Model networks with probabilistic connectivity. (a) The network has a single sensory RB neuron exciting neurons in the right half-centre, which also has sensory pathway dlc and dla interneurons. There are ten of each neuron type in each half-centre. The broad pattern of connections is shown by the axons from groups of neurons onto the half-centres (triangles are excitatory and circles are inhibitory synapses). The actual synaptic connections are determined probabilistically for each neuron. Resistor symbols show electrically coupled neuron groups. (b-d) Examples of activity of selected neurons in response to a single stimulus to the sensory RB neuron for networks with connection probabilities based on experiments: (b) sustained swimming; (c) synchronous activity on each side; and (d) no long-lasting response.