Motor units as tools to evaluate profile of human Renshaw inhibition

Key Points

To uncover synaptic profile of Renshaw inhibition on motoneurons we stimulated thick motor axons and recorded from voluntarily‐activated motor units. Stimuli generated 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 an idea of extrapolation which 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 for this is the lack of reliable methods to study 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. A total of 54 distinct SMUs from 12 subjects were analysed. Stimuli that generated only the direct motor response (M‐only) on surface electromyography (SEMG) induced an inhibitory response in the low‐threshold SMUs. Since the properties of the 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 the RI was found to be inversely proportional to the discharge rate of SMUs. Using this important finding, we have developed a method of extrapolation to estimate RI as it develops on motoneurons (MNs) in the spinal cord. The frequency methods indicated that the duration of the RI was between 30 to 40 ms depending on the background firing rate of units, and the extrapolation indicated that the RI on silent MNs was around 55 ms. This study establishes a novel methodology for studying RI in human subjects and hence may serve as a tool towards improving our understanding of RI involvement in human motor control.

This article is protected by copyright. All rights reserved

from Physiology via xlomafota13 on Inoreader http://bit.ly/2Wg8Vj3
via IFTTT