Question

Describe the membrane potential and explain how it is produced

Answer 1

Membrane Potential

*When two electrodes are connected through a suitable amplifier and placed on the surface of a single axon, no potential difference is observed. However, if one electrode is inserted into the interior of the cell, a constant potential differences observed, with the inside negative relative to the outside of the cell at rest.

*A membrane potential results from separation of positive and negative charges across the cell membrane. In neurons, the resting membrane potentialis usually about –70 mV, which is close to the equilibrium potential for K+. Every cell shows a potential difference, with the inside being negative. Its value varies from cell to cel

*This is also termed as steady potential and indicates the resting state i.e. state of polarisation of the cell membrane.

*It is written with a minus sign, signifying that inside is negative relative to the exterior.

*Its magnitude varies considerably from tissue to tissue ranging from -10 m V to
-100m

GENESIS OF RESTING MEMBRANE POTENTIAL (RMP)

(i) It is due to distribution of ions across the cell membrane. Some K+ diffuses out of the cell along its concentration gradient while non-diffusible anions (e.g. proteins etc.) stay in the cell, creating a potential difference across the membrane. Therefore, there is a slight excess of cations outside the membrane and slight excess of anions inside. (It is worth noting that the actual number of ions responsible for the membrane potential is a minute fraction of the total number present across the cell membrane in each respective compartment.)

(ii) Na+ -K+ pump is electrogenic at rest with a coupling ratio of 3/2 . It throws 3 Na+ outisde and Move 2 K+ inside.lt does not generate the membrane potential when the coupling ratio is 1. However, it maintains the concentration gradient on which the existence of the membrane potential depends.

iii) Na+ influx does not compensate for K+ efflux, because membrane at rest is much less permeable to Na+ than K+.

(iv) K+ diffuses inwards down its concentration gradient but its movement is balanced by the electrical gradient.