Question

Action potential is generated by
A. $Na^{+}$
B. $K^{+}$
C. $Ca^{2+}$
D. $Cl^{-}$

Answer 1

Hint: When a nerve fibre is stimulated by a stimulus of adequate strength then its polarity is reversed and a new potential difference called action potential is developed. This depolarization is self-propagated and is transmitted in a particular direction in a non-decremental manner.

Complete answer: When a nerve fibre is stimulated by a stimulus of adequate strength, the stimulated area of the nerve fibre comes in the local excitatory state. In this area, the transmembrane potential increases, when the membrane potential becomes -50 mV, the sodium channels come in activated state due to opening of outside lying activation gates. Si the membrane becomes several times more permeable to sodium ions than to potassium ions. Sodium ions rapidly diffuse from outside to intracellular fluid due to electro-chemical gradient so that inside becomes positively charged which further increases the opening of more and more sodium ion channels and the permeability of membrane to sodium ion and sodium conductance.
This reversal of polarity is called depolarization and the membrane with reversed polarity is called depolarized nerve fibre. It is electropositive inside while electronegative outside. It lasts for less than $\dfrac{1}{1,000}$ of a second. So a nerve fibre can conduct about 1,000 impulses per second.
Due to rapid inflow of sodium ions, the potential increases first to zero, then rises to +45 to +50mV and this newly developed potential difference is called action potential. It is developed due to the inflow of sodium ions.
So, the correct answer is “Option A”.

Note: The action potential travels as a wave of depolarization along the length of a nerve fibre in a particular direction and is called a nerve impulse. After the peak of the action potential called spike potential, the permeability of the membrane to sodium ion decreases.