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| Introduction | |
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| Channels and ions are needed for excitation | |
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| Channels get names | |
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| Channels have families | |
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| Ohm's law is central | |
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| The membrane as a capacitor | |
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| Equilibrium potentials and the Nernst equation | |
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| Current-voltage relations of channels | |
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| Ion selectivity | |
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| Signaling requires only small ion fluxes | |
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| Description of Channels | |
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| Classical Biophysics of the Squid Giant Axon | |
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| The action potential is a regenerative wave of Na[superscript +] permeability increase | |
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| The voltage clamp measures current directly | |
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| The ionic current of axons has two major components: I[subscript Na] and I[subscript K] | |
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| Ionic conductances describe the permeability changes | |
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| Two kinetic processes control g[subscript Na] | |
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| The Hodgkin-Huxley model describes permeability changes | |
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| The Hodgkin-Huxley model predicts action potentials | |
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| Do models have mechanistic implications? | |
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| Voltage-dependent gates have gating charge and gating current | |
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| The classical discoveries recapitulated | |
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| The Superfamily of Voltage-Gated Channels | |
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| Drugs and toxins help separate currents and identify channels | |
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| Drugs and toxins act at receptors | |
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| Gates open wide at the cytoplasmic end of the pore, and the pore narrows at the outside | |
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| Early evidence for a pore came from biophysics | |
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| There is a diversity of K channels | |
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| Voltage-gated Na channels are less diverse | |
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| Ion channels can be highly localized | |
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| Voltage-gated channels form a gene superfamily | |
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| The crystal structure shows a pore! | |
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| Patch clamp reveals stochastic opening of single ion channels | |
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| Recapitulation | |
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| Voltage-Gated Calcium Channels | |
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| Early work found Ca channels in every excitable cell | |
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| Ca[superscript 2+] ions can regulate contraction, secretion, and gating | |
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| Ca[superscript 2+] dependence imparts voltage dependence | |
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| Multiple channel types: Dihydropyridine-sensitive channels | |
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| Neurons have many HVA Ca-channel subtypes | |
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| Voltage-gated Ca channels form a homologous gene family | |
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| A note on Ca-channel nomenclature | |
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| Permeation and ionic block require binding in the pore | |
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| Do all Ca channels inactivate? | |
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| Channel opening is voltage-dependent and delayed | |
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| Overview of voltage-gated Ca channels | |
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| Potassium Channels and Chloride Channels | |
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| Fast delayed rectifiers keep short action potentials short | |
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| Slow delayed rectifiers serve other roles | |
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| Transient outward currents space repetitive responses | |
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| Shaker opens the way for cloning and mutagenesis of K channels | |
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| Ca[superscript 2+]-dependent K currents make long hyperpolarizing pauses | |
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| Spontaneously active cells can serve as pacemakers | |
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| Inward rectifiers permit long depolarizing responses | |
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| What are K[subscript ir] channels used for? | |
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| The 4TM and 8TM K channels | |
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| The bacterial KcsA channel is much like eukaryotic K channels | |
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| An overview of K channels | |
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| A hyperpolarization-activated cation current contributes to pacemaking | |
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| Several strategies underlie slow rhythmicity | |
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| Cl channels stabilize the membrane potential | |
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| Cl channels have multiple functions | |
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| Ligand-Gated Channels of Fast Chemical Synapses | |
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| Ligand-gated receptors have several architectures | |
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| Acetylcholine communicates the message at the neuromuscular junction | |
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| Agonists can be applied to receptors in several ways | |
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| The decay of the endplate current reflects channel gating kinetics | |
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| Fluctuation analysis supported the Magleby-Stevens hypothesis | |
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| The ACh receptor binds more than one ACh molecule | |
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| Gaps in openings reveal slow agonist unbinding | |
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| Agonist usually remains bound while the channel is open | |
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| Ligand-gated receptors desensitize | |
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| An allosteric kinetic model | |
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| Recapitulation of nAChR channel gating | |
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| The nicotinic ACh receptor is a cation-permeable channel with little selectivity | |
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| Fast chemical synapses are diverse | |
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| Fast inhibitory synapses use anion-permeable channels | |
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| Excitatory amino acids open cation channels | |
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| Recapitulation of fast chemical synaptic channels | |
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| Modulation, Slow Synaptic Action, and Second Messengers | |
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| cAMP is the classic second messenger | |
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| cAMP-dependent phosphorylation augments I[subscript Ca] in the heart | |
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| Rundown could be related to phosphorylation | |
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| cAMP acts directly on some channels | |
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| There are many G-protein-coupled second-messenger pathways | |
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| ACh reveals a shortcut pathway | |
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| Synaptic action is modulated | |
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| G-protein-coupled receptors always have pleiotropic effects | |
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| Encoding is modulated | |
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| Pacemaking is modulated | |
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| Slow versus fast synaptic action | |
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| Second messengers are launched by other types of receptors | |
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| First overview on second messengers and modulation | |
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| Sensory Transduction and Excitable Cells | |
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| Sensory receptors make an electrical signal | |
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| Mechanotransduction is quick and direct | |
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| Visual transduction is slow | |
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| Vertebrate phototransduction uses cyclic GMP | |
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| Phototransduction in flies uses a different signaling pathway | |
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| Channels are complexed with other proteins | |
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| Chemical senses use all imaginable mechanisms | |
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| Pain sensation uses transduction channels | |
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| What is an excitable cell? | |
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| Calcium Dynamics, Epithelial Transport, and Intercellular Coupling | |
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| Intracellular organelles have ion channels | |
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| IP[subscript 3]-receptor channels respond to hormones | |
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| Ca-release channels can be studied in lipid bilayers | |
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| The ryanodine receptor of skeletal muscle has recruited a voltage sensor | |
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| Voltage-gated Ca channels are the voltage sensor for ryanodine receptors | |
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| IP[subscript 3] is not the only Ca[superscript 2+]-mobilizing messenger | |
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| Intracellular stores can gate plasma-membrane Ca channels | |
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| The extended TRP family is diverse | |
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| Mitochondria clear Ca2+ from the cytoplasm by a channel | |
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| Protons have channels | |
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| Transport epithelia are vectorially constructed | |
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| Water moves through channels as well | |
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| Cells are coupled by gap junctions | |
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| All cells have other specialized intracellular channels | |
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| Recapitulation of factors controlling gating | |
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| Principles and Mechanisms of Function | |
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| Elementary Properties of Ions in Solution | |
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| Early electrochemistry | |
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| Aqueous diffusion is just thermal agitation | |
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| The Nernst-Planck equation describes electrodiffusion | |
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| Uses of the Nernst-Planck equation | |
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| Brownian dynamics describes electrodiffusion as stochastic motions of particles | |
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| Electrodiffusion can also be described as hopping over barriers | |
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| Ions interact with water | |
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| The crystal radius is given by Pauling | |
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| Ion hydration energies are large | |
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| The "hydration shell" is dynamic | |
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| "Hydrated radius" is a fuzzy concept | |
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| Activity coefficients reflect weak interactions of ions in solution | |
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| Equilibrium ion selectivity can arise from electrostatic interactions | |
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| Recapitulation of independence | |
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| Elementary Properties of Pores | |
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| Early pore theory | |
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| Ohm's law sets limits on the channel conductance | |
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| The diffusion equation also sets limits on the maximum current | |
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| Summary of limits from macroscopic laws | |
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| Dehydration rates can reduce mobility in narrow pores | |
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| Single-file water movements can lower mobility | |
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| Ion fluxes may saturate | |
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| Long pores may have ion flux coupling | |
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| Ions must overcome electrostatic barriers | |
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| Ions could have to overcome mechanical barriers | |
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| Gramicidin A is the best-studied model pore | |
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| Electrostatic barriers are lowered in K channels | |
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| A high turnover number is good evidence for a pore | |
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| Some carriers have pore-like properties | |
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| Recapitulation of pore theory | |
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| Counting Channels and Measuring Fluctuations | |
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| Neurotoxins count toxin receptors | |
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| Gating current counts mobile charges within the membrane | |
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| Digression on the amplitudes of current fluctuations | |
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| Fluctuation amplitudes measure the number and size of elementary units | |
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| A digression on microscopic kinetics | |
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| The patch clamp measures single-channel currents directly | |
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| Summary of single-channel conductance measurements | |
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| Thoughts on the conductance of channels | |
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| Channels are not crowded | |
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| Structure of Channel Proteins | |
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| The nicotinic ACh receptor is a pentameric glycoprotein | |
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| Complete amino acid sequences were determined by cloning | |
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| Ligand-gated receptors form a large homologous family | |
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| Determining topology requires chemistry | |
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| Electron microscopy shows a tall hourglass | |
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| A partial crystal structure shows a pentameric ring | |
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| Voltage-gated channels also became a gene superfamily | |
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| Are K channels tetramers? | |
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| Auxiliary subunits change channel function | |
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| KcsA is a teepee | |
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| Electron paramagnetic resonance probes structure | |
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| Kv channels have a lot of mass hanging as a layer cake in the cytoplasm | |
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| Excitatory GluRs combine parts of two bacterial proteins | |
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| Is there a pattern? | |
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| Selective Permeability: Independence | |
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| Partitioning into the membrane can control permeation | |
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| The Goldman-Hodgkin-Katz equations describe a partitioning-electrodiffusion model | |
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| Uses of the Goldman-Hodgkin-Katz equations | |
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| Derivation of the Goldman-Hodgkin-Katz equations | |
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| A more generally applicable voltage equation | |
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| Voltage-gated channels have high ion selectivity | |
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| Other channels have low ion selectivity | |
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| Ion channels act as molecular sieves | |
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| Selectivity filters can be dynamic | |
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| First recapitulation of selective permeability | |
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| Selective Permeability: Saturation and Binding | |
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| Ionic currents do not obey the predictions of independence | |
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| Simple models for one-ion channels | |
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| Na channel permeation can be described by state models | |
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| Some channels must hold more than one ion at a time | |
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| Single-file multi-ion models | |
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| Multi-ion pores can select by binding | |
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| Anion channels have complex transport properties | |
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| Recapitulation of selective permeation | |
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| What do permeation models mean? | |
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| Classical Mechanisms of Block | |
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| Affinity and time scale of the drug-receptor reaction | |
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| Binding in the pore can make voltage-dependent block: Protons | |
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| Some blocking ions must wait for gates to open: Internal TEA | |
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| Local anesthetics give use-dependent block | |
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| Local anesthetics alter gating kinetics | |
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| Antiarrhythmic action | |
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| State-dependent block of ligand-gated receptors | |
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| Multi-ion channels may show multi-ion block | |
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| STX and TTX are the most potent and selective blockers of Na channels | |
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| Some scorpion toxins plug K channel pores | |
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| Recapitulation of blocking mechanisms | |
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| Structure-Function Studies of Permeation and Block | |
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| Charges in the M2 segment help nAChR channels conduct | |
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| What can a charged residue do? | |
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| Channel blockers interact with M2 and M1 segments | |
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| Cysteine substitution can test accessibility of residues | |
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| The S5-S6 linker forms the outer funnel and pore in K channels | |
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| The S5-S6 linker forms the outer funnel and pore in Na channels | |
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| Divalent/monovalent selectivity depends on charge density and electrostatics | |
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| The S6/M2 segment contributes to the inner pore | |
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| Inward rectification is voltage-dependent block | |
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| Functions are not independent | |
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| Recapitulation of structure-function studies | |
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| Gating Mechanisms: Kinetic Thinking | |
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| First recapitulation of gating | |
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| Proteins change conformation during activity | |
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| Events in proteins occur across the frequency spectrum | |
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| Topics in classical kinetics | |
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| Additional kinetic measures are essential | |
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| Most gating charge moves in significant steps | |
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| A new round of kinetic models for Shaker K channel gating | |
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| For BK channels we need three-dimensional kinetic models | |
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| Na[subscript v] and Ca[subscript v] channels require more complex models | |
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| Channels can have several open states | |
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| Conclusion of channel gating kinetics | |
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| Gating: Voltage Sensing and Inactivation | |
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| Simple equilibrium principles of voltage sensing and charge movement | |
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| Early mutagenesis points to the S4 segment | |
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| The S4 segment does carry much of the gating charge | |
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| Several residues in S4 move fully across the membrane | |
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| Movements around S4 are observed optically | |
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| Recapitulation of voltage sensing | |
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| What is a gate? | |
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| Pronase clips inactivation gates | |
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| Inactivation is coupled to activation | |
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| Microscopic inactivation can be rapid and voltage-independent | |
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| Fast inactivation gates are tethered plugs | |
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| Fast inactivation of Na channels involves a cytoplasmic loop | |
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| Slow inactivation is distinct from fast inactivation: A new gate? | |
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| Recapitulation of inactivation gating | |
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| Modification of Gating in Voltage-Sensitive Channels | |
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| Many peptide toxins slow inactivation | |
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| A group of lipid-soluble toxins changes many properties of Na channels | |
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| Reactive reagents eliminate inactivation of Na channels | |
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| External Ca[superscript 2+] ions shift voltage-dependent gating | |
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| Surface-potential calculations | |
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| Much of the negative charge is on the channel | |
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| Surface-potential theory has shortcomings | |
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| Recapitulation of gating modifiers | |
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| What are models for? | |
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| Cell Biology and Channels | |
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| Channel genes can be identified by classical genetics | |
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| Expression of channels is dynamic during development | |
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| Transcription of nAChR genes is regulated by activity, position, and cell type | |
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| Channel mRNA can be alternatively spliced and edited | |
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| Channel synthesis and assembly occurs on membranes | |
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| Sequences on channel subunits are used for quality control | |
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| Membrane proteins can be localized and immobilized | |
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| nACh receptors become clustered and immobilized | |
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| Multivalent PDZ proteins cluster channels at glutamatergic synapses | |
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| Channels are sorted and move in vesicles | |
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| Recapitulation | |
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| Evolution and Origins | |
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| Channels of lower animals resemble those of higher animals | |
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| Channels are prevalent in eukaryotes and prokaryotes | |
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| Channels mediate sensory-motor responses | |
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| Channel evolution is slow | |
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| Gene duplication and divergence create families of genes | |
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| Proteins are mosaics | |
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| Speculations on channel evolution | |
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| Conclusion | |
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| References | |
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| Index | |