CSB332 Lecture 7
- When there is no net movement of ions through the plasma membrane, then it is similar to a
state of the neuron called the resting membrane potential. At resting membrane potential,
there is no net flow of ions.
- Equilibrium potential is reflective of the resting membrane potential.
- Is the equilibrium potential of K+ ions always equivalent to the resting membrane potential of a
given neuron? Is the resting membrane potential solely dependent on the equilibrium potential
- The resting membrane potential is not only dependent on the equilibrium potential of K+. The
resting membrane potential of a typical neuron is dependent on a number of factors. There are
other ions and their respective equilibrium potentials that are involved in the generation and
the maintenance of the resting membrane potential.
- The resting membrane potential of a neuron is dependent on Na+, K+, and the ATPase pump.
- The currents that contribute to the resting membrane potential are of two entities.
o K+ leak current
Voltage-gated K+ channels are open when the resting membrane potential is
o Na+ leak current
NALCN channels are the predominant Na+ leak current channels.
- HCN channels are non-selective for K+ and Na+.
- Nernst equation
o K+ dictates the resting membrane potential of a typical neuron because there is a
greater concentration of K+ inside the neuron and there is a high number of resting K+
channels on the plasma membrane.
o The resting membrane potential would always be equivalent to the equilibrium
potential for K+, but this is if only K+ ion channels are important in maintaining the
resting membrane potential.
o The equilibrium potential for K+ is dependent on the external concentration and the
internal concentration of K+.
o There is a direct linear relationship between the external concentration of K+ and the
membrane potential of the neuron. The membrane potential changes by 58 mV per
10-fold change in the external concentration of K+ ions.
- The equilibrium potential for K+ is not sufficient to determine the resting membrane potential of
- Hodgkin and colleagues were able to record the resting membrane potential by using a squid
giant axon to verify if (or not) Vr is equivalent tK E .
o The axon is about 800 microns in diameter, which is 1000X larger than the diameter of a
typical mammalian neuronal axon. o They stuck electrodes in to the giant axon. You can manipulate the external
concentrations of K+ in the bath solution and record the resulting change in membrane
potential. You can examine the relationship of external concentration to the change in
o There is a clear deviation from the expected equilibrium potential based on the Nernst
equation. The membrane potential deviates significantly from expected equilibrium
potential when the concentration of K+ is less than 100 mM.
- Therefore, the resting membrane potential is not always equivalent to the equilibrium potential
o Na+ contributes to the generation of resting membrane potential.
o Cl- does not contribute to the resting membrane potential.
o This is the condition when only resting K+ channels are present in the plasma
membrane. K+ is at equilibrium, so there is no net movement of K+ ions in and out of
the neuron. The membrane potential is equivalent to the equilibrium potential of K+.
- What if only leak Na+ channels are present?
o If only leak Na+ channels are present, then Vr =Na .
o There is a large concentration of Na+ outside of the neuron. The membrane potential
that is being recorded at that moment is the membrane potential that defines
equilibrium potential for K+.
o The internal potential is -85 mV. It is more negative inside the neuron. There is a larger
concentration of Na+ ions outside of the neuron. Therefore, there is an inward
movement of Na+ ions because there is a large net driving force that is composed of the
chemical driving force (e.g., concentration gradient) and the electrical driving force (e.g.,
more negative potential inside the neuron attracts positive io