Motor units as tools to evaluate profile of human Renshaw inhibition

Abstract

Key points To uncover the synaptic profile of Renshaw inhibition on motoneurons, we stimulated thick motor axons and recorded from voluntarily‐activated motor units. Stimuli generated a direct motor response on the whole muscle and an inhibitory response in active motor units. We have estimated the profile of Renshaw inhibition indirectly using the response of motor unit discharge rates to the stimulus. We have put forward a method of extrapolation that may be used to determine genuine synaptic potentials as they develop on motoneurons. These optimized techniques can be used in research and in clinics to fully appreciate Renshaw cell function in various neurological disorders. Abstract Although Renshaw inhibition (RI) has been extensively studied for decades, its precise role in motor control is yet to be discovered. One of the main handicaps is a lack of reliable methods for studying RI in conscious human subjects. We stimulated the lowest electrical threshold motor axons (thickest axons) in the tibial nerve and analysed the stimulus‐correlated changes in discharge of voluntarily recruited low‐threshold single motor units (SMUs) from the soleus muscle. In total, 54 distinct SMUs from 12 subjects were analysed. Stimuli that generated only the direct motor response (M‐only) on surface electromyography induced an inhibitory response in the low‐threshold SMUs. Because the properties of RI had to be estimated indirectly using the background discharge rate of SMUs, its profile varied with the discharge rate of the SMU. The duration of RI was found to be inversely proportional to the discharge rate of SMUs. Using this important finding, we have developed a method of extrapolation for estimating RI as it develops on motoneurons in the spinal cord. The frequency methods indicated that the duration of RI was between 30 and 40 ms depending on the background firing rate of the units, and the extrapolation indicated that RI on silent motoneurons was ∼55 ms. The present study establishes a novel methodology for studying RI in human subjects and hence may serve as a tool for improving our understanding of the involvement of RI in human motor control.

Transcranial magnetic stimulation induced early silent period and rebound activity re-examined

“…The time delay for recurrent inhibition to show its effect should be several milliseconds following MEP and recurrent IPSP can last for about 40–50 ms on firing motoneurons [54]. At around the time onset of IPSP reported in this study, the effect of recurrent inhibition would wither away.…”

“…Our suggestion about the net EPSP during CSP is made up of a contribution from the falling phase of MEP-induced EPSP and TMS-induced IPSPs in cortical and spinal networks. One of the contributing IPSPs may come from the Renshaw circuitries in the spinal cord [54] which may explain prolongation in CSP after GABA-specific drug administration that may cause enhanced/longer recurrent inhibition.…”

Post-activation depression of primary afferents reevaluated in humans

“…However, with the current results, these explanations should stay only at the hypothetical level. On the other hand, high level of P-AD at ISIs lower than 200 msecs may be due afterhyperpolarization of motoneurons, recurrent inhibition and/or other oligosynaptic pathways as these systems may reduce motoneuron activity and result in reduced H-reflex in addition to classical PSI (Chamma et al, 2012;Eccles et al, 1963;Özyurt et al, 2019;Pierrot-Deseilligny and Burke 2005;Schupp et al, 2016).…”

Spatial and Temporal Arrangement of Recurrent Inhibition in the Primate Upper Limb

“…First, the distribution of recurrent inhibition has not been directly studied in primate upper limb motoneurons to date. Indirect studies in man, primarily through conditioning of stretch reflexes, report patterns of recurrent inhibition resembling those seen in cats ( Illert and Kümmel, 1999 ; Katz and Pierrot-Deseilligny, 1999 ), but the necessarily indirect nature of these measures can lead to ambiguities in interpretation ( Özyurt et al, 2019 ). Given the great flexibility and dexterity of the primate arm compared with the limbs of the cat, with augmented motor cortical control, it is important to know whether the organization of recurrent inhibition follows the principles first elucidated in cat hindlimb.…”