English: Describes the structure and function of the mauthner cell axon cap. The Red arrows represent the influx of sodium ions through VGNaCs along their electrochemical potential depolarising the PHP cell axon. Ions passively leave the axon into the axon cap space to balance the charge influx of the sodium current. The potential drop by the resistances of the axon cap and membrane hyperpolarize the M cell. Though there is an influx of positive charge into the M cell axon hillock, the extracellular face of the membrane becomes even more positive leading to a net hyperpolarization.