Researchers are studying how the membrane potential of a postsynaptic neuron changes in response to neurotransmitter stimulation. Baseline measurements determine that the resting membrane potential is generated by high membrane permeability for a particular ion. When neurotransmitter stimulation begins, ligand-gated ion channels open (black arrow) and increase the membrane permeability for a different ion, causing a change in membrane potential. This triggers the delayed opening of voltage-gated ion channels (red arrow), which increase the membrane permeability for a third type of ion. The results of the experiment are shown in the graph below.
Show Explanatory Sources
The equilibrium potentials of different ions under physiologic conditions are as follows:
| ENa | = | +60 mV |
| EK | = | −90 mV |
| ECl | = | −75 mV |
| ECa | = | +125 mV |
Which of the following options would best explain the changes in this neuron's membrane potential during the experiment?
Equilibrium potentials of cellular ions reflect how they affect the membrane potential if the membrane were permeable solely for that ion. The resting membrane potential shown in the graph is negative, indicating that at rest, the membrane is permeable to an ion with a negative equilibrium potential (potassium or chloride). Opening of ligand-gated ion channels in response to neurotransmitter binding (black arrow) causes an increase in membrane potential to above zero. This indicates that the membrane has become permeable for an ion with a positive equilibrium potential (sodium or calcium). Opening of voltage-gated ion channels in response to the change in membrane potential (red arrow) causes a drop in membrane potential, indicating that the membrane becomes permeable to an ion with a negative equilibrium potential (potassium or chloride).
(Choices A and B) If the membrane were permeable for sodium at rest, the resting membrane potential would be positive as the equilibrium potential of sodium is +60 mV.
(Choices C and D) Had the membrane become more permeable for calcium or sodium following the opening of the voltage-gated ion channels, the membrane potential would have remained positive as both ions have positive equilibrium potentials.
Educational objective:
Changes in membrane potential occur in response to changes in neuronal membrane permeability to various cellular ions. The more permeable the membrane becomes for a cellular ion, the more that ion's equilibrium potential contributes to the total membrane potential.