Abstract
Cross-correlation of neural discharges was used to investigate the connections between expiratory bulbospinal neurons (EBSNs) in the caudal medulla and expiratory motoneurons innervating thoracic and abdominal muscles in anesthetized cats. Peaks were seen in the cross-correlation histograms for around half of the EBSN-nerve pairs for the following: at T8, the nerve branches innervating internal intercostal muscle and external abdominal oblique muscle and a more distal branch of the internal intercostal nerve; and at L1, a nerve branch innervating internal abdominal oblique muscle and a more distal branch of the ventral ramus. Fewer peaks were seen for the L1 nerve innervating external abdominal oblique, but a paucity of presumed α-motoneuron discharges could explain the rarity of the peaks in this instance. Taking into account individual EBSN conduction times to T8 and to L1, as well as peripheral conduction times, nearly all of the peaks were interpreted as representing monosynaptic connections. Individual EBSNs showed connections at both T8 and L1, but without any discernible pattern. The overall strength of the monosynaptic connection from EBSNs at L1 was found to be very similar to that at T8, which was previously argued to be substantial and responsible for the temporal patterns of expiratory motoneuron discharges. However, we argue that other inputs are required to create the stereotyped spatial patterns of discharges in the thoracic and abdominal musculature.
Highlights
IN THE NEUROPHYSIOLOGY OF RESPIRATION, expiration is often regarded as the poor relation of inspiration, even by authors who champion its “rediscovery” in the central pattern generator of the rat brain stem (Feldman et al 2013)
This study investigates the nature of the linkages between expiratory bulbospinal neurons (EBSNs) and both intercostal and abdominal motoneurons, which are presently not well documented
EBSNs were recorded in two types of experiments, first in two control animals where only projection frequencies were investigated (n ϭ 34), and second in those where the connections from EBSNs to motoneurons were assessed (n ϭ 27)
Summary
IN THE NEUROPHYSIOLOGY OF RESPIRATION, expiration is often regarded as the poor relation of inspiration, even by authors who champion its “rediscovery” in the central pattern generator of the rat brain stem (Feldman et al 2013). Anatomic experiments have shown that the neurons of this nucleus in the cat project widely to spinal motor nuclei (Holstege 1991) and make extensive direct connections to motoneurons at cervical, thoracic, and lumbar levels With regard to EBSNs the anatomic evidence is only indirect, as emphasized by observations in the lower lumbar and sacral cord For these segments, Boers et al (2005) showed that the projections from the NRA included EBSNs and many non-EBSNs, the non-EBSNs being more likely to project to the motor nuclei than the EBSNs. The experiments described in this article clarify the situation by showing the presence of similar direct connections from EBSNs to thoracic and to upper lumbar expiratory motoneuwww.jn.org
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