To better understand the neuronal mechanism of the inspiratory pattern formation, we examined the spatiotemporal activity profile of phrenic motoneurons. We applied an voltage-imaging technique to newborn rat brainstem-spinal cord preparations. The preparations were placed in a recording chamber with a transversely cut plane of the spinal cord at the C4/C5 level horizontal. We recorded respiratory-related depolarizing optical signals on the transversely cut plane of the spinal cord simultaneously with the inspiratory discharge from the C4 ventral root using a high-speed optical recording system MiCAM01. We succeeded in visualizing the dynamic processes of the respiratory-related neuronal excitation propagation on the cut plane of the spinal cord. We found that the activity of phrenic motoneurons consisted of spatially distinct two subpopulations; the inspiratory activity first appeared in the ventrolateral portion of the ventral horn (VL) with a rapid onset and decrementing pattern starting simultaneously with the C4 ventral root discharge, and then another inspiratory activity appeared in the ventromedial portion of the ventral horn (VM). The activity in VL was larger and persisted longer than that in VM. We conclude that the phrenic motoneuron pool is composed of topographically and functionally different two subpopulations and they together generate the phrenic inspiratory discharge pattern. [J Physiol Sci. 2008;58 Suppl:S115]
