Resting potential and action potential

An excitable cell

A neuron is an excitable cell. Its membrane is in a polarised state: the two sides carry different electrical charges. When a stimulus acts, this state changes for a moment, and the change is the signal. To see how, we start with the resting neuron.

Ions and channels

The neural membrane has different types of ion channels. Some are voltage-gated: they open or close when the voltage across the membrane changes. Each channel lets only certain ions pass.

When the neuron is not conducting an impulse, it is at rest. At rest:

• The axonal membrane is more permeable to K+ and almost impermeable to Na+.
• The membrane is impermeable to the negatively charged proteins inside the axon.
• So the axoplasm (the cytoplasm of the axon) has a high concentration of K+ and of negatively charged proteins, and a low concentration of Na+.
• The fluid outside has a low concentration of K+ and a high concentration of Na+.

These gradients are kept up by the sodium-potassium pump. It uses ATP to move 3 Na+ out of the axon and 2 K+ in for each turn. As a result, the outer surface of the membrane is positively charged and the inner surface is negatively charged. The membrane is polarised.

The electrical potential difference across the resting membrane is the resting potential.

NCERT gives no number for the resting potential. The usual value in textbooks for a mammalian neuron is about -70 mV (the inside is 70 millivolts lower than the outside). The model values in the table below are typical, and we use them in this chapter.

Why about -70 mV? (not in the NEET syllabus)

This part is not in the NEET syllabus, but A-Level and IB ask it. For one kind of ion, the voltage at which it would be in balance is given by the Nernst equation:

At body temperature (310 K), $RT/F$ is $26.7\ \text{mV}$. For K+ with the table above, $E_K = 26.7\ln(4/140) \approx -95\ \text{mV}$. For Na+, $E_{Na} = 26.7\ln(145/12) \approx +67\ \text{mV}$.

The membrane at rest is much more permeable to K+, so its voltage lies close to $E_K$, but a little Na+ leaks in and lifts it to about $-70\ \text{mV}$.

You should find that a membrane which is more permeable to sodium moves towards the sodium potential, which is positive. You should also find that more potassium outside lifts the resting potential, so the cell is closer to firing.

The action potential

A stimulus is applied at a site on the axon.

1. The membrane at that site becomes freely permeable to Na+. Sodium ions rush in along their gradient.
2. The inside becomes positive and the outside becomes negative. The polarity is reversed. The membrane is depolarised.
3. The potential difference across the membrane at this site is the action potential. It is also called the nerve impulse.
4. The rise in the permeability to Na+ is extremely short-lived. It is quickly followed by a rise in the permeability to K+.
5. Within a fraction of a second, K+ diffuses out and the resting potential is restored at the site. The fibre is ready for the next stimulus.

The graph is a model with stated values: rest at -70 mV, threshold at -55 mV, peak at +30 mV. A real recording has the same shape, and the exact numbers depend on the cell and the temperature. Four words describe it.

• Depolarisation: the rise, when Na+ enters.
• Repolarisation: the fall back towards rest, when K+ leaves.
• Hyperpolarisation: a short dip below the resting level, which is the extra K+ that has left.
• Threshold: the potential that a stimulus must reach to start a spike.

NCERT does not use these four words or give any of the numbers. The next ideas are also from A-Level and IB and not in the NEET syllabus. The spike is all or none: a stimulus below the threshold gives no spike, and a stronger stimulus does not give a taller spike. After the spike, the pump works to put the ions back in their places.

Try it in Python

The program uses the Nernst equation for the model concentrations.

Practise with new numbers

Common mistakes

• Saying that the resting membrane is more permeable to Na+. It is more permeable to K+ and almost impermeable to Na+.
• Saying that the inside of the resting axon is positive. The inside is negative and the outer surface is positive.
• Saying that the pump moves equal numbers of ions. It moves 3 Na+ out and 2 K+ in.
• Saying that K+ rushes in during the rising phase. Na+ rushes in during the rising phase. K+ leaves later.
• Saying that the pump makes the action potential. The action potential comes from the movement of ions through channels. The pump restores the gradients.

Remember these

• Resting: more permeable to K+, almost impermeable to Na+. Outside positive, inside negative. Pump: 3 Na+ out, 2 K+ in.
• Stimulus: Na+ enters, polarity reverses (depolarisation). The potential difference is the action potential, the nerve impulse.
• Then K+ leaves and the resting potential returns.
• Model values: rest -70 mV, threshold -55 mV, peak +30 mV.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why is the outer surface of a resting axon positive?

2. How many sodium and potassium ions does the pump move in one turn, and in which directions?

3. What happens to the membrane when a stimulus acts on a site of the axon?

4. What brings the potential back to the resting level?

5. In the model graph, a stimulus lifts the membrane from -70 mV to -60 mV. A second stimulus lifts it to -50 mV. Which one gives a spike?

References

• Clark M. A., Douglas M. and Choi J. (2018). Biology 2e, OpenStax, chapter 35, The Nervous System, section 35.2, How Neurons Communicate.
• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 12, The Nervous System and Nervous Tissue, sections 12.4 and 12.5.
• NCERT (2023). Biology, Textbook for Class XI, chapter 18.