An action potential at one point on an axon triggers an action potential in the region next to it, so the impulse moves along the neurone as a wave of depolarisation. In an unmyelinated axon this happens as follows.
1. At the site of an action potential, voltage-gated sodium ion channels are open and Na+ diffuses into the axon. The inside of this region is positive.2. The Na+ ions diffuse sideways inside the axon towards the neighbouring region, which is still at resting potential and negative inside. On the outside of the membrane, positive ions move the opposite way. These movements of charge are called local currents.3. The local currents make the neighbouring region of membrane less negative. When it reaches threshold, its voltage-gated Na+ channels open and a new action potential is generated there.4. Meanwhile the first region repolarises as K+ channels open and K+ diffuses out. This region is now in its refractory period.The process repeats along the whole length of the axon. Each action potential is regenerated at full size, so the impulse does not fade as it travels, unlike an electrical signal in a long wire.The impulse only moves forwards because the region behind it is refractory: its Na+ channels are inactivated, so the local currents cannot trigger another action potential there.In an unmyelinated axon every part of the membrane must depolarise in turn, so transmission is relatively slow. In myelinated axons the local currents spread between nodes of Ranvier, which is much faster.
1. At the site of an action potential, voltage-gated sodium ion channels are open and Na+ diffuses into the axon. The inside of this region is positive.2. The Na+ ions diffuse sideways inside the axon towards the neighbouring region, which is still at resting potential and negative inside. On the outside of the membrane, positive ions move the opposite way. These movements of charge are called local currents.3. The local currents make the neighbouring region of membrane less negative. When it reaches threshold, its voltage-gated Na+ channels open and a new action potential is generated there.4. Meanwhile the first region repolarises as K+ channels open and K+ diffuses out. This region is now in its refractory period.The process repeats along the whole length of the axon. Each action potential is regenerated at full size, so the impulse does not fade as it travels, unlike an electrical signal in a long wire.The impulse only moves forwards because the region behind it is refractory: its Na+ channels are inactivated, so the local currents cannot trigger another action potential there.In an unmyelinated axon every part of the membrane must depolarise in turn, so transmission is relatively slow. In myelinated axons the local currents spread between nodes of Ranvier, which is much faster.