IB Biology Sub-topic C2.2 Notes

In the SL syllabus, you learned about neurons and the basics of nerve impulses. However, you need to know how a nerve impulse is generated within a neuron and how it propagates along it.

This is called the formation of an action potential, whereby the resting membrane potential of an axon is lost. It is officially defined as the depolarization and repolarization of a neuron due to the movement of ions. Let's explore these terms:

1. A positive impulse from a prior portion of the neuron elevates the potential from -70 mV to -50mV, the threshold potential.
2. This causes depolarization of the axon, during which voltage-gated Na⁺ channels are opened to allow the facilitated diffusion of Na⁺ into the axon.
3. This causes the inside of the axon to become more positive, so the potential rises to +30 mV.
4. Next, repolarization occurs. First, the Na⁺ channels close and prevent further entry of sodium.
5. Then, voltage-gated K⁺ channels open to allow the facilitated diffusion of out of the axon.
6. This causes the inside of the axon to become more negative, so the membrane potential falls to -80 mV. This overshoot past resting potential is known as hyperpolarization.
7. To re-establish resting membrane potential of -70 mV, the Na⁺/K⁺ pump exchanges ions back to their original levels.