rhythm on both sides of the medulla in newborn rats

26 Each half of the medulla contains respiratory neurons which compose two 27 generators that control respiratory rhythm. One generator consists of the inspiratory 28 neurons in the preBötzinger Complex (preBötC); the other of the preinspiratory (Pre-I) 29 neurons in parafacial respiratory group (pFRG) rostral to the preBötC. We investigated 30 the contribution of the commissural fibers connecting the respiratory rhythm generators 31 located on the opposite side of the medulla, to the generation of respiratory activity in 32 brainstem-spinal cord preparation from 0-1 day-old rats. Pre-I neuron activity and the 33 facial nerve and/or first lumbar (L1) root activity were recorded as indicators of the 34 pFRG driven rhythm. C4 root and/or hypoglossal nerve activity were recorded as 35 indicators of preBötC driven inspiratory activity. We found that a midline section which 36 interrupted crossed fibers rostral to the obex irreversibly eliminated C4 and XII root 37 activity, while the Pre-I neurons, facial nerve and L1 roots remained rhythmically active. 38 The facial and contralateral L1 nerve activities were synchronous, whereas right and left 39 facial (and right and left L1) nerves lost synchrony. Optical recordings demonstrated 40 that pFRG-driven burst activity was preserved after a midline section while the preBötC 41 neurons were no longer rhythmic. We conclude that in newborn rats, crossed excitatory 42 interactions (via commissural fibers) are necessary for the generation of inspiratory 43 bursts, but not for the generation of rhythmic Pre-I neuron activity. 44 45