Abdominal and internal intercostal motoneurones are strong synergists for expiration but are not synergists for Group I monosynaptic afferent inputs
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Internal intercostal and abdomininal motoneurones are strongly co-activated during expiration (Saywell et al. 2007; Road et al. 2013). We investigated whether that synergy was paralleled by synergistic Group I reflex excitation. Intracellular recordings were made from motoneurones of the internal intercostal nerve of T8 in anaesthetized cats and the specificity of the monosynaptic connections from afferents in each of the two main branches of this nerve was investigated by observing the presence or absence of short latency EPSPs from stimulation of each nerve branch. Cats were anaesthetized with sodium pentobarbitone (initially 37.5 mg/kg I.P., subsequently as required, I.V.). Subsequent to surgery they were subjected to neuromuscular blockade (gallamine triethiodide, I.V., repeated doses 24 mg, as required). Ventilation was supplemented with CO2 to enhance the respiratory drive. Data come from the same animals as in Saywell et al. (2007), which should be consulted for procedures for ensuring adequate anaesthesia. Motoneurones were shown by antidromic excitation to innervate three muscle groups: external abdominal oblique, EO (innervated by the lateral branch, Lat), the region of the internal intercostal muscle proximal to the branch point (IIm) and muscles innervated from the distal remainder (Dist). Most motoneurones showed an expiratory drive potential (13/13 for EO, 12/12 for IIm and 22/32 for Dist), with 8/34 from Dist showing an inspiratory drive potential. Strong specificity was observed in the connections, only 2/54 motoneurones showing EPSPs from both Lat and Dist. No EO motoneurones showed an EPSP from Dist and no IIm motoneurones showed an EPSP from Lat. The expiratory Dist motoneurones fell into two groups. Those with EPSPs from Dist but not from Lat (Group A) were assumed to innervate distal internal intercostal muscle. Those with Lat EPSPs (Group B) were assumed to innervate abdominal muscle (transversus abdominis or rectus abdominis). Inspiratory Dist motoneurones (assumed to innervate interchondral muscle) showed EPSPs from Dist. Stimulation of dorsal ramus nerves gave EPSPs in 11 instances, 9 being in Group B Dist motoneurones. The complete absence of heteronymous monosynaptic Group I reflex excitation between muscles that are synergistically activated in expiration leads us to conclude that such connections from muscle spindle afferents of the thoracic nerves have little role in controlling expiratory movements but, where present, support other motor acts.
|Title of host publication
|Proceedings of The Physiological Society
|Number of pages
|Published - 2014