Abstract
Internal intercostal and abdominal motoneurons are strongly coactivated during expiration. We investigated whether that synergy was paralleled by synergistic Group I reflex excitation. Intracellular recordings were made from motoneurons of the internal intercostal nerve of T8 in anesthetized cats, and the specificity of the monosynaptic connections from afferents in each of the two main branches of this nerve was investigated. Motoneurons 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). Strong specificity was observed, only 2 of 54 motoneurons showing excitatory postsynaptic potentials (EPSPs) from both Lat and Dist. No EO motoneurons showed an EPSP from Dist, and no IIm motoneurons showed one from Lat. Expiratory Dist motoneurons fell into two groups. Those with Dist EPSPs and none 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 motoneurons (assumed to innervate interchondral muscle) showed Dist EPSPs. Stimulation of dorsal ramus nerves gave EPSPs in 12 instances, 9 being in group B Dist motoneurons. 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.
Highlights
THE MONOSYNAPTIC CONNECTION between the Group I afferents of muscle spindles and spinal motoneurons is one of the bestknown synapses in the CNS and is of fundamental historical importance in the understanding of synaptic mechanisms
The following nerves were prepared for stimulation via platinum wire electrodes on the left side of T8: 1) a bundle of dorsal ramus (DR) nerves (Kirkwood et al 1988); 2) the external intercostal nerve; 3) the most proximal point on the internal intercostal nerve (Int; in continuity but arranged to be lifted away from the volume conductor separately from the external intercostal nerve, so as to avoid stimulus spread); 4) the lateral branch of the internal intercostal nerve (Lat); 5) the distal remainder of the internal intercostal nerve (Dist). These nerves were used for antidromic identification of motoneurons, but here we report only those motoneurons identified from the internal intercostal nerve
Recordings were made from 57 motoneurons, all initially identified from the proximal internal intercostal nerve
Summary
THE MONOSYNAPTIC CONNECTION between the Group I afferents of muscle spindles and spinal motoneurons is one of the bestknown synapses in the CNS and is of fundamental historical importance in the understanding of synaptic mechanisms. The intercostal muscles are historically significant in the development of concepts for one particular aspect of motor control in involving spindles, alpha-gamma coactivation This was described for respiration in simultaneous studies by Sears (1963) and by Critchlow and von Euler (1963), showing the importance of signals from the muscle spindles of these muscles in this motor act. We have extended the analyses of the heteronymous Group I input to another pair of muscles active in respiration These are the internal intercostal and the external abdominal oblique, which are well recognized as being close synergists during expiration. We looked for heteronymous monosynaptic EPSPs from the afferents of each muscle in various motoneurons and found connections that were remarkably specific but with a complete absence of monosynaptic excitation between these two particular muscles This leads us to reject the hypothesis and raises further doubts as to the role of the monosynaptic reflex in respiratory movements
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