OK, they are trying to sell stuff at this web site, but I found their explanation as something I could understand, so I thought someone else may find it of value as well.
The nervous system is made up of groups of cells called neurons. A neuron is a cell body with a nucleus, a dendrite (which receives input) and an axon (which relays signals to other cells). Neurons transmit information through the body in a manner similar to the way electricity flows through a wire.
A nervous impulse begins with a reversal of the concentrations of sodium and potassium both inside and outside the neuron. Sodium rushes into the cell and potassium rushes out, generating momentum that conducts an impulse down the axon and toward the dendrite of the next neuron. This transmission continues from cell to cell until the message is delivered. All this takes place at incredible speeds.
Some of the spaces between nerve cells, called junctions, are so small that the impulse can travel from one nerve to the next very quickly. These are called gap junctions. However, most neurons relay signals across larger spaces called synaptic junctions. These require chemical messengers called neurotransmitters.
Neurotransmitters are released into a synaptic junction by one cell and bond with receptors on the membrane of the neighboring cell. After the electronic signal has been relayed, the neurotransmitters are deactivated and recycled. Some neurotransmitters include serotonin, acetylcholine, dopamine, GABA (gamma amino butyric acid) an norepinephrine (noradrenaline).
Abnormal levels of neurotransmitters can cause an assortment of complications. For instance, abnormally low levels of serotonin are associated with obesity, depression, sleep disorders, anxiety and migraine headaches.
Many neurons are surrounded by special cells that form an outer layer called the myelin sheath. The myelin is composed mostly of lipids. It insulates the neurons to speed up the conduction of impulses, and it prevents "short circuits" between adjacent neurons. Deterioration of the myelin sheath is associated with many degenerative disorders of the nervous system.