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An availability change associated with the outer pore and selectivity filter of certain potassium ion channels. Its kinetics and structural origin differ from ball-and-chain inactivation, although the two processes can interact.

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.

An ion channel is a selective permeation pathway, not simply an aqueous hole of fixed diameter. A hydrated ion entering a narrow selectivity filter must lose or reorganize some hydration water. The energetic cost can be offset by interactions with charged or polar groups in the pore. Different ionic radii, valences, hydration free energies, and coordination geometries therefore produce different permeation barriers and binding affinities.

A classical potassium selectivity filter presents oxygen ligands that stabilize relatively dehydrated potassium in configurations resembling its aqueous coordination. Smaller sodium does not automatically pass more easily: its hydration is stronger and the filter need not provide a compensating configuration at equally low free energy. Filter flexibility and multi-ion occupancy matter, so the explanation is an energetic balance, not the assertion that a rigid hole simply blocks every larger or smaller sphere. Ion-ion interactions and coupled displacement can permit rapid conduction while maintaining selectivity.^[ref]

Other ion channels use different arrangements. Fixed negative groups can strongly favor divalent calcium over monovalent ions; occupancy and competition then influence both selectivity and throughput. Selectivity is distinct from activation or ion-channel inactivation, which control access or availability. Selective passive flux follows an electrochemical potential gradient; it does not by itself pump ions uphill. Selectivity allows cells to establish particular membrane potentials, generate action potentials, regulate calcium signals, and control solute balance without dissipating every ionic gradient whenever one pathway opens.

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.

The average electric current is the number of channels times their ion-channel open probability times the electric current carried by an open channel:Consequently the identifiable answer isTODO (missing input): The source does not supply $V_K$ or the potassium thermodynamic activities for this experiment. A unique numerical electrical conductance cannot follow from the listed electric current, count and ion-channel open probability alone. For the explicitly additional choice $V_K=-90$ mV, ${\gamma }_K=125$ pS. Taking $V_K=-100$ mV instead gives $100$ pS: both match the measured electric current. The table in question 3 describes a skeletal-muscle resting state, and cannot silently supply the conditions of this separate recording.

Rapid permeation is possible because the hydrated pore and coordinating groups of the selectivity filter provide a favorable free-energy landscape for repeated ion transit. Several ions can occupy the filter and their interactions favor onward passage, while the channel remains open for many successive transits. Selectivity need not mean that each transported ion remains tightly trapped for a long time. This is passive downhill transport; it does not consume one ATP molecule per ion crossing.

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.

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.

The hydrophobic core of a lipid bilayer makes transfer of a hydrated ion into bare lipid energetically costly. An ion channel supplies a water-accessible route across that barrier. Its narrow selectivity filter presents coordinating chemical groups that compensate for loss or rearrangement of the ion's hydration shell. Selectivity depends on the free-energy balance of hydration, coordination, electrostatics and geometry, rather than a sieve that passes all smaller ions.

A potassium-channel filter presents backbone carbonyl oxygens in a geometry favourable for potassium; smaller sodium does not automatically gain the same energetic compensation. Sodium and calcium channels use different filter architectures: charged side chains and their coordination environments favour particular ions, with ion occupancy and competing species also affecting permeation. In canonical vertebrate voltage-gated sodium channels, the DEKA selectivity ring illustrates chemical discrimination distinct from potassium-channel carbonyl coordination. The precise mechanism therefore depends on the channel family.

Rapid flux requires low successive free-energy barriers, not permanent binding of the preferred ion. Several occupied sites, nearby water and ion–ion interactions can facilitate passage without requiring the ion to cross the lipid. For scale, a $100\mathrm{pS}$ open channel driven by $100\mathrm{mV}$ carries $10\mathrm{pA}$, approximately $6\times 10^7$ monovalent ions per second. A nanometre-scale route with favourable coordination can support such rapid, thermally driven motion. An ion channel is not an ATP-driven pump: net flux follows the electrochemical potential difference,which vanishes at the species' Nernst equilibrium potential. ATP-consuming pumps maintain the gradients that make sustained channel currents possible.

Permeation selectivity and opening probability are separate properties. Voltage-sensor movement, ligand binding or mechanical tension can regulate access to the pore, while inactivation can terminate conduction during a maintained stimulus. Patch-clamp unitary currents establish discrete conducting states; reversal potentials under changed ionic compositions quantify selectivity. Structure determination locates the filter and gates, and mutations or specific blockers test their functional roles. Agreement between these measurements is stronger evidence than structure or current shape alone.

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.

An ion channel supplies a water-accessible pathway through the otherwise strongly insulating, hydrophobic lipid bilayer. Moving a hydrated ion directly into that low-permittivity region is energetically expensive. The channel's polar pore and coordinating groups reduce the barrier while retaining selectivity. Transport is passive: the electrochemical potential difference drives net movement, and the channel does not itself pump an ion uphill. For an ideal species of charge number $z$,The net electric current of a pathway permeable only to that species reverses at its Nernst equilibrium potential. An open channel can carry zero net electric current at that voltage even while individual ions cross both ways.

The selectivity filter balances loss of the hydration shell against favorable coordination within the pore. In a typical potassium channel, backbone carbonyl oxygen atoms create appropriately arranged binding sites for a substantially dehydrated potassium ion. The smaller sodium ion is not automatically preferred simply because it fits geometrically: its hydration is stronger and the filter does not compensate its dehydration in the same way. Geometry, flexibility, coordination chemistry and ion occupancy all matter. Sodium-selective and calcium-selective channels use different arrangements, often involving charged side chains; a single rigid-sieve explanation is inadequate.

Rapid passage coexists with transient binding because several nearby sites have comparable free energies and relatively low barriers between them. Multiple ions occupying a pore influence one another through electrostatic interactions, enabling correlated exchange and displacement rather than irreversible trapping at one deep site. High throughput therefore requires favorable passage kinetics as well as strong discrimination against unwanted species. A more tightly bound species can even block a pathway instead of being its fastest permeant.

Gating and selectivity are distinct. A voltage-sensing domain couples membrane potential to activation-gate conformation; ligand-gated ion channels couple binding to opening; mechanosensitive ion channels couple membrane stress to opening. Ion-channel inactivation introduces additional unavailable states. None of these mechanisms alone identifies which ion the open pore prefers.

For $N$ similar independent channels, a local Ohmic approximation with outward-positive electric current isHere open probability controls availability, single-channel electrical conductance controls current per open pore, and $V-E_{\mathrm{rev}}$ is the electrical driving factor in this approximation. Nonlinear current-voltage relations and mixed-ion permeation may require a fuller model. Single-channel recording resolves discrete openings; reversal shifts when ionic concentrations change test selectivity; gating electric currents test sensor motion; structural evidence identifies the pore and its coordinating groups. Together these distinguish rapid selective permeation from the separate process of opening the gate.

A selectivity filter is a channel-pore region that preferentially stabilizes permeating ions through geometry and chemical interactions. Selectivity balances dehydration and coordination energies rather than following pore diameter alone.