The Ionic Basis of the Resting Membrane Potential
The membrane potential (V ) im the voltage across the neuronal membrane at any
moment.
Vmcan be measured by inserting a microelectrode into the cytosol
o A typical microelectrode is a thing glass tube with an extremely fine tip that will
penetrate the membrane of a neuron with minimal damage
o This method reveals that electrical charge in unevenly distributed across the
neuronal membrane
The inside of the neuron is electrically negative with respect to the outside
Equilibrium Potentials
o The electrical potential differences that exactly balances an ionic concentration
gradient is called an ionic equilibrium potential and it is represented asion.
o There are form points that are raised in the topic of equilibrium potentials
1) Large changes in membrane potentials are caused by miniscule
changes in ionic concentrations
2) The net difference in electrical charge occurs at the inside and outside
surfaces of the membrane
3) Ions are driven across the membrane at a rate proportional to the
difference between the membrane potential and the equilibrium
potential
4) If the concentration difference across the membrane is known for an
ion, an equilibrium potential can be calculated for that ion
o The Nernst Equation
The exact value of an equilibrium potential in mV can be calculated using
an equation derived from the Nernst equation. It takes into consideration
the charge of the ion, the temperature, and the ration of the external and
internal ion concentrations
The Distribution of Ions Across the Membrane
o K is more concentrated on the inside, and Na and Ca 2+are more concentrated
on the outside
o Ionic concentrations gradients are established by the actions of ion pumps in the
neuronal membrane
o Two ion pumps are especially important in cellular neurophysiology: the sodium
and the calcium pumps
o The sodium-potassium pump is an enzyme that breaks down ATP in the
presence of internal Na+
The chemical energy released by this reaction drives the pump, which
exchanges internal Na for external K .
+ +
The pump ensures that K is concentrated inside the neuron and that Na
is concentrated outside
The sodium-potassium pump expends as much as 70% of the total
amount of ATP utilized by the brain
2+
o The calcium pump actively transports Ca out of the cytosol across the cell
membrane
o Ion pumps work in the background to ensure that the ionic concentration
gradients are established and maintained Without ion pumps, the resting membrane potential would not exist and
the brain would not function
Relative Ion Permeabilities at Rest
o See Fig. 3.15
o An equilibrium potential for an ion is the membrane potential that results if a
membrane is selectively permeable to that ion alone
In reality, neurons are
More
Less
