Question

What is the resting membrane potential and why is it important? Describe 2 ways the cell maintains the resting membrane potential.

Answer 1

Resting membrane potential is the result of a difference in the concentration of ions inside and outside the cell.

• All cells establish a resting membrane potential.
• Let us discuss the cell membrane of a neuron as an example
• When a neuron is not sending a signal, it is "at rest." When a neuron is at rest, the inside of the neuron is negative relative to the outside.
• Although the concentrations of the different ions attempt to balance out on both sides of the membrane, they cannot because the cell membrane is selectively permeable and allows only some ions to pass through channels (ion channels).

The cell membrane separates two electrolyte fluids

• ICF(Intracellular fluid), and

• ECF(Extracellular fluid, with different ionic concentrations.

The difference in the number of positively charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential

Thus, Resting Membrane Potential can be defined as the difference in the electrical potential between the interior and exterior of cell.

Importance: The significance of the resting membrane potential is that it allows the body’s excitable cells (neurons and muscles) to experience rapid changes to perform their function.

• The resting membrane potential of a neuron is about -70 mV (mV=millivolt) this means that the inside of the neuron is 70 mV less than the outside. At rest, there are relatively more sodium ions outside the neuron and more potassium ions inside that neuron.
• Upon excitation, these cells deviate from their resting membrane potential to undergo a rapid, temporary action potential before coming back to rest.
• This mechanism helps in the transmission of signals and is gravely important for the functioning of the entire body.

Mechanism: The development and maintenance of resting membrane potential in a cell are carried out by some mechanisms, which produce ionic imbalance across the cell membrane.

• These mechanisms increase the concentrations of positive ions outside the cell, thus creating ionic imbalance.

Mainly two transport mechanisms are involved:

1. Sodium Potassium Pump and

2. Selective Permeability of the cell membrane.

Sodium Potassium Pump: There is active transport of Na+ and K+ ions in the opposite direction across the cell membrane by means of an electrogenic pump, called Sodium Potassium Pump.

It moves 3Na+ ions out of the cell and 2K+ ions inside the cell by using energy from ATP.

At resting membrane all voltage-gated Na+ channels and most voltage-gated K+ channels are closed, the Na+K+ pump transports K+ into the cell and 3Na+ out.

Since more positive ions are pumped outside and inside a net deficit of positive ions occurs inside the cell.

Selective Permeability of cell membrane: The permeability of the cell membrane depends largely on transport channels.

These transport channels are selective for the movement of some specific ions.

Their Permeability to these ions also vary.

Most of the channels are gated channels and ions can move across only once these channels are open.

• Channels for major anions like Proteins: Channels for major anions(A) that are negatively charged like proteins, organic phosphate compounds, and sulfate compounds are either absent or closed.
• such substances remain inside the cell and play a major role in developing the RMP
• Leak Channels: The channels for three important ions Na+, Cl-, and K+ also play an important role in maintaining the resting membrane potential.
• These three ions are unequally distributed across the cell membrane.
• Na+ and Cl- being more outside and K+ inside.
• Since Cl- channels are mostly closed in resting conditions, Cl- ions are mostly retained outside.
• Thus, only Positive ions Na+ and K+ can move across the membrane.
• Na+ is actively transported against the concentration gradient outside the cell.
• K+ is actively transported against the concentration gradient inside the cell.
• However, again because of the concentration gradient, Na+ and K+ have a tendency to diffuse back, through their respective channels.
• However in resting conditions, all K+ channels are open, and only very few Na+ channels are also open.
• This results in a greater passive efflux of K+ ions than Na+ influx.
• Resulting in more positive ions outside compared to inside, thus creating the RMP of -70mV.