Respiratory neurons in medullary reticular formation which directly project to hypoglossal motoneurons in cats

The present study was carried out to determine the location of the excitatory premotor neurons projecting to the hypoglossal motoneurons (XII MNs) that show respiratory activities in pentobarbital-anesthetized cats. Projections from these hypoglossal premotor neurons to the phrenic motoneurons were also investigated. The following results were obtained. (1) Injection of a fluorescent dye (Fast Blue, FB) into the hypoglossal nucleus and another fluorescent dye (Nuclear Yellow, NY) into the phrenic nucleus the cells in the medullary reticular formation labelled retrogradely with FB and/or NY mainly in the region ventrolateral to the nucleus of the tractus soritarius (vl-NTS) and dorsomedial to the nucleus ambiguus (dm-AMB). (2) There were respiratory neurons in the region vl-NTS and dm-AMB which antidromically responded to the stimulation of the hypoglossal nucleus. Some of them antidromically responded also to the stimulation of the phrenic nucleus. (3) Averaging of the hypoglossal and phrenic nerve discharges by spontaneous spikes of single respiratory neurons in the region vl-NTS and dm-AMB revealed a facilitation in the hypoglossal nerve discharge. In some of these excitatory hypoglossal premotor neurons, a facilitation was also revealed in the phrenic nerve discharge. It was concluded that there were respiratory neurons in the region vl-NTS and dm-AMB which were excitatory premotor neurons projecting to the XII MNs showing respiratory activities. Some of them had excitatory projections to both the hypoglossal and phrenic motoneurons via the bifurcating axons.