Simple Pharmanotes: Electrophysiology of Neurons: Action Potential.

Electrophysiology of Neurons: Action Potential

ELectrophysiology of Neurons:

The intracellular and extracellular fluid in the nervous system contains many charged ions like Na+, K+, Ca++, Cl- etc.
The presence of these ions imparts an electric potential inside the cell and outside the cell.
Membrane Potential (Transmembrane Potential, Membrane Voltage) is the difference of voltage between inside the cell membrane and outside the cell membrane.
The resting membrane potential is -70mv.
Nerve impulse (Action Potential) is generated by a change in ion concentrations across the cell membrane.
The three phases in which action potential takes place are,

On a receiving stimulus the Na+ ion channels open causing entry of Na+ ions inside the cell.
As Na+ ions are positively charged, the resting membrane potential now starts shifting to “0”.
When the membrane potential reaches “-55mv” it is said that the “Action Potential is generated or threshold is generated”.
Due to the higher concentration gradient of Na+ ions the membrane potential reaches upto “+30mv”.
Depolarization means shifting of membrane potential from” -70mv to 0”.

As the membrane potential reaches “+30mv” it causes the opening of “K+” ion channels.
The K+ ions start leaving the cell to extracellular fluid ( Concentration of K+ ions is less outside the cell).
The loss of K+ ions causes the membrane potential to come down.
The opening of K+ ion channels is slower at the same time the Na+ ion channels start getting inactivated.
Due to this the membrane potential drops to its resting stage i.e -70mv.
The return of membrane potential to -70 mv is called “Repolarization”.

Due to the slow closing of K+ ion channels the further loss of positivity the membrane potential lowers more to -90mv.
Hyperpolarization means dropping of membrane potential to -90mv.
After closing of K+ ion channels the membrane potential comes back to its resting stage and is now ready to receive new stimulus.