  A self-organising model is proposed to explain the criticality in cortical
networks deduced from recent observations of neuronal avalanches. Prevailing
understanding of self-organised criticality (SOC) dictates that conservation of
energy is essential to its emergence. Neuronal networks however are inherently
non-conservative as demonstrated by microelectrode recordings. The model
presented here shows that SOC can arise in non-conservative systems as well, if
driven internally. Evidence suggests that synaptic background activity provides
the internal drive for non-conservative cortical networks to achieve and
maintain a critical state. SOC is robust to any degree $\eta \in (0,1]$ of
background activity when the network size $N$ is large enough such that $\eta
N\sim 10^3$. For small networks, a strong background leads to epileptiform
activity, consistent with neurophysiological knowledge about epilepsy.
