From BEAM Robotics Wiki
A Nervous Network can be as simple as a stand alone Central Pattern Generator, or it can be receive and respond to sensory input. In either case, it is generally used to control the motor functions, and therefore the behaviour of BEAM robots.
Pulses traveling though the network are influenced by the sensory data and generate signals that are sent to the actuators (generally small motors), resulting in feedback (e.g changes in the sensory input caused by the work the actuators have performed).
Nervous Networks can be represented in the form of Biomorphic Maps and are realized by implementating the concepts (represented by the maps) in the form of BEAM circuits. These can then be combined with simple mechanical assemblies to create BEAM robots.
Details about Nervous Networks are described in these articles: