A ganglion is a cluster of neuron cell bodies which occur outside the central nervous system (the brain and spinal cord). Neurons are made up of the cell body, with dendrites to conduct nerve impulses towards it and an axon to conduct nerve impulses away from it. The cell body contains the usual organelles, as well as the cell's nucleus (containing genetic information). When a group of neuron cell bodies exists within the central nervous system, it is called a nucleus; when such a group exists outside of the central nervous system (within, therefore, the peripheral nervous system), it is referred to as a ganglion.

In very primitive organisms, no central nervous system can be identified. No distinct brain structure exists. Instead, nervous information is totally relayed through the existence of ganglion bundles.