Most of the nerve cells are gathered together into a long central column forming the brain and spinal cord and known as the cerebro-spinal axis. A few nerve cells, however, exist in groups apart from this - notably those connected with the organs of nutrition (sympathetic nervous system) and those belonging to peripheral sensory neurons.

These aggregations of nerve cells are bound together by connective tissue and form small irregular bodies called ganglia. Small isolated groups of nerve cells within the cerebro­spinal axis are usually designated by the term 'nuclei.'