Question

Discuss the steps in generating an action potential in a neuron. (Make sure to include which ion channels are activated and how, changes in membrane potential as a result of the movement of those ions, the different phases seen in an action potential, and the movement of the action potential in relation to the anatomy of a neuron.).

Answer 1

Action potential is caused by either threshold or supra threshold stimuli upon a neuron. It consists of four phases; hypopolarization, depolarization, overshoot, and repolarization. An action potential propagates along the cell membrane of an axon until it reaches the terminal button.

The sequence of events

• Initially the cell is resting
• The cell becomes excited.
• Channels open and the membrane permiability to sodium is suddenly increase greatly.
• Sodium(Na+) rushes into the cell.
• Voltage-activated Potassium channels open.
• Permeability to Potassium increases slowly
• Positive charges accumulate within the cell.
• The membrane potential approaches the equilibrium potential for Sodium
• Na channels close

Hypopolarization is the initial increase of the membrane potential to the value of the threshold potential. The threshold potential opens voltage-gated sodium channels and causes a large influx of sodium ions. This phase is called the depolarization.

During depolarization, the inside of the cell becomes more and more electropositive, until the potential gets closer the electrochemical equilibrium for sodium . This phase of extreme positivity is the overshoot phase.

After the overshoot, the sodium permeability suddenly decreases due to the closing of its channels. The overshoot value of the cell potential opens voltage-gated potassium channels, which causes a large potassium efflux, decreasing the cell’s electropositivity. This phase is the repolarization phase.

Repolarization always leads first to hyperpolarization, a state in which the membrane potential is more negative than the default membrane potential.