Action potentials. Conductances channels

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1 Action potentials Conductances channels

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4 Cole and Curtis AC Wheatstone bridge resistance decreased during action potential R1 & R2 divide one path, Rv (variable) and Ru divide the other Galvanometer between two nodes Ru = Rv x R2/R1 Goldman is like Nernst equation relative permeabilities of Na+ and K+ change

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6 Alumnus neuro research Joel Geerling, aldosterone from adrenal cortex regulation of sodium in the kidney sodium appetite Aldosterone sensitive neurons confocal microscopy & neuroanatomy.

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8 Passive spread At any place along the axon, a spike would depolarize the axon to threshold for a spike a certain distance ahead of it, and that distance depends on the square root of the radius. Spike at one place would also depolarize the axon behind it to threshold, but it does not generate a retrograde action potential because of the refractory period, explained (below) by the inactivation of the Na+ channel.

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13 Summary action potential depolarizes membrane ahead of it to threshold Passive, current leaks out through membrane resistance and capacitance space constant varies w/ square root of radius time constant independent of radius. invertebrates use giant axons myelinated axons also have faster propagation for larger axons.

14 Coding frequency of action potentials, not size since they are all-or-none sometimes action potentials come in bursts or at beginning of depolarization because of "adaptation"

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16 Hodgkin - Huxley relative opening & closing of channels "sodium pump" already established ion gradients oscilloscope graphs voltage as a function of time action potentials can also be listened to on a loud speaker activation, inactivation, voltage gating

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18 Notes from outline differential amplifier compares 2 voltages and puts out current operational amplifier is a differential amplifier to clamp voltage space clamp - really just do whole axon at once voltage clamp - change voltage then pump and monitor current needed to keep it there I - t curves

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21 Notes from outline divide into early and late components as I - V curve Ohm's law: E=IR axes of an I-V curve reversed slope is conductance = 1/R in units of Siemens (formerly "mho") ina = gna(v-vna) - driving potential

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23 Toxins tetrodotoxin (TTX) puffer fish sodium careful preparation is important sushi best if enough TTX left to make mouth numb saxitoxin from dinoflagellates (red-tides are "blooms" and filter feeding shellfish can become poisonous) dose of TTX show few sodium channels works only if applied to outside of axon potassium TEA (tetraethyl ammonium)

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25 summary resting potential is based on predominant K+ permeability then Na+ channels activate then Na+ channels inactivate then a late K+ channel activates action potential is based on Na+ and K+ there are MANY other channel types

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28 Notes from outline "heterologous expression" Xenopus oocytes inject exogenous mrna into clawed African frog egg Channels are at a low "concentration" (except in post-synaptic membrane) It takes little tetrodotoxin to block action potential so they are proteins that are not highly expressed.

29 Genetic approach channel mutants might be lethal conditional mutants like temperature sensitive permissive temperature restrictive temperature

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32 Ether a-go-go human homologue HERG = human ether-a-go-go-related-gene. HERG inactivates so quickly that it only opens after voltage is over. Contributes to long action potential in cardiac muscle. Long QT syndrome is mutation of HERG, QRS in EKG (electrocardiogram) is ventricular depolarization, T is repolarization.

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34 Shaker K+, A-type conductance sea slug Anisodoris, Drosophila fruit fly - Shaker mutant Tetramer makes channel, each component crosses membrane 6 times hydrophobic domains, S1-S6, inactivation is "stopper" on chain at N- terminal, voltage gating is S4 with + charged arginines or lysines every 3 or 4 amino acids Pore between S5 and S6, not hydrophobic.

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38 Chloride channels 12-transmembrane protein for Cl- Cl- cystic fibrosis transmembrane conductance regulator (CFTR) cystic fibrosis is most common genetic disorder in Caucasians (1/2000), lungs fill up with thick mucus. One presumes the channel is two components.

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40 Sodium channel now diverse (human 10 genes) electric eel Electrophorus electricus 600 V book 1820 amino acids - "pseudotetramer S4 - gating - positively charged (basic) arginine (R) or lysine (K)

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43 Notes from outline low current (1-2 pa), low conductance - 10 ps stopper to inactivate. different types of Na+ channels some are targets of local anesthetics benzocaine and lidocaine.

44 Potassium channels lots of them, 100 Leak (resting potential) 20 ps Delayed rectifier (repolarization of action potential) 10 ps Anomalous rectifier - maintain depolarization - cardiac, fertilization HERG human ether-a-go-go related gene

45 Calcium channels 16 genes Ca2+ regulation by parathormone, calcitonin and vitamin D synaptic terminal vesicle release Ca2+ channel is receptor for IP3 (inositol trisphosphate "second" messenger) on smooth endoplasmic reticulum Ca2+ channel in muscle sarcoplasmic reticulum Ca2+ channel in t- (transverse-) tubule in muscle

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49 Genetic diseases myotonia (stiffness too much excitation) Cl- paralysis from Ca2+ channel defect CSNB from Ca2+ defect Congenital stationary night blindness myotonia, paralysis or stiffness from Na+ channel Long QT syndrome from Na+ or K+ channel

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52 Notes from outline selectivity by pore size interesting that non-hydrated ion passes. Hydrated - size is inverse Li > Na > K > Rb > Cs (lyotropic series)

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