Dissociated neuronal cultures provide a useful platform to study behavior and development of biological neural networks. Isolated from external inputs neural cultures generate electrical activity of their own, showing several features. The most striking feature is the phenomenon of, more or less regular, network bursts, i.e. simultaneous firing of many neurons in a relatively short time window. In this paper we address the issue of spontaneous bursting activity in cortical neuronal cultures and explain what might cause this collective behavior using computer simulations of two different neural network models. While the common approach to activate a passive network is done by introducing synaptic noise, we show that a small subset of pacemaker neurons can trigger network bursts which better resemble experimental bursts.