How do you explain the transmission of nerve impulses across an axon
As you are IGCSE, it is a bit difficult to get into detail. I do this in simple words, don't worry!
Nerve impulses are caused by an exchange of ions. You must distinguish two different ions: sodium ions and potassium ions. The potassium ions are inside of a nerve cell (intracellular space), the sodium ones outside of it (extracellular space). A potassium-sodium-pump transfers the potassium ions outside of the nerve cell and the sodium ions inside, so vice versa. That causes an exchange of charges. This procedure goes on the whole axon. Thus the axon is stimulated and a nerve impulse is transmitted through it.