Rhythmic motor patterns for locomotion in vertebrates are generated in spinal cord neural networks known as spinal Central Pattern Generators (CPGs). A key element in pattern generation is the role of glycinergic synaptic transmission by interneurons that cross the cord midline and inhibit contralaterally-located excitatory neurons. The glycinergic inhibitory drive permits alternating and precisely timed motor output during locomotion such as walking or swimming. To understand better the evolution of this system we examined the physiology of the neural network controlling swimming in an invertebrate chordate relative of vertebrates, the ascidian larva Ciona intestinalis.
Nishino, A., Okamura, Y., Piscopo, S., & Brown, E. R. (2010). A glycine receptor is involved in the organization of swimming movements in an invertebrate chordate. BMC Neuroscience, 11(n/a), . https://doi.org/10.1186/1471-2202-11-6