Neural networks biological origins

Whereas the former requirements are inspired by real, biological neural networks, the latter one has been introduced for purely technical reasons — to make a formal description of the artificial neural network easier. On the other hand, that condition does not actually impose any real restriction on the ANN, in the sense that any architecture in which it does not hold can be extended to an architecture in which it does hold, while between any two neurons from the original, non-extended architecture, a connection exists in the extended architecture if and only if it already exists in the original one. 

---