Lecture 8 - Electrical properties of excitable cells: Capacitance & Conductance

Why are cell membranes a leaky capacitor?

• ion channels are embedded in cell membrane

• this movement of charge is the current through the ion channel

How is current measured?

• measured in picoamperes (pA) = movement of 1 picocoulomb of charge per second

What is the role of channel proteins?

• allow specific ions to pass across the membrane

• act as conductors

  • membrane’s ionic conductance depends on presence of open ion channels

  • together with capacitance —> contributes to electrical activity of excitable cells

• ion channels are more resistant to ion flow than ECF and ICF —> act as resistors

Explain current flow of potassium channel in cell membrane

• lipid bilayer has ability to separate charge

• current that flows through channel is dependent on the electrical potential difference across the cell membrane (especially Ek)

• 
  

Define conductance

• ability of channel to pass current is characterized as conductance

• movement of ions through an ion channel is measured in siemens (S)

  • yion = 1/R

  • S is inversely proportional to resistance (R, measured in Ohms)

Ohms Law

• I = V/R

• wires connecting voltage source and resistor have negligible resistance

• electrons that flow through circuit product current of electricity

How does Ohms Law apply to cells?

• gion = 1/R

• I = gion * V

• At resting Vm does not equal Ek

• Iion depends on conductance (g) AND Vm - Eion

• driving force = Vm - Eion

• consider current for each ion separately

The following data were obtained from a non-excitable cell: Resting membrane potential = -75 mV
K+ equilibrium potential = -109 mV
Na+ equilibrium potential = 65 mV
Q: For which ion is the driving force greater? Why? Also, in what direction will current move when ion channels open?

Explain passive and active currents in a neuron at steady-state

• an equivalent circuit that shows the passive and active currents that flow across the membrane of a cell will include pathways that represent…

  • Na, K and Cl conductance through passive transport

  • Na, K conductance via active transport

• at steady-state, passive and active currents are balanced and sum of ionic currents = 0

How do we know if current is moving in or out of the cell?

• current is movement of positive charge

• driving force provides info whether ions will move in or out of cell

• driving force = negative —> ion will enter the cell if it is a positive ion

• driving force = positive —> ion will leave the cell if it is a positive ion

Which ions flow through channel can be determined using what technique and how?

• patch clamp

• micropipette with diameter of 1 micro metre placed against cell

• gentle suction applied to form tight seal between pipette and plasma membrane

• only one or few channels will be in there

• flow of ions is recorded

How to create current-voltage (I - V) plot?

• measure current across the ion channel as function of applied voltage

• plot data

• should see linear relationship if the channel behaves like Ohmic conductor

What is reversal potential?

• The reversal potential (also known as the Nernst or equilibrium potential for a single ion) is the membrane voltage at which the net flow of ionic current through a channel or pore is zero.

What do we know about data from single channel I-V plot?

• the point where there is no current = Eion

• E_ion = E_R = Reversal Potential for single channel

• if Vm = Eion —> no current flows because no driving force

• if Vm set on other side of Eion, current flows again, but will occur through channels in opposite direction