⭐ Model answer: An action potential begins when the membrane reaches its threshold, causing sodium ion channels to open. Sodium ions enter the neurone, making the inside of the membrane more positive. Depolarisation occurs because the membrane potential becomes less negative and then positive. Potassium ions leave the neurone, causing the membrane to become negative again, known as repolarisation. Myelinated neurones have a myelin sheath which insulates the axon and increases the speed of impulse transmission. Nodes of Ranvier are gaps in the myelin sheath where the action potential is regenerated. Saltatory conduction occurs because the impulse effectively jumps from node to node, allowing it to travel much faster.

Initiation: An action potential begins when the neurone membrane reaches its threshold potential, causing voltage-gated sodium ion ( $\text{Na}^+$ ) channels to open.
Depolarisation:
- Sodium ions ( $\text{Na}^+$ ) diffuse rapidly into the neurone down their electrochemical gradient.
- This makes the inside of the membrane less negative and temporarily positive.
Repolarisation:
- Potassium ion ( $\text{K}^+$ ) channels open while sodium channels close.
- Potassium ions ( $\text{K}^+$ ) diffuse out of the neurone, returning the membrane potential to a negative state.

Myelin Sheath: An insulating layer around the axon that prevents ion leakage and increases the speed of electrical impulse transmission.
Nodes of Ranvier: Gaps in the myelin sheath containing high densities of ion channels where the action potential is regenerated.
Saltatory Conduction: The mechanism by which the electrical impulse jumps between adjacent Nodes of Ranvier, allowing it to travel significantly faster than in unmyelinated neurones.