The nervous system of all vertebrates is built up of long conducting, specialized cells, termed neurons. In order to function, these neurons, or nerve-cells, must be linked together in chains or arcs. Each neuron in the chain or arc is structurally separated from other neurons by a break termed a synapse. The structural elements of a neuronic arc are, therefore, as follows:

1. 1. A sensitive receiving organ—the receptor or sense-organ.

2. 2. A centrally conducting receptor bipolar neuron.

3. 3. One or more short connector or internuncial neurons.

4. 4. A peripherally conducting effector multipolar neuron.

5. 5. An effector apparatus—for example, a muscle or a gland.