Expiratory abdominal muscle nerve is active at flexor phase, while inspiratory phrenic nerve is not active during locomotion evoked by 5-HT and NMDA in the neonatal rat.

Abdominal muscles are involved in respiration and locomotion. In the isolated pons-spinal cord-rib attached preparation from neonatal rat, the phrenic nerve and abdominal muscles show inspiratory and expiratory activity, respectively. Using this preparation, we investigated whether the bath application of NMDA and 5-HT could evoke locomotor activities in the fourth cervical ventral root (C4VR), phrenic nerve, and abdominal muscle nerve (ilioinguinal nerve, IIG-n). We also observed rib and abdominal muscle movements visually. The phrenic nerve and C4VR showed inspiratory activity consistently under the control conditions, whereas IIG-n showed expiratory activity only at the beginning of the experiment. During the chemically-induced locomotion, both C4VR and IIG-n showed locomotor activity, and IIG-n in particular showed flexor activity. During the flexor activity, lateral bending of the rib cage to the recording site was observed. The phrenic nerve showed weak or no apparent locomotor activity. We concluded that the central pattern generator (CPG) for locomotion provides stronger excitatory synaptic inputs to C4 motoneurons innervating neck and shoulder muscles than the inputs to the phrenic motoneurons. Thus, the locomotor CPG provides a suitable amount of inputs to the functionally proper motoneurons. This preparation will be useful to explore how the respiratory and locomotor CPGs select proper motoneurons to give synaptic inputs and are coordinated with each other.