Motoneurone synchronization for intercostal and abdominal muscles: interneurone influences in two different species

J D Road,A T R De Almeida,P A Kirkwood

doi:10.1007/s00221-020-05924-6

Abstract

The contribution of branched-axon monosynaptic inputs in the generation of short-term synchronization of motoneurones remains uncertain. Here, synchronization was measured for intercostal and abdominal motoneurones supplying the lower thorax and upper abdomen, mostly showing expiratory discharges. Synchronization in the anaesthetized cat, where the motoneurones receive a strong direct descending drive, is compared with that in anaesthetized or decerebrate rats, where the direct descending drive is much weaker. In the cat, some examples could be explained by branched-axon monosynaptic inputs, but many others could not, by virtue of peaks in cross-correlation histograms whose widths (relatively wide) and timing indicated common inputs with more complex linkages, e.g., disynaptic excitatory. In contrast, in the rat, correlations for pairs of internal intercostal nerves were dominated by very narrow peaks, indicative of branched-axon monosynaptic inputs. However, the presence of activity in both inspiration and expiration in many of the nerves allowed additional synchronization measurements between internal and external intercostal nerves. Time courses of synchronization for these often consisted of combinations of peaks and troughs, which have never been previously described for motoneurone synchronization and which we interpret as indicating combinations of inputs, excitation of one group of motoneurones being common with either excitation or inhibition of the other. Significant species differences in the circuits controlling the motoneurones are indicated, but in both cases, the roles of spinal interneurones are emphasised. The results demonstrate the potential of motoneurone synchronization for investigating inhibition and have important general implications for the interpretation of neural connectivity measurements by cross-correlation.

Highlights

It is widely accepted that two motoneurones engaged in a common task will be more likely to fire within a few ms of each other than chance would predict, i.e., their discharges will be, to an extent, synchronized
Whereas the timedomain approach concentrated on “short-time synchronization” (STS), regarded by Sears and Stagg (1976) and by Milner Brown et al (1973) as an inevitable consequence of the known branching of presynaptic axons to innervate a high proportion of the motoneurones of a given motor nucleus, the frequency-domain approach has revealed other features of motor drives, such as rhythmicities, that likely originate in higher centres
Three examples, illustrating some of the variation seen in the time courses of the peaks, are included in Fig. 3, which illustrates the measurements of the baseline count, m, the amplitude of the peak, k, and the half-width

Summary

Introduction

It is widely accepted that two motoneurones engaged in a common task will be more likely to fire within a few ms of each other than chance would predict, i.e., their discharges will be, to an extent, synchronized This is the case for motoneurones innervating the same muscle, but is true for close agonists and even sometimes for anatomical antagonists. Whereas the timedomain approach concentrated on “short-time synchronization” (STS), regarded by Sears and Stagg (1976) and by Milner Brown et al (1973) as an inevitable consequence of the known branching of presynaptic axons to innervate a high proportion of the motoneurones of a given motor nucleus, the frequency-domain approach has revealed other features of motor drives, such as rhythmicities, that likely originate in higher centres. The interest of these studies has been in rather general properties of motor control, together with the possible functional significance of the synchronization itself

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Conclusion