What ion depolarizers the membrane when it diffuses into the axon of a neuron?

Sodium ions (Na⁺) are the primary ion that depolarizes the membrane when they diffuse into the axon of a neuron. During an action potential, voltage-gated sodium channels open in response to a threshold stimulus, allowing Na⁺ to flow into the cell. This influx of positively charged sodium ions reduces the negative charge inside the neuron, leading to depolarization and the propagation of the nerve impulse.