Neuron function: Depolarization and Repolarization

Sodium potassium pump

• Specialized molecule to help maintain cell resting state

• Pumps Na+ to the outside 

• Pumps K+ to the inside

Resting Membrane potential

• Difference in electrical impulse across neurons membrane

• Net negative charge

Resting state

• Neuron is not being stimulated

• NA+ has pumped out of the cell

• K+ has pumped into the cell

• Net negative charge inside cell membrane

Depolarization

• Neuron receives external stimulus

• Na+ channels open on neuron cell membrane

• Na+ ions flow into the cell by passive diffusion

Charge is now positive inside cell membrane

Switching of electrical charge from negative to positive is known as an action potential.

Threshold stimulus

Stimulus is strong enough to cause complete depolarization and to generate nerve impulse (all or nothing principle)

Nerve impulse:

• Electrical charges “flip” across cell membrane (depolarization), followed by “unflipping” of the electric charges (repolarization)

• Adjacent area is being stimulated, spreading a wave of depolarization

Repolarization

• Na+ channels are closing

• K+ channels are opening

• K+ ions flow out of the cell (net charge is returning to negative direction)

• Sufficient outflow of K+ ions restored net negative charge to inside of cell

• Na+ and K+ ions are on opposite sides of cell membrane from where they started

• Resting state is restored

Hyperpolarization (refractory period)

Very brief time period when a neuron is insensitive to additional stimuli

• Cell is still in depolarization/early repolarization

Absolute refractory period 

• During Na+ influx and early K+ outflow

Relative refractory period

• During end of repolarization period

• Possible to stimulate another depolarization if stimulus is very large

Saltatory conduction

• Rapid means of conducting an action potential (each Na+ channel opening stimulates opening of adjacent channel)

• Depolarization in myelinated axons can only take place at the nodes of ranvier