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Onimaru H, Tsuzawa K, Nakazono Y, Janczewski WA. Midline section of the medulla abolishes inspiratory activity and desynchronizes pre-inspiratory neuron rhythm on both sides of the medulla in newborn rats. J Neurophysiol 113: 2871-2878, 2015. First published February 25, 2015 doi:10.1152/jn.00554.2014.-Each half of the medulla contains respiratory neurons that constitute two generators that control respiratory rhythm. One generator consists of the inspiratory neurons in the pre-Bötzinger complex (preBötC); the other, the pre-inspiratory (Pre-I) neurons in the parafacial respiratory group (pFRG), rostral to the preBötC. We investigated the contribution of the commissural fibers, connecting the respiratory rhythm generators located on the opposite side of the medulla to the generation of respiratory activity in brain stem-spinal cord preparation from 0-to 1-day-old rats. Pre-I neuron activity and the facial nerve and/or first lumbar (L1) root activity were recorded as indicators of the pFRGdriven rhythm. Fourth cervical ventral root (C4) root and/or hypoglossal (XII) nerve activity were recorded as indicators of preBötC-driven inspiratory activity. We found that a midline section that interrupted crossed fibers rostral to the obex irreversibly eliminated C4 and XII root activity, whereas the Pre-I neurons, facial nerve, and L1 roots remained rhythmically active. The facial and contralateral L1 nerve activities were synchronous, whereas right and left facial (and right and left L1) nerves lost synchrony. Optical recordings demonstrated that pFRG-driven burst activity was preserved after a midline section, whereas the preBötC neurons were no longer rhythmic. We conclude that in newborn rats, crossed excitatory interactions (via commissural fibers) are necessary for the generation of inspiratory bursts but not for the generation of rhythmic Pre-I neuron activity.parafacial; pre-inspiratory; respiratory rhythm; pre-Bötzinger; in vitro THE PARAFACIAL RESPIRATORY group (pFRG) around the ventrolateral part of the facial motor nucleus and the pre-Bötzinger complex (preBötC) located ventrolateral to the subcompact part of the ambiguous motor nucleus are the two generators capable of pacing respiratory rhythm. The rhythmogenic circuit of the pFRG is formed of the pre-inspiratory (Pre

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