Just as transistors are fundamental to processors, neurons are fundamental to brains, and it is important that we understand these basic building blocks if we ever hope to understand the brain and intelligence as a whole.
Propagation delay and circular loops. No it doesn't have to be "external" stimuli. There's no explicit input and output layer in how a real brain is constructed. That is an artificial constraint put on artificial neural nets to make a programmers job easier. A real brain can and likely does have self stimulating loops.
>Just as transistors are fundamental to processors, neurons are fundamental to brains
Transistors are also quite simple as individual units and the computational complexity comes about in how they are connected.
This is what I understand causes epilepsy. At least when they are out of control.