In the brain we usually distinguish between two types of networks, functional and structural. Although the network theoretical background can be applied to both of them, the definition of nodes and edges can be different.
Viewing the human brain as a network originated at the end of the 19th century with the so called disconnection syndrome, where the connection between parts of the brain were disrupted. However the combination of using neuroimaging, for example with MRI, with network theory is relatively recent. The use of network theory in the human brain increased in particular, once it was discovered that the brain topology shares aspects of small-world and rich-club organisation.
For example, structural brain networks are often defined using the following steps. First, the grey matter is divided into regions of interest (nodes). Using diffusion MRI and tractography, the structural connectivity (edges) are estimated between the individual brain regions. Although the exact techniques used to define nodes and edges might differ, the result is a structural brain network which can be used to investigate developmental aspects and the effects of diseases.