A positive feedback loop is a cycle in which the factor that initiates the cycle is generated again at the end of the cycle, restarting the loop. In neuron signaling, the cycle of rapid membrane depolarization during the action potential is a positive feedback loop. Here, the reception of a signal from a previous cell depolarizes the region around the first set of voltage-gated sodium channels along the axon. This causes the sodium channels to open, allowing sodium ions in. This further depolarizes the surrounding area, causing the next set of sodium channels to open. This continues in a chain down the axon.
(“Positive feedback,” 1999)