Final answer:
The propagation of electric impulses in neurons is primarily caused by the action potential. It begins when the neuron receives a chemical signal, which opens ion channels and allows positive ions to flow into the cell.
Step-by-step explanation:
The propagation of electric impulses in neurons is primarily caused by the action potential. An action potential is a sudden reversal of the electrical charge across the membrane of a resting neuron. It begins when the neuron receives a chemical signal, which opens ion channels and allows positive ions to flow into the cell. This reversal of charge, known as the action potential, then propagates down the axon to transmit the electrical impulse.