Alternation of agonists and antagonists during turtle hindlimb motor rhythms

In a variety of vertebrates, including turtle, many classical and contemporary studies of spinal cord neuronal networks generating rhythmic motor behaviors emphasize a Reciprocal Model with alternation of agonists and antagonists, alternation of excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), and reciprocal inhibition. Some studies of spinal cord neuronal networks, including some in turtle during scratch motor rhythms, describe a Balanced Model with concurrent EPSPs and IPSPs. The present report reviews turtle spinal cord studies and concludes that there is support for a Combined Model with both alternating and concurrent excitation and inhibition, that is, characteristics of both the Reciprocal and the Balanced Models, in the same spinal cord neuronal network for scratch reflex in turtle. Studies of spinal cord neuronal networks for locomotion in a variety of vertebrates also support a Combined Model.