Resting membrane potential (RMP)

The difference in charges with more positive outside and more negative ions inside is called the resting membrane potential (RMP).

A " resting neuron" is not coordinating a nerve impulse. The plasma membrane of a that is polarized. The substance on the inner side of the membrane is negatively charged. It is positively charged outside the membrane.

Two factors cause this unequal distribution of ions

  1. There is difference in relative numbers of positive and negative ions between the inside and outside fluids.

  2. The plasma membrane has different permeability for these ions. So there is unequal distribution of charges inside and outside.

All cells have such a resting potential.  But neurons and muscle cells are specialized to transmit and recycle it rapidly. The resting potential is calculated in millivolts (mV). A millivolt is 1/1,000 of volt. Normally, the resting membrane potential is about -70mV.

Following ions are present inside and outside the membrane

1. Sodium [Na+]

They are highly concentrated in the fluid outside the plasma membrane. Cl- ions are also present outside the membrane.

2. Potassium [K+]

They are negative protein ions . They are highly concentrated internally. Ion channels are present in the plasma membrane. The Na+ and K+ ions constantly diffused through these ion channels. They move from regions of higher concentrations to regions of lower concentrations. The Cl- ions and negative protein ions have larger size. So they cannot move easily through the membrane.

Sodium-potassium ATPase pump

  • It is present in the plasma membrane. It is powered by ATP. It keeps the concentrations of Na and K ions constant on the two side of the membrane. The pump actively move Na+ ions outside of the cell. It pumps K ions inside of the cell. It moves three Na+ out for each two K+moves inside. Thus this pump establishes resting potential across the membrane. Both ions also leak back across the membrane. But K+ move more easily back to the outside. It adds to the positive charge there. Thus membrane potential of -70mV is established.

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