It causes degeneration of the myelin sheaths. It prevents the release of ACh by presynaptic cells. It prevents the removal of dopamine from the synaptic cleft. An action potential is brought about by a sudden membrane impermiability the movement of negatively-charged proteins throught the plasma membrane. Multiple sclerosis is caused by a common soil bacteria and can be prevented by vaccination.
Many axons make up a nerve.
Many nerves make up an axon. Many axons make up a myelin sheath. Many myelin sheaths make up a dendrite. All neurotransmitters have an excitatory effect. Neuromodulators can magnify or reduce the effects of neurotransmitters.