In cells other than those in the SA and AV nodes, sodium ions enter the cell via the voltage-gated sodium channel to generated the inward-going rapid sodium current (Ina), which is the dominant depolarizing current. These cells also contain calcium channels through which calcium ions flow from the outside to the inside of the cell generating the slowly activated calcium current. This calcium current is "turned on" after the sodium current is activated and contributes to the terminal portion of the action potential upstroke and to the action potential plateau.