Neurons

     The axon hillock of a neuron is dense with sodium voltage-gated channels, which acts as a spike-initiation zone. Once the charge is increased significantly, an action potential propagates across the neuron in an all-or-nothing action. This means that once the signal is going to go through, nothing can stop it.
    Neurons have myelin sheaths along their axons which facilitate current flow. These are found in Schwann cells in the PNS, oligodendroglia in the CNS, and help with saltatory conduction. The space in between each myelin sheath is called a Nodes of Ranvier. When a neuron is sending an action potential, the signal jumps across nodes of ranvier, to allow faster propagation of the signal. If the insulation is lost, the conduction of velocity slows. This can be seen in multiple sclerosis, which is an autoimmune degeneration of myelin sheaths.