction. Intracellular ligands can gate other channels, including ATP-dependent regulation, so “ligand-gated” need not imply an extracellular neurotransmitter.

The decisive tests separate binding, gating and permeation. Patch clamp resolves individual opening and closing events; rapid ligand application measures activation and desensitization; ion substitution and reversal-potential measurements constrain relative permeability. Changing a filter residue while retaining agonist binding and surface expression can implicate selectivity, while changing binding-site affinity without changing unitary conductance suggests a different role. Appropriate controls include leakage currents, solution junction potentials, channel abundance and ionic activities. A structure supplies candidate mechanisms; state-dependent electrophysiology establishes what those mechanisms do.

In classical ball-and-chain inactivation, a cytoplasmic N-terminal part of the voltage-gated potassium channel acts as a tethered blocking particle. Once the pore opens, this particle reaches and occludes its intracellular entrance. The flexible connecting segment allows access; removal of the block permits recovery. Thus opening and subsequent loss of conduction can occur even while the activating voltage remains applied.

Several independent observations support this mechanism. Removing or disrupting the N-terminal inactivation segment suppresses fast ion-channel inactivation while leaving activation possible. An appropriate soluble peptide applied from the cytoplasmic side restores an open-channel block in otherwise noninactivating channels. Changes in the blocking segment alter block kinetics, while changing its access or tether can alter how rapidly it finds the pore. Rescue in trans is particularly informative: the blocking particle need not remain covalently attached to produce the effect.^[ref][ref]

This mechanism is N-type inactivation. It should not be generalized to every potassium ion channel: C-type inactivation instead involves changes associated with the outer pore and selectivity filter.

A typical voltage-gated potassium channel has four subunits, each with six membrane-spanning segments. S1–S4 form a voltage-sensing domain; S5–S6 and their intervening loop form the pore and selectivity filter. Conserved positively charged side chains along S4 move through part of the membrane electric field when the voltage changes. Countercharges and hydrated crevices in the surrounding sensor help make this movement possible. Coupling between the sensor and pore, including the S4–S5 region and S6 gate, changes the probability of opening. A voltage-gated sodium channel usually combines four related domains in one large protein, rather than four identical independent proteins.

The evidence separates charge movement, structural arrangement and functional coupling. A transient ion-channel gating current can persist when permeating ionic electric current is suppressed: some charged parts of the channel itself move. Comparing integrated ion-channel gating charge with channel opening tests their coupling. Charge-neutralization experiments change both total moved charge and voltage dependence, implicating S4 basic residues and countercharges elsewhere; they do not show that S4 alone supplies every charge. ^[ref]

Crystal structure of a mammalian voltage-dependent Shaker family K+ channel by Stephen B. Long and others supplies structural evidence for the arrangement. Voltage-dependent accessibility and optical reports of sensor motion provide complementary functional tests. No one static structure is a movie of gating, and a perturbation that abolishes current can disrupt folding or pore function rather than selectively remove voltage sensing. Agreement between structural, electrical and motion-sensitive measurements supports the mechanism more strongly than any one observation alone.

Ion-channel deactivation and ion-channel inactivation are different. Ion-channel deactivation closes an activation gate when the voltage returns toward rest. Ion-channel inactivation makes a channel unavailable even while depolarization remains favorable for activation. A minimal kinetic picture is a closed state, an open state and an unavailable state, with recovery required before reopening. This helps terminate inward electric current and produces a neuronal refractory period.

In rapid voltage-gated potassium channel N-type inactivation, a cytoplasmic terminal segment, sometimes supplied by an auxiliary subunit, blocks the open inner pore. The classic ball-and-chain inactivation picture is supported by loss of rapid ion-channel inactivation after removing the relevant terminal segment and restoration by an appropriate soluble peptide. This links a mobile structural element to open-pore blockade; it does not imply that all potassium channels use that mechanism.^[ref]

Slower C-type inactivation involves rearrangement near the selectivity filter and external pore. Dependence on pore structure, ion occupancy and extracellular ions distinguishes it from a simple cytoplasmic plugging particle. Different channels can combine several unavailable states, so a single recovery time constant need not describe every trace.

Fast ion-channel inactivation in canonical animal voltage-gated sodium channels involves the intracellular linker between domains III and IV, including its hydrophobic IFM motif, and is coupled to domain-IV voltage-sensor activation. The modern structural interpretation is an allosteric latch acting on the gate, rather than a universal literal particle sitting in the ion pathway. The older hinged-lid analogy is useful only with that qualification.^[ref]

Functionally, a voltage clamp trace shows electric current declining during a maintained step; a second test pulse measures recovery of availability. Holding-potential dependence separates an altered fraction of available channels from a mere change in maximal open electrical conductance. Persistent electric current after defective ion-channel inactivation, or delayed recovery that reduces subsequent responses, illustrates why both entry into and exit from unavailable states matter for excitability. A decaying macroscopic electric current alone does not uniquely identify the molecular mechanism.

The sodium trace is a rapid inward current after depolarization, followed by loss of current even while voltage remains depolarized. A voltage-gated sodium channel therefore activates rapidly and then becomes unavailable through inactivation. Under ordinary cardiac ionic gradients, the test potential is below the sodium reversal potential, so opening carries inward positive charge. Rapid sodium entry supports the fast action-potential upstroke and propagation in working myocardium; inactivation and subsequent recovery contribute to refractoriness.

The potassium trace develops more slowly, is outward during the positive step, and remains approximately sustained until repolarization. It is consistent with a delayed outward potassium conductance: activation counteracts depolarization, and deactivation follows the return towards the holding voltage. Potassium currents contribute to repolarization and stable diastolic voltage. The trace does not identify a unique cardiac potassium-channel subtype, and not all potassium channels lack inactivation.

Together, fast inward activation initiates excitation and slower outward activation terminates it. Current direction is set by electrochemical driving force as well as gating: $I=N_{\mathrm{ch}}{g}_{\mathrm{single}}{p}_o(V_m-E_{\mathrm{ion}})$ in an ohmic approximation. A voltage clamp fixes $V_m$ and reveals those currents; in a freely firing cell the currents instead jointly determine the voltage through the membrane-capacitance equation. Calcium current is also important for the cardiac plateau, so these two traces alone are not a complete ventricular action-potential model. The sodium-potassium pump maintains long-term gradients but is not the fast gating mechanism seen in these recordings.

Voltage gating and ion selectivity are different structural functions. A typical voltage-gated potassium channel has four pore-forming subunits; their voltage-sensing regions move charged residues in the membrane electric field and couple that motion to a pore gate. Carbonyl coordination in the selectivity filter compensates potassium dehydration in an ion-specific geometry. The smaller sodium ion is not automatically passed more readily: its hydration and coordination energetics differ. Structural work on a mammalian Kv channel resolves the sensor/pore architecture and an open activation gate.^[ref]

A sodium-channel alpha subunit contains four homologous domains assembled into one pore, with separate voltage sensors. Rapid activation and fast inactivation are distinguishable conformational transitions; an intracellular inactivation apparatus stabilizes nonconducting states after activation. Repolarization favours recovery of availability. A channel can therefore be closed but activatable, or inactivated and temporarily unavailable, despite zero current in both cases.

Several types of evidence test these ideas. Ionic reversal potentials shift when ionic gradients are changed, establishing selectivity and driving force. Single-channel recordings show discrete conducting levels and stochastic dwell times; whole-cell current changes often reflect changed open probability rather than a continuously varying single-channel current. Small gating-charge transients can remain when ionic permeation is blocked, demonstrating sensor movement rather than ion flux. Conditioning and paired-pulse records separate activation, inactivation and recovery. Structural perturbations of voltage sensors and of inactivation machinery alter different kinetic properties, supporting their functional distinction.

These observations constrain models but do not make an individual macroscopic trace a unique structural reconstruction. Different distributions of closed and inactivated states can produce similar averages. In particular, the sustained potassium trace supports little inactivation on the displayed interval; it does not justify assigning every potassium channel a universal absence of an inactivation mechanism.

Represent one permissive voltage-gated potassium channel gate by an available closed state and an activated state, with voltage-dependent transition rates ${\alpha }_n(V)$ and ${\beta }_n(V)$. Gain minus loss givesDuring a voltage clamp step the imposed voltage and hence these rates are constant. Integration with $n(0)=n_0$ givesThe fourth power in the electrical conductance describes four permissive activation variables in the Hodgkin-Huxley model phenomenology. If all must be permissive and are statistically independent, their joint probability is $n^4$. Even a simple exponential rise in $n$ then gives a delayed-looking macroscopic potassium electric current. The second panel of the model figure in solution illustrates this. A continuously changing voltage makes the rates time dependent and requires solving the differential equation along that voltage trajectory; one fixed exponential is not the general action-potential solution.

selective permeability sets the driving-force term. Potassium and sodium channels provide narrow coordinating pathways whose free energy balance of hydration and chemical binding favours the appropriate ion. voltage-gated potassium channel carbonyl coordination differs from the charged filter environment in canonical sodium channels, so selective permeation is not a pore-size ranking. Ion substitution shifts the reversal potential, and selective blockers or filter mutations alter the corresponding electric current. An open channel conducts down the electrochemical gradient; ATP-driven pumps maintain the gradients rather than pushing each ion through the pore.

Voltage-dependent activation sets channel availability. Charged residues in the S4-containing voltage-sensing domains move in the membrane electric field and couple to pore opening. A classical voltage-gated potassium channel is a tetramer; a voltage-gated sodium channel pore-forming polypeptide has four homologous domains. ion-channel gating current measurements and sensor-residue mutations connect electric charge motion to opening, while structures separate the sensor from the permeation pathway. The historical $n^4$ and $m^3$ variables describe kinetics phenomenologically and need not map one-to-one onto individual resolved structural movements.^[ref][ref]

Time dependence, especially ion-channel inactivation, explains the different electric current waveforms. Sodium activation is rapid, but sustained depolarization reduces the availability variable $h$, so the sodium electric current decays despite maintained driving voltage. Fast sodium ion-channel inactivation involves intracellular machinery including the interdomain III–IV region. Potassium activation is slower, supplying delayed outward electric current; the classical delayed rectifier is relatively sustained on this time scale. Some voltage-gated potassium channel variants do show rapid N-terminal pore-blocking ion-channel inactivation or slower filter-associated ion-channel inactivation, which should not be assumed identical to sodium's mechanism. voltage clamp steps separate activation from decay; recovery protocols, single-channel recordings and targeted perturbations distinguish gating, ion-channel inactivation and ion conduction. Removal of the Shaker N-terminal blocker and restoration by a free peptide are particularly direct evidence for one voltage-gated potassium channel mechanism.^[ref][ref]

Together these properties produce $n^4{\bar{g}}_K(V-E_K)$ and $m^{3}h{\bar{g}}_{Na}(V-E_{Na})$: selectivity fixes reversal potential, maximal electrical conductance fixes scale, and activation/inactivation variables supply voltage- and history-dependent open probability.

At the initial negative membrane potential, most voltage-gated potassium channels have a closed activation gate. Depolarization moves charged residues in each voltage-sensing domain, particularly the S4 helices, through the electric field. Coupling between the voltage-sensing domains and the pore opens the intracellular gate. A classical voltage-gated potassium channel has four subunits, each with six transmembrane helices; S5, the pore loop and S6 contribute to the central pathway and selectivity filter. The rising open probability produces increasing outward potassium electric current, since the imposed positive membrane potential is above the potassium Nernst equilibrium potential. Here outward electric current is positive: $I_K=N\gamma p_O(V-E_K)$, where $N$ is channel number and $\gamma$ is single-channel electrical conductance.

The electric current subsequently falls because the rapidly inactivating Shaker form exhibits ball-and-chain inactivation. An intracellular N-terminal domain, tethered to the channel, blocks the open pore. The ion channel can therefore stop conducting while the membrane potential remains depolarized and the activation machinery remains activated. This is ion-channel inactivation, rather than reversal of the voltage step, loss of the ionic driving force, or ordinary gate closure. Repolarization favors removal of the block and recovery of channel availability. Slower C-type inactivation involving the outer pore can coexist, but is not the principal explanation of the rapid decay described here.

Several independent observations connect this functional explanation to structure. Small transient gating electric currents report movement of sensor electric charge before ionic conduction; charge-neutralization studies of S4/S2 change the measured gating charge and activation. Structural studies resolve separate sensor and pore regions.^[ref][ref] Historical N-terminal removal abolished rapid ion-channel inactivation while preserving activation, and the corresponding free peptide restored block from the cytoplasmic side. These results separate the activation gate from the tethered blocker and identify which side of the cell membrane contains the blocking domain.^[ref][ref]

Source-gap TODO: the supplied PDF's figure invokes an undefined image object, so its actual pulse duration, pulse sequence, current scale, peak amplitude, decay time and any tail-current waveform cannot be read. The account above uses only the stated change from $-60$ to $+20\mathrm{mV}$ and the stated rise and rapid decay; no numerical kinetic fit or reconstruction of the missing trace is claimed.

The rising sigmoid shows activation by depolarization: $P_o$ is almost zero at very negative membrane potential, rises steeply around $-20\mathrm{mV}$, and approaches about $0.8$ at positive membrane potential. The graph describes the fraction of time a single ion channel is open, not a continuously variable opening amplitude. A convenient phenomenological fit isHere $V_{1/2}$ refers to half the maximum, about $0.4$, not absolute $P_o=0.5$. The effective gating charge describes the steepness; a sigmoid alone does not identify the microscopic gating sequence or explain why the observed maximum is below one.

A voltage-gated potassium channel has four pore-forming subunits. Each canonical subunit contains an S1–S4 voltage-sensing domain and S5–S6 pore elements. Positive charges in S4 respond to the electric field; their displacement during depolarization changes the energetic preference of sensor conformations. Coupling through the sensor–pore interface favors opening of the intracellular pore gate. The potassium selectivity filter is structurally distinct from this voltage sensor.

Several independent observations support this account. Voltage clamp detects transient gating electric currents before pore conduction, even when ionic conduction is blocked: charge movement precedes opening. Charge-neutralization studies change total gating charge and voltage dependence, with contributions from charged S4 residues and an S2 residue. Accessibility and voltage-sensitive optical measurements support conformational motion rather than permeating potassium itself acting as the sensor. Structural measurements locate the charged sensor beside the pore. These observations constrain a coupled moving-sensor mechanism, while the exact displacement cannot be deduced from this $P_o$ curve alone.^[ref][ref]

In ball-and-chain inactivation, also called N-type inactivation, a flexible intracellular N-terminal region carries an inactivation particle. Once a voltage-gated potassium channel opens, this tethered particle can enter and occlude the inner pore, stopping electric current even while the activation machinery favors an open configuration. Recovery requires the particle to leave, after which the activation gate can return to its resting conformation. This supplies an additional unavailable state, not merely reversal of activation.

The decisive evidence separates necessity from sufficiency. Removing the relevant N-terminal region greatly reduces fast ion-channel inactivation while preserving activation. Applying a peptide corresponding to the missing N-terminal region on the cytoplasmic side restores blocking with similar properties. Changes in the blocking region alter blocking kinetics, and altering the tether affects the rate at which the particle reaches the pore. Open-pore access and competition with intracellular pore blockers support occlusion of the inner cavity. A tether makes the particle's local effective concentration high, explaining rapid intramolecular block.^[ref][ref]

This mechanism is distinct from slower filter-associated C-type inactivation. An N-terminal particle is a mechanism for certain potassium ion channels, not a universal explanation of all ion-channel inactivation.

In a classical action potential, delayed activation of voltage-gated potassium channels supplies outward electric current that drives repolarization. In the Hodgkin-Huxley model, for example, $I_K={\bar{g}}_K{n}^4(V-E_K)$ and $\dot{n}={\alpha }_n(V)(1-n)-{\beta }_n(V)n$. Increasing the opening rate at depolarized membrane potential, other properties fixed, recruits outward electric current earlier: the action potential generally becomes narrower and may have a smaller peak. Slower opening delays repolarization and broadens the action potential.

Slower closing after repolarization retains excess potassium electrical conductance, lengthening the after-hyperpolarization and often the interval of reduced excitability. Faster closing shortens this tail and permits earlier recovery. If activation or the available electrical conductance is insufficient, normal repolarization may fail; changing rates can also change the steady open fraction. Duration is therefore determined by the whole balance of inward and outward electric currents, not by a closing rate in isolation.

A voltage-gated potassium channel couples a voltage-sensing region to a potassium-selective pore. The sensor determines voltage-dependent opening, whereas filter coordination chemistry contributes to which ions permeate.