The Neuron chip is a complete LonWorks system on a chip, and contains three CPU’s.
One is the Media Access Processor (MAC), and is responsible for the sending and receiving of messages on the network.
The Network Processor is responsible for the middle layers of the protocol. Doing things like packet routing, destination addressing, end-to-end acknowledgements, retries, duplicate message detection, etc.
The third processor is the Application processor, which runs the users custom application.
Having separate processors allows the application to still send and receive messages even if the network processor gets very busy.