Segmental oscillators in axial motor circuits of the salamander : 1 distribution and bursting mechanisms . 2 3 4

34 The rhythmic and coordinated activation of axial muscles that underlie trunk 35 movements during locomotion are generated by specialized networks in the spinal cord. The 36 operation of these networks has been extensively investigated in limbless, swimming 37 vertebrates. But little is known about the architecture and functioning of the axial locomotor 38 networks in limbed vertebrates. We investigated the rhythm-generating capacity of the axial 39 segmental networks in the salamander (Pleurodeles waltlii). We recorded ventral root activity 40 from hemisegments and segments that were surgically isolated from the mid-trunk cord and 41 chemically activated with bath-applied N-methyl-D-aspartate (NMDA). We provide evidence 42 that the rhythmogenic capacity of the axial network is distributed along the mid-trunk spinal 43 cord, without an excitability gradient. We demonstrate that the burst generation in a 44 hemisegment depends on glutamatergic excitatory interactions. Reciprocal glycinergic 45 inhibition between opposite hemisegments ensures left-right alternation and lowers the 46 rhythm frequency in segments. Our results further suggest that persistent sodium current 47 contributes to the rhythmic regenerating process both in hemisegments and segments. Burst 48 termination in hemisegments is not achieved through the activation of apamine-sensitive 49 Ca-activated K channels and burst termination in segments relies on crossed glycinergic 50 inhibition. Together, our results indicate that the basic design of the salamander axial network 51 is similar to most of axial networks investigated in other vertebrates, albeit with some 52 significant differences in the cellular mechanism that underlies segmental bursting. This 53 finding supports the view of a phylogenetic conservation of basic building blocks of the axial 54 locomotor network among the vertebrates. 55 56