Voltage gated channelsMolecular structureNa+, K+, Ca++Cl-Voltage sensingAction potentialCalcium signaling

Core voltage-gating functional unit6 transmembrane segmentsOne chargedPore facingIon selectivity & V-dependenceTetrameric organization4x separate, 6 pass proteins1 protein with 4, 6 pass domainsTransmembrane domainPDB: 2r9rPotassium channel has 4 separate subunits

Voltage gated sodium channelIon selectivity and voltage sensitivity from S4 helicesLong cytoplasmic loops btw domainsIntracellular domains subject to modificationConductivityOpen probabilitySodium channel has 4 functional domains

Domain organizationCommon prokaryotic ancestorS5-S64 subunit/domainPore forming motifOrganizationS1S2S3S4S5-6Canonical subunitK+ structure

Voltage sensingTransmembrane potential stabilizes S4S4 moves S5/S6Pore open/close213456

Chloride ChannelDouble barreled, 2 subunit channelEach subunit has 3 charged helices with anti-parallel arrangement forming V-sensorPDB: 1kpl

Whole cell recordingClamp voltage Record currentAggregate channel activity & densityG=1/R=I/VDerived I-VDerived ConductanceRectification:Current diverges from straight-line conductanceCOVState Model

Channel InactivationFeedback mechanismMembrane depolarizationReduces driving forceSecondary conformational change

DepolarizationVoltage stepsPreconditionedDepolarizedChannel opens with depolarizationChannel becomes refractory with depolarizationCOVIState Model

State transitions with voltage clamp

Characteristics of voltage gated channelConductanceIon selectivityThresholdOpen timeInactivation time

Anatomy of Action potentialVoltage gated channels selectively drive intracellular potential between different ionic equilibrium potentialsK+ -90mVNa+ +60mVThreshold for V-gated Na+ channelsNeural APCardiac AP

Ionic currents in APStep voltage to increasing depolarizationNet currentNa+ currentK+ currentSub-thresholdDepolarizing currentInactivatesLarge depolarization opens new K+ channelsDelayed rectifier

Ionic currents in APCurrent declines over time, even though potential remains constant

Ionic contributions to APKleak (Kir) set resting potentialInactivate at thresholdNaVOpen at thresholdRapid, large gKVOpen at thresholdDelayed rectifier (slow)Large g

Anatomy of Cardiac APLeaky membranes (7) give slow depolarization toThreshold opens CaV (3) & NaV (1)KV (4) and KCa repolarizeProlonged AP vs neuronCa currentMuch delayed K+

NaV causes local depolarizationMembrane capacitance of 10-6 F/cm210-6 (p r2)Na influx: n (1.6 10-19 C)Threshold ~-40mVV=Q/C

20-30 channels/micron2~ 400 ions/channel to depolarize neighborsNa+r-90 mV-40 mV104 ions/mm2

Equivalent CircuitBorrowed from cable theoryBreak cell into parallel compartmentsPropagation depends on resistance/capacitanceExtracellularCmCmRmRmRiIntracellular

Neural cable theoryNeuron size vs conduction velocityLarge diameter, low internal resistanceMyelinated/UnmyelinatedInsulates membraneIncreases RMDecreases CMIncrease VNode of Ranvier

NaV Modulation10 genesAlternative splicingPhosphorylationProtein binding

AltersThresholdConductivityKineticsSelectivityCnRPTPPKAPKCPKAPKCCnRPTPPhosphatases increase conductionKinases decrease conduction-28 identified binding partnersCytoskeletalAdhesionSignaling

Calcium channelMost common effector of APSame basic structure as other VG channelsMajor classesN-type NeuronalL-Type LongT-Type Tiny

NeuronsIonotropic = channelsMetabotropic = receptorsNeurotransmitter release depends on [Ca2+]IMultiple inputsNerve terminals & presynaptic vessicles

N-type calcium channelsNeurotransmitter release (presynaptic)Calcium dynamics same time scale as firing (10 ms)Highly localized changes (50-5000 nm)Post-synaptic, Ca-dependent remodeling

Striated MuscleCardiacSkeletalTwitch force50-200 msAll-or-noneTension depends on [Ca2+]ISpontaneousNeural

L-type calcium channelsExcitation contraction couplingLong open time (100 ms)ModulationCalcium dependent inhibitionOxidationPhosphorylation

T-Type calcium channelsTiny conductance (6 vs 25 pS)Low threshold (-50 vs -30 mV)Regulatory roleCell differentiationModulation of phenotypeNeuronal bursting

Smooth muscleTonicVascularRespiratoryPhasicGIBladderTension depends on [Ca2+]IHormonalMechanicalNeuralSmooth muscle cells in vasculature, gut, sphincter

Smooth Muscle CalciumLigand gated Ca channelsVoltage gated Ca channelsSecond messengers