Topic 4 – The action potential in nerve fibers (phases, the role of voltage-gated ion

channels, changes in excitability during the action potential)

Action potential

Wave of electrical discharge that travels along the membrane of a cell.

for communication between neurons and other tissues- muscles, glands. When

stimulus arrives its energy causes a temporary reversal of the charges on the neuron

cell surface membrane. As a result, the negative potential of 70 mV inside the

membrane becomes a positive potential of around +40mV. This is known as the

action potential, and in this condition the membrane is said to be depolarized. All or

nothing reaction

Phases

1. Resting potential –potassium leak channels are open, voltage-gated Na+

channels closed. Though, no net current is flowing, major ion species moving

across membrane is potassium, pulling resting potential close to the K+

equilibrium potential.

2. Stimulation - local membrane depolarization caused by an excitatory stimulus

causes voltage-gated sodium channels to open, therefore, sodium ions diffuse

in through the channels along their electrochemical gradient. Being positively

charged, they begin a reversal in the potential difference across the membrane

from negative-inside to positive-inside.

3. Rising phase – depolarization - As sodium ions enter membrane potential

becomes less negative; more sodium channels open, causing an even greater

influx of sodium ions. Sodium current dominates over potassium leak current

and membrane potential becomes positive inside.

4. Peak - By the time the membrane potential has reached a peak value of around

+45 mV voltage-sensitive inactivation gates on the sodium channels have

already started to close, reducing and finally

preventing further influx of sodium ions. While this

happens, voltage-sensitive activation gates on the

voltage-gated potassium channels begin to open.

5. Falling phase – repolarization - voltage-gated

potassium channels open, large outward movement of

potassium ions driven by the potassium concentration

gradient and favored by the positive-inside electrical

gradient. As potassium ions diffuse out, this movement

of positive charge causes a reversal of the membrane

potential to negative-inside and repolarization of the

neuron back towards the large negative-inside resting

potential.

6. Undershoot – hyperpolarization - As potassium exits

the cell, the resulting membrane repolarization initiates

the closing of voltage-gated potassium channels. These

channels don't close immediately, rather, voltage-gated

potassium channels have delayed response, and

potassium continues to flow out of the cell even after

the membrane has fully repolarized. potential dips below normal resting

membrane potential of the cell for a brief moment;

the role of voltage-gated ion channels

inactivation and activation gate in voltage gated Na channels. One gate in voltage

gated K channels. The depolarization occurs because channels in the axon membrane

change shape, and hence open or close, depending on the voltage. • Both the speed and complexity of action potentials vary between different types of cells. However, the amplitudes of

the voltage swings tend to be roughly the same.

changes in excitability during the action potential

Refractory period Where membrane has undergone an action

potential, a refractory period follows.

Absolute refractory period, virtually all

sodium channels are inactivated and thus it is

impossible to fire another action potential in

that segment of membrane.

Relative refractory period - With time,

sodium channels are reactivated in a stochastic

manner. As they become available, it becomes

possible to fire an action potential.

Nerve – 1ms

Skeletal – 10 ms

Cardiac – 200 ms