Question

In your own words describe what is taking place during an action potential. Be sure to include the following terms: sodium ions Na+, potassium ions K+, Threshold, depolarization, repolarization, hyperpolarization, voltage gates. This is not an exhaustive list, you will need to use additional terms when necessary.  

Answer 1

Action potential -

Definition - The brief sequence of changes which occur in the membrane potential following excitation is called action potential.

Events takes place during action potential -

-Resting membrane potential - It is a potential difference between the inside and outside of the cell at rest and indicates the resting state i.e state of polarisation of the cell membrane.

Resting membrane potential is negative inside and positive outside of the cell. It is due to the distribution of ions across the cell Increased flow of the positively charged potassium(k+) ions diffuses out of the cell (relative to entry of sodium (Na+) ions into the cell) while non diffusable anions (eg:proteins) stay in the cell.

Therefore there is slight excess of cations outside the membrane and slight excess of anions inside the membrane, causing negative resting membrane potential inside the cell.

The resting membrane potential ranges from - 40 to - 90 mV.

Depolarization - when nerve fibre is stimulated, the plasma membrane becomes more permeable to sodium ions than to potassium ions as the voltage gates of the sodium channels open and voltage gates of potassium channels close. As a result the rate of flow of sodium ions into the cell exceeds the rate of flow of potassium ions out of the cell. Hence, inside the cell positively charged and negatively charged outside. This is known as depolarization.

An action potential occurs in the cell membrane when depolarization reaches a certain level called threshold potential (-55 mV).This particular stimulus which is able to bring the membrane potential to threshold is called threshold stimulus.

Repolarization-Repolarization starts with potassium efflux (movement of k+ ions outside the cell) due to opening of voltage gated potassium channles and decrease in further movement of sodium ions into the cell.

The opening of voltage gated K+ channels is more prolonged than opening of voltage gated Na+ channels.

K+ efflux cause net transfer of positive charge out of the cell i.e exit of potassium ions cause a reversal of membrane potential to negative inside. This is called repolarization.

Hyperpolarization - The repolarization typically goes more negative than the resting potential to about - 90mV.This is called hyperpolarization.

This occurs due to the increased flow of potassium ions out of the cell as voltage gated K+ channels are open and voltage gated Na+ channels remain closed.