Final answer:
When the threshold is reached at the SA node, slow calcium channels open causing further depolarization of the membrane.
Step-by-step explanation:
When the threshold is reached at the SA node (an autorhythmic cell), slow calcium channels open causing further depolarization of the membrane. Conductive cells in the heart have sodium ion channels that allow a slow influx of sodium ions, leading to a slow rise in the membrane potential. Once the threshold is reached, calcium ion channels open, allowing an influx of calcium ions that further depolarizes the membrane. This rapid depolarization is followed by repolarization caused by the opening of potassium channels.