Tinbergen added complexity to this model, a model now known as Tinbergen's hierarchical model. He suggested that motivational impulses build up in nervous centres in the brain which are held in check by blocks. The blocks are removed by an innate releasing mechanism that allows the energy to flow to the next centre (each centre containing a block that needs to be removed) in a cascade until the behaviour is expressed. Tinbergen's model shows multiple levels of complexity and that related behaviours are grouped.