Neuron at Rest – Resting Potential

A neuron at Rest – Resting Potential

Imagine you have a dead frog. Ever wondered what would happen to its leg if you applied an electrical stimulus to the attached nerve? Creepily enough, the dead leg would move. It is because nerve cells (neurons) carry information via electrical signals.

At a cellular level, neurons generate electrical signals by controlling the movement of charged ions in and out of the cell membrane. But the plasma membrane of most cells is impermeable. Then how do ions move? Some proteins act as ion channels and ion pumps that allow the ions to move selectively across the membrane, creating a voltage difference (resting membrane potential).

Resting Membrane Potential

What is a nerve impulse?

A nerve impulse is any physiological change in a neuron due to external or internal stimuli. In simple words, a nerve impulse is a signal transmitted along with a nerve fibre.

Transmission of a nerve impulse occurs in three steps – (1) Resting potential (polarisation), (2) Action potential (depolarization), and (3) repolarisation. Here, we’ll discuss the resting membrane potential of a neuron.

Why are neuron membranes polarised?

For the nervous system to function, it must be able to send and receive signals, determine an appropriate response, and act accordingly. Each neuron has a charged cellular membrane that generates electrical signals.

Electric signals are formed due to voltage differences resulting from the uneven distribution of ions across the membrane.

What are the most common ions present in a neuron?

• Cations (positively charged ions): sodium (Na+) and Potassium (K+)
• Anions (negatively charged ions): Chlorine (Cl-) and organic ions

How do ions cross the membrane?

The cell membrane of a neuron is naturally more permeable to potassium than to sodium. Similarly, the membrane is almost impermeable to negatively charged ions present in the axoplasm. The fluid outside the axon contains a high concentration of sodium ions, and a low concentration of potassium ions, thus, forming a concentration gradient. Below are four steps on how ions cross the membrane.

• Na+ movement

When a neuron is at rest, two forces attract Na+ ions towards the inside of the cell.

• Concentration force pushes Na+ ions into the cell because the extracellular concentration of Na+ ions is high
• Negative charges inside the neuron pull Na+ ions into the cell using electrical force

However, since the neural membrane selectively allows the passage of Na+ions, the outside of the cell keeps the high concentration of Na+ ions intact.

• K+ movement

The diffusion force enables K+ ions to move out freely across the neural membrane. But simultaneously, the increased negative charge inside the cell keeps K+ ions from leaking out. Consequently, the concentration of K+ ions remains higher inside the cell, as before.

• Cl- movement

Cl- ions can easily pass through the neural membrane. The negative charge inside the neuron pushes Cl- ions outside the cell, increasing the extracellular negative charge concentration. Once this happens, Cl- ions tend to move back to the neuron down the concentration gradient.

• Protein molecules movement

Though the protein molecules tend to move out of the cell, these are too big to cross the neural membrane.

Additionally, there is a pump that uses energy to move three Na+ ions out of the cell for every two K+ ions it diffuses inside. Finally, when the concentration forces and electrical forces balance out, we receive a voltage difference of – 70mV, called resting membrane potential.

Highlights: Polarised Membrane and Resting Potential

The resting potential of a neuron cell membrane arises due to the movement of ions across the neural membrane in a cell at rest.

We already know that ions are unevenly distributed on either side of the cell membrane. Sodium ions are almost ten times more concentrated on the outside of the cell than the inside. Conversely, potassium ions are 30 times more concentrated on the inside of the cell. This uneven distribution initiates the diffusion of ions when stimulated by external stimuli.

• A neuron at rest doesn’t conduct. However, it still carries an electric charge
• The plasma membrane of a neuron at rest is negatively charged compared to the interstitial fluid
• When the neuron is at rest, its plasma membrane allows K+ ions to pass freely
• Na+ and Cl- ions selectively pass through the membrane
• The negatively charged protein molecules are too big to pass through the plasma membrane
• When all the forces balance out, a voltage difference of 70 mV appears between the inside and outside of the cell, with the intracellular space being negative

Resting Potential of a Neuron

A neuron is said to be at rest(-70mV) if it is not generating any action potential on its membrane.

Conclusion

The plasma membrane of a neuron at rest is more permeable to K+ ions than to Na+ and Cl- ions. The electrochemical equilibrium that develops due to the distribution of ions across the membrane is called the resting potential of a neuron, as explained above. We hope this article helps you with your exam preparations.