Depolorisation: When an action potential happens, some of the sodium ions that enter the neurone diffuse sideways. THis causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse into that part. This caused a wave of depolorisation away from the parts of the membrane in the refactory (recovery) period because these parts can't fire an action potential - the membrane can't be excited again straight away. The ions channels are recovering and can't be made to open - sodium ion channels are closed during repolorisation and potassium ion channels are closed during hyperpolorisation. It can only travel in one direction cause it occurs at the start of the axon.