Using Computational Neuroscience to Define Common Input to Spinal Motor Neurons

Common input is a widely used concept in motor neurophysiology. It embodies the notion that inputs to individual spinal motor neurons (MNs) are not unique, but partly shared across MNs, and is considered the main explanation for synchronized activity of MNs (Bremner et al. Motor-unit synchronization was first observed in the time domain using cross-correlation histograms from pairs of MNs (Sears and Stagg, 1976). This was later extended to the frequency domain by estimating coherence between spike trains to reveal the frequency content of common input (Farmer et al., 1993). In addition to measurements of individual MNs, coherence can also be estimated between the surface EMG of different muscles—referred to as intermuscular coherence—to assess common input shared across motor-unit pools (Boonstra and Breakspear, 2012). Common input is considered relevant for motor control as it may provide a mechanism to reduce the dimensionality of the control signal, thereby simplifying motor control (Farmer, 1998). Recently the existence of common input has been debated: while some argue that synergistic muscles share most of their synaptic input (Laine et al., 2015), others have argued that common input provides no explanation for MN synchronization (Kline and De Luca, 2016). Apart from common input being used to explain different types of MN synchronization, this dispute mainly arises from the absence of a clear definition of common input (cf. Kirkwood, 2016). Initially, Sears and colleagues defined common input structurally, that is, as resulting from branched presynaptic axons (Sears and Stagg, 1976; Kirkwood and Sears, 1978). Later, presynaptic synchronization—synchronization of neurons that project to the MNs—was proposed as an explanation for broad-peak MN synchronization (central peak in cross-correlogram with a duration of 40–60 ms), which is observed following a lesion of the direct mono-synaptic inputs to MNs and cannot be explained by branched presynaptic axons (Kirkwood et al., 1982). This is a functional definition as it involves correlations between input activities, rather than shared anatomical connections. Although the authors themselves did not refer to this mechanism as common input, the term common input has been invoked quite loosely in recent years, reflecting either shared structural or functional inputs. To resolve this dispute and determine whether MN synchronization is caused by common input, we first need to agree on the definitions of common input and MN synchronization. Computational models are particularly useful as they provide a quantitative and unambiguous description of variables. Computational neuroscience can be used to define …