Action potentials along axon plasma membrane

• Action potentials are transmitted along axon plasma membrane by local currents; involve positive ion diffusion, down electrochemical gradient, from region in axon, where action potential is occurring, to local regions in cell, where there is no action potential
• Axon membranes where action potential occurs, Na+ ions are diffusing in rapidly, positive ion concentration is very high, compared with cytoplasm regions nearby
• Axon membrane next to depolarised membrane is polarised, having a much lower Na^+ concentration inside so Na+ ions start to diffuse down electrochemical gradient in axon cytoplasm; local flow of current
• As more Na+ ions diffuse to next cytoplasm region and potential difference across membrane increases, this region of plasma membrane is depolarising
• Na+ voltage gated channels will start to open; will lead to action potential, theoretically, current flow could cause it on either side undergoing an action potential
• However, impulse can only be transmitted one direction, since membrane region behind action potential region is in refractory period