Proceedings of The Physiological Society

Physiology 2016 (Dublin, Ireland) (2016) Proc Physiol Soc 37, PCA280

Poster Communications

The differential recruitment of parasternal intercostal motor units during inspiration is not preserved for a voluntary postural task

A. L. Hudson1, S. C. Gandevia1, J. E. Butler1

1. Neuroscience Research Australia and University of New South Wales, Sydney, New South Wales, Australia.

  • Table 1. Respiratory and rotation parameters and onset recruitment behaviour of single motor units (SMU) in the 2nd and 4th parasternal intercostal (PSIC) muscles during quiet breathing or rotations. * significantly different to the 4th interspace.

The human parasternal intercostal muscles are obligatory inspiratory muscles, but there is differential recruitment of parasternal intercostal motor units in the first-to-fifth interspaces (Gandevia et al., 2006). The parasternal intercostals are also active in ipsilateral rotation of the chest wall and we showed previously that the same motor units are active in quiet breathing and voluntary rotations (Hudson et al., 2010). However, it is not known if the differential recruitment of motor units across interspaces, as observed during inspiration, is preserved in a non-respiratory task of chest wall rotation. Healthy volunteer subjects (n=5; all male) were seated and breathed through a pneumotachometer. Intramuscular recordings were made from the parasternal intercostal muscles in the 2nd and 4th interspaces while subjects performed quiet breathing and ramped ‘isometric' ipsilateral rotations of the chest wall during apnoea. The recruitment behaviour of single motor units was compared during quiet breathing and rotations. For breathing, the onset of each motor unit (TO) was determined and expressed relative to total inspiratory time (TI) to represent the proportion of TI that the unit was active onset (i.e. [TI-TO/TI%]). For rotations, the torque at which the motor unit began to discharge was determined as a percentage of maximal rotation torque for each subject. Data were compared between the 2nd and 4th interspaces using t-tests and median [IQR] values are shown. Single motor units active in both quiet breathing and ipsilateral rotations were discriminated from the 2nd (n=44) and 4th (n=56) interspaces. Respiratory parameters during these recordings were matched for inspiratory time and mean flow, but tidal volume was higher during recordings from the 2nd interspace. The inspiratory onset time of motor units in the 2nd interspace was significantly earlier than the 4th interspace. However, for the same motor units, the rotation force at which motor units were recruited was similar in the 2nd and 4th interspaces, for rotations of similar peak and rate of torque. With voluntary drive for the rotation task, there was divergence from the differential recruitment observed during inspiration. This suggests that parasternal intercostal motoneurone output at different spinal levels can change depending on task and supports a spinal mechanism that integrates and distributes descending drive to different human inspiratory muscles.

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