Connections between inspiratory medullary neurons and phrenic or intercostal motoneurones
Hilaire, G.; Monteau, R.
Journal de Physiologie 72(8): 987-1000
1976
ISSN/ISBN: 0021-7948 PMID: 1030738 Document Number: 103714
The activity of 107 medullary inspiratory neurons was recorded extracellularly in anesthetized cats (urethane-chloralose). According to their localization in the medulla and to their axonal pathways (tested by antidromic activation), these neurons were classified as: bulbo-spinal neurons (NBSI) which send their axons to the spinal cord; they are located in the dorsal or the ventral respiratory nucleus; propriobulbar neurons (NPBI) whose axons are probably entirely located within the medulla; they are found only in the ventral nucleus. Summation of gross discharges from phrenic (C5 ventral root) or inspiratory intercostal (from T4-T10) nerves was triggered by spikes from medullary respiratory neurons. If the studied neuron controls the activity of the recorded motor fibers, after summation of a great number of sweeps (3000-5000), the summed nerve activity displays a short delay excitatory wave, indicating that the activity of the motoneurons is increased after the onset of the medullary potential. On the other hand, there is a flat summed nerve activity if the nerve discharge is not directly dependent on the medullary recorded activity. The dorsal nucleus NBSIs send their axon to the contralateral phrenic motoneurons and not to the intercostal motoneurons. The ventral nucleus NBSIs control contralateral intercostal motoneurons. Some of them send their axons only to these motoneurons, but the majority also act on phrenic motoneurons. Connections between NBSIs and phrenic motoneurons are monosynpatic; those between NBSIs and intercostal motoneurons are probably also monosynaptic. No excitatory wave observed on phrenic or intercostal summed activity when summation was triggered by NPBI spikes. This confirms the accuracy of the antidromic stimulation test used to classify medullary respiratory neurons.