axon guide

axon guide third edition

authors: Molecular Devices

Chapter I: bioloelectricity

• electric properties of cells come from the properties of the membrane

  • membrane gets properties from lipids and proteins like ion channels

• the electrical potential difference is between the interior and exterior of the cell

  • an ion loses energy as it moves down its gradient

• units of volts

• transmembrane potentials are generated by difference across the membrane established by pumps

  • usually less than 0.1 V in animal cells (less than 100mV)

• electrical current is the flow of charge

  • units of amperes

• currents always flow in complete circuits

  • can flow through capacitors, resistors, ion channels, amplifiers, electrodes, etc but must be in complete circuit

• current is conserved at branch points

  • if current $I_{total}$ splits into $I_1$ and $I_2$ then $I_{total}=I_1+I_2$

• resistors = barrier to current flow

• conductors = pathway for flow

• resistance is the inverse of conductance in Ohms law

• parallel conductances sum

  • for example with ion channels, when multiple are open simultaneously, the total conductance is the sum of the individual open channels

• the reversal potential of a channel equals the nernst potential for the permeant ion

• Ohm’s law

  • the potential difference between two points linked by a current path with a conductance and current is
    • $\Delta V=IR=I/G$

• in extracellular recording - current is what flows between parts of a cell through external resistance produces the change in voltage. as the impulse propagates, current changes and so does the voltage

• in voltage clamp experiment - the driving force = membrane potential minus reversal potential which produces a current. As channels open and close, the conductance changes which changes the current. voltage clamp current is proportional to the number of open channels at any given time

  • each channel can change conductance incrementally

• when two resistors are connected in series, the same current passes through each of them so the voltage across each = the potential difference provided by a battery

  • potential difference is divided in proportino to 2 resistance values

• ephys recordings should:

  • accurately measure parameter of interest
  • should produce no perturbation of the parameter

• best way to measure electrical potential difference is to use voltmeter with infinite resistance

  • the best way to measure current is to open the path and insert an ammeter. if the ammeter has zero resistance it will not perturb the circuit.

• Ohms law is a linear relation between potential difference and current flow

  • applies to aqueous ionic solutions

• current is carried by anions and cations (at least one of each usually more)

• current must be transformed smoothly from a flow of electrons in the copper wire to a flow of ions in solution.

  • most common electrode is a silver/silver chloride

• electrical field is a property of each point in space proportional to the force experienced by a charge placed at that point

  • the greater the potential difference the greater the electrical field

• the cell membrane is important for understanding concept of electrical fields

  • the transmembrane resting potential is huge but typical ephys equipment cannot measure these fields
  • instead we assume that voltage gating domains of ion channels as a proxy of the electrical excitability because it will change the conductance when channels are open or closed.
  • the membrane is thick and a good capacitor - ability to store charge

• charge is stored in a capacitor only when there is a change in voltage across the capacitor.

  • current flowing through is proportional to the voltage change with time
  • as long as voltage is constant, you can ignore membrane capacitance on currents
  • any change in the voltage across the membrane is accompanied by a change in the stored charge.
    • new current first satisfies requirement for charging membrane capacitance, then it changes membrane voltage
  • the charging time constant increases when the membrane capacitance or resistance increases

• current clamp

  • apply a known constant or time varying current and measure change in membrane potential
  • mimics current produced by synaptic input

• voltage clamp

  • control membrane voltage and measure current required to maintain that voltage
  • does not mimic nature… but
  • it eliminates capacitive current
  • currents that flow are proportional only to membrane conductance and to number of open channels
  • channel gating is determined by transmembrane voltage alone, so voltage clamp allows you to determine the opening and closing of ion channels

• patch clamp

  • type of voltage clamp that allows one to resolve currents flowing through single ion channels
  • currents measured are very small (picoamps in single channel to nanoamps in whole cell)

• glass microelectrodes and tight seals

  • currents through the seal do not distort measured voltage or current but they do add to noise

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