Some fraction of an excitatory voltage may reach the axon hillock and may (in rare cases) depolarize the membrane enough to provoke a new action potential.
Like the regions of the sheath near the glial nucleus, the regions of the sheath at the axon hillocks are thicker than those surrounding the rest of the neuron.
This staining method is useful to localize the cell body, as it can be seen in the soma and dendrites of neurons, though not in the axon or axon hillock.
Besides being an anatomical structure, the axon hillock is also the part of the neuron that has the greatest density of voltage-dependent sodium channels.
This all-or-nothing phenomenon originates at the axon hillock, resulting in a depolarization of the intracellular environment which propagates down the axon.