Basis of Electrical Activity: Channels, Bilayer, Ions
Ion gradients store energy which is used for signaling to produce action potential
Resting potential is approximately -60mV
Action Potential is +40mV
Action potential is "all-or-nothing"
3 main sources of electrical events
(1) lipid bilayer: very good insulator (impermeable to ions); capacitator – stores charge
(2) ion channels: act as conductance, how easily current (ions) can flow
(3) ion gradient: acts as a battery, creates an electrical charge which is stored on the membrane
separation of electrical charge produces a voltage called the membrane potential
qualitatively determined by the combination of electrical and chemical forces
quantitatively determined for a single ion by the Nernst equation
EK = 60 log (Ko/Ki)
If the cell is totally impermeable to an ion, that ion would not affect the membrane potential; Ko = KI hence EK = 0
In reality, more than one ion effects the membrane potential (mostly EK and ENa)
qualitatively, VM must be between EK and ENa
quantitatively determined by parallel batteries model (or Goldman equation)
VM = GNaENa + GKEK (weighted average of the equil potentials of the permeant ions)
In a real cell, K and Na conc change, determined by:
current = conductance x driving force, so IK = GK x (VM - EK)
however this is kept in a steady state due to the Na+/K+ pump
Single Channels
patch clamp recordings demonstrated existence of discrete ion channels
spontaneous fluctuations in the current are due to random opening and closing of channels
the behavior of a single channel appears random, but a large population of channels behaves deterministically as a result of first order rate constants for the opening and closing of the ion channels
the rate constants reflect the depth of the energy wells
current thru a single channel
I = G (VM – VR) = 2 pA = 107 ions/sec
thus thought of as a pore due to this extremely fast flux rate
Gating of Ion Channels
the opening and closing of channels is called gating
2 main stimuli affect gating by affecting the rate constants
(1) binding of chemicals for ligand-gated channels
(2) changes in voltage for voltage-dependent channels
if a protein has 2 conformational states that differ in the location of charge, the energy levels of the states will depend on the membrane potential