Final answer:
The sodium (Na+) ion is responsible for local currents that depolarize parts of the axon to threshold, enabling the propagation of the action potential along the axon.
Step-by-step explanation:
The diffusion of the sodium (Na+) ion across the neuronal membrane is responsible for the local currents that depolarize regions of the axon to threshold. When a stimulus from a sensory cell or another neuron depolarizes the neuron to its threshold potential, voltage-gated Na+ channels in the axon hillock open, allowing these positive ions to rush into the cell. This results in the downward propagation of the action potential along the axon.
The entry of Na+ into the neuron is an all-or-nothing event that leads to complete depolarization of the neuron, with the membrane potential reversing from around -70mV to about +40mV. Once the Na+ channels open and the action potential is triggered, they can't open again until they have been reset, starting the neuron's refractory period.