The physiological viability of the model.

<p>Active pathways at stopping. The three large boxes in the centre of the figure are the three neuro-muscular systems as indicated from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone-0078246-g001" target="_blank">Fig. 1</a>. Their existence is physiologically well established <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Bschges2" target="_blank">[15]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Bschges3" target="_blank">[27]</a>. Their inner structure has been partially confirmed in experiments <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Bssler3" target="_blank">[10]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Bssler4" target="_blank">[11]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Westmark1" target="_blank">[24]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Bschges3" target="_blank">[27]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Laurent2" target="_blank">[31]</a>. The small boxes CC and S1–S6 are CPG-like functional units consisting of mutually inhibitory neurons. Such small networks of mutually inhibitory neurons do exist in the nervous system of athropods <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Mullroney1" target="_blank">[32]</a> and other animals <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Marder1" target="_blank">[33]</a>. However, such units with the specific roles assigned to them in the present model have not been identified. Triangle CI represents the common inhibitory motoneuron. Its existence is proven in experiments <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Wolf1" target="_blank">[17]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Ballantyne1" target="_blank">[19]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Iles1" target="_blank">[21]</a>. N1-N5 are neurons whose existence with the specific function they have in the present model has not yet been established. Pentagons , , are sense organs where signals reflecting the position (angle) are generated; pentagons , are sense organs where signals reflecting the (angular) velocity are generated. Such sense organs do exist in insects (e.g. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Bssler2" target="_blank">[6]</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone.0078246-Hustert1" target="_blank">[8]</a>). Small empty triangle on neurons or on system boxes are excitatory synapses. Filled black squares symbolize inhibition by CI of the slow muscle fibres. Filled black circles are inhibitory synapses. Very small black circles along paths (neuronal connections) are branching points. Red pathways are the connections that are active at stop of stepping. Grey pathways are the connections that are active at start of stepping (see. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone-0078246-g008" target="_blank">Figs. 8</a>–<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0078246#pone-0078246-g009" target="_blank">9</a>). For more explanation as to the workings of this network, see text.