橋呼吸調節中枢の機能 : 特に延髄腹側呼吸性ニューロン群(VRG)の橋への軸索投射様式について

This study was carried out to clarify the function of the pontine "pneumotaxic center" with special reference to the axonal projections of the medullary respiratory neurons to the pons. Adult cats were anesthetized with pentobarbital and immobilized with gallamine triethiodide. The unit discharges of respiratory neurons were extracellulary recorded in the pons and medulla. The effects of electrical stimulation of the pons on the discharges of medullary inspiratory neurons and the phrenic nerve were examined. The pattern of the ascending axonal projections and their branches of medullary respiratory neurons （ventral respiratory group, VRG） were systematically explored by antidromic responses to electrical microstimulation of the rostral pons. In addition, the descending projections of the pontine respiratory neurons were examined. To obtain histological evidence of the axonal connections between the pons and medulla, HRP （horseradish peroxidase） or WGA-HRP （wheat germ agglutinin-conjugated HRP） was injected into the VRG where inspiratory unit discharges were recorded and HRP was injected into the rostral pons. The results were as follows: 1） Unit discharges of the phasic type respiratory neurons were recorded in and around the ambigual nucleus and the retrofacial nucleus in the medulla. In the rostral pons, unit discharges of the phasic type and frequency-modulation type neurons were recorded. There was no obvious difference in the localization between the two types of neurons. Unit discharges of the inspiratory neurons were recorded in and around the medial parabrachial nucleus （Pbm）. On the other hand, those of the expiratory and phase-spanning neurons were recorded more medially and ventrally. 2） Train pulse stimulation of the lateral edge of the Pbm inhibited the discharges of the medullary inspiratory neuron and the phrenic nerve, as well as switched-off the in?spiratory phase to the expiratory phase when the stimulation was given during the second half of the inspiratory phase. 3） Six out of 55 （10.9%） medullary respiratory neurons examined were found to project to the rostral pons and were considered to have many axonal branchings. On the other hand, only 4 out of 98 （4.1%） pontine respiratory neurons examined had axonal projections to the medulla at the obex level and axonal branching was observed in only one neuron. 4） When HRP was injected into Pbm and its surrounding area in the pons, many retrogradely labeled neurons were observed in the bilateral ambigual nuclei and their surrounding areas and in the ipsilateral nucleus of the solitary tract. When HRP was injected into VRG, many retrogradely labeled neurons were observed in the ipsilateral Kolliker-Fuse nucleus （KF） and lateral parabrachial nucleus. When WGA-HRP was injected into VRG, anterogradely labeled axons, terminal fibers and retrogradely labeled neurons were found mainly in the ipsilateral Pbm and KF. The present results strongly suggest that some pontine respiratory neurons receive respiratory rhythmic input from the medullary respiratory neurons and may play a role in respiratory rhythm control.