Action potential

Excitatory and inhibitory signals compete to decrease or increase the likelihood of an action potential firing.

Excitatory signals depolarize the membrane; bring the membrane closer to threshold potential (make more positive).

Inhibitory signals hyperpolarize the membrane; take the membrane farther from threshold potential (make more negative).

Excitatory postsynaptic potential (ESPS)

Depolarize the membrane by letting positive ions into the cell.

• E.g., glutamate lets Na+ into the cell, which depolarizes from -70 mV to -60 mV.

Inhibitory postsynaptic potential (ISPS)

Hyperpolarize the membrane by letting negative ions in the cell.

• E.g., GABA lets Cl- into the cell, which hyperpolarizes from -70 mV to 80 mV.

What happens if extracellular K+ concentration is increased?

It would increase the resting potential (membrane becomes more positive).

The inside of the cell is more negative compared to the outside. K+ already prefers to stay in the negatively-charged cell. Increasing extracellular K+ would make the outside even more positive, which keeps K+ inside the cell, thus raising the membrane potential.

• I.e., more positive ions in the cell = membrane potential becomes more positive

What happens if extracellular Na+ concentration is increased?

It would cause a small depolarization, as well as increase the amplitude of the action potential.

There is already a larger concentration of Na+ outside the cell then inside. Raising extracellular Na+ would make the Na+ gradient b/w inside and outside of the cell even larger, which would drive slightly more Na+ to enter the cell, which raises the membrane potential.

Also due to the larger gradient, the influx of Na+ will be stronger during opening of the VG Na+ channels, which makes the amplitude higher.