Potassium channels opens, allowing potassium ions to move out. Some sodium channels open in the dendrite. All ion channels close and the Na+/K+ exchange pump starts working. Sodium ions diffuse toward the first node of Ranvier in the axon Lights hit the retina. The threshold level is reached at the first node of Ranvier in the axon Sodium channels open in the neural membrane of the axon, allowing the remaining sodium ions to move in.