Electrophysiology cheat sheet.Electrical termUnitsUnitssymbol typicalneuronal DescriptionCharge (Q) CoulombsC nCProperty of matter that causes it to experiencea force in an electromagnetic fieldCurrent (I) Amperes A pA Flow of charge per unit timeResistance (R) Ohms Ω MΩ Resistance to current flowConductance (G) Siemens S pS Reciprocal of resistance.Capacitance (C) Farads F pF Ability to separate, store chargeVoltage (V, E) Volts V mV A separation of charge, potential differenceFor intracellular recordings, look at the calibration bars in a figure. If the vertical calibration is current (pA, nA, µA), the recording is a voltage clamp recording. If the vertical calibration is voltage (mV), the recording is current clamp.In current clamp, the experimenter controls current, measures voltage (membrane potential). In voltage clamp the experimenter controls voltage, measures current.In voltage clamp, downward current =negative current, =inward current, =depolarizing current at the clamped membrane potential. Upward current = positive current, =outward current, =hyperpolarizing current at the clamped membrane potential.Convention of current polarity in voltage clamp is easier to remember if you think of the plotted current as what the amplifier is doing to keep Vm clamped, i.e., to counteract ion flow across the membrane. For instance a sodium current at -20 mV carries positively charged sodium into the cell from outside. This cation influx would normally depolarize the cell away from -20 mV. To keep Vm clamped at -20 mV, the amplifier must pass currentexactly equal and opposite (negative, downward-plotted) current. The physiologist’s version of Ohm’s Law is very handy: Iion = Gion * (Vm-Eion)So, if the amount of current flowing changes, the change could result from a change in Gion (# channels conducting * unit conductance of each channel) or a change in driving force (Vm-Eion)Under physiological conditionsENa ~ +60 mV, ECa ~ +80 mV, ECl ~ -70 mV (important exceptions exist), EK ~ -90 mVErev for glutamate ionotropic or nicotinic receptors ~ 0 mVErev for GABAA receptors ~ -70 mV (important exceptions exist)Erev for a resting neuron = the resting potential ~-65 mV (but varies) A Nernst potential corresponds to a single ion species. A reversal potential is a more general term that includes the zero-current potential for mixed conductances (e.g., Na+ and K+ through glutamate receptor channels or through Na+, K+ and Cl- through resting leak channels)A few advantages of voltage clamp:- Because of the explosive nature of the action potential it is very difficult to measure contributions of time vs. voltage to changes in membrane selective permeability. By clamping the membrane potential to a fixed value, the time course of development of a voltage-gated conductance can be studied at a particular membrane potential.- Contributions of Cm to slowing Vm changes are eliminated because dV/dt = 0 (remember that Ic = C*(dV/dt)). Therefore, ionic currents (the currents through channels) are isolated.- For isolating synaptic conductances, clamping Vm to a fixed value allows you to isolate signals generated by transmitter gated conductances, without contamination from voltage-gated channels that are not of direct interest.Lecture supplementsSome helpful YouTube videos and other resources for basic electrophysiology concepts. Chapters refer to “From Neuron to Brain” 5th editionUT Health Sciences Center Houston, Online textbook with some lectures. http://nba.uth.tmc.edu/neuroscience/s1/index.htmEstablishment of the resting potential, role of potassiumhttps://www.youtube.com/watch?v=YTXv_zEkFsUhttps://www.youtube.com/watch?v=MtstzOM5G9cChapter 6. Action potentialsInitiation: https://www.youtube.com/watch?v=iSn_g9genlIPropagation: https://www.youtube.com/watch?v=D8zUGVxeNSsChapters 7, 5. Archival videos of squid giant axon, voltage clamphttps://www.youtube.com/watch?v=omXS1bjYLMIhttps://www.youtube.com/watch?v=k48jXzFGMc8Voltage clampinghttps://www.youtube.com/watch?v=wAG dvuZ6fwUPatch clamp configurations. https://www.youtube.com/watch?v=U2UnLoxMjAMSingle-channel analysis, vintage clip from Fred Sachshttps://www.youtube.com/watch?v=ahRSOvdGyeUReview of the exponential functionhttps://www.youtube.com/watch?v=oo1ZZlvT2LQSee reading: Chapter 2.1 from Gutfreund: Kinetics for the Life Sciences. Receptors, transmitters, and catalysts. Cambridge University Press.