Investigating human motor control by transcranial magnetic stimulation

Abstract

In this review we discuss the contribution of transcranial magnetic stimulation (TMS) to the understanding of human motor control. Compound motor-evoked potentials (MEPs) may provide valuable information about corticospinal transmission, especially in patients with neurological disorders, but generally do not allow conclusions regarding the details of corticospinal function to be made. Techniques such as poststimulus time histograms (PSTHs) of the discharge of single, voluntarily activated motor units and conditioning of H reflexes provide a more optimal way of evaluating transmission in specific excitatory and inhibitory pathways. Through application of such techniques, several important issues have been clarified. TMS has provided the first real evidence that direct monosynaptic connections from the motor cortex to spinal motoneurons exist in man, and it has been revealed that the distribution of these projections roughly follows the same proximal-distal gradient as in other primates. However, pronounced differences also exist. In particular, the tibialis anterior muscle appears to receive as significant a monosynaptic corticospinal drive as muscles in the hand. The reason for this may be the importance of this muscle in controlling the foot trajectory in the swing phase of walking. Conditioning of H reflexes by TMS has provided evidence of changes in cortical excitability prior to and during various movements. These experiments have generally confirmed information obtained from chronic recording of the activity of corticospinal cells in primates, but information about the corticospinal contribution to movements for which information from other primates is sparse or lacking has also been obtained. One example is walking, where TMS experiments have revealed that the corticospinal tract makes an important contribution to the ongoing EMG activity during treadmill walking. TMS experiments have also documented the convergence of descending corticospinal projections and peripheral afferents on spinal interneurons. Current investigations of the functional significance of this convergence also rely on TMS experiments. The general conclusion from this review is that TMS is a powerful technique in the analysis of motor control, but that care is necessary when interpreting the data. Combining TMS with other techniques such as PSTH and H reflex testing amplifies greatly the power of the technique.

Age-related decrease in motor cortical inhibition during standing under different sensory conditions

“…In contrast, studies examining wrist flexors and extensors found more SICI in old than in young adults (Kossev et al, 2002; McGinley et al, 2010). As the TA exhibits nearly similar density of corticospinal projections as hand muscles (Brouwer and Ashby, 1992; Petersen et al, 2003), results of the present study support the hypothesis of a specific reduction in SICI for muscles with rich corticospinal projections, suggesting that aging further strengthens the corticomotoneuronal pathway by diminishing intracortical inhibition. Nonetheless, this may also reflect a greater contribution of this pathway during upright standing, regardless of leg muscles (Baudry et al, 2014b).…”

Non cell-autonomous role of DCC in the guidance of the corticospinal tract at the midline

“…MEPs elicited by stimulation of the motor cortex with TMS result from the transmission of a neuronal signal from the cortex to the peripheral muscles via fast-conducting CST fibers and spinal motoneurons 38–40 . Direct connections of CST axons to motoneurons (direct cortico-motoneuronal connections) make a significant contribution to MEPs in humans 41–44 . In our patients, the presence of ipsilateral MEPs with latencies similar to those of contralateral MEPs indicates direct connections of CST axons from one hemisphere to hand muscle motoneurons on both sides of the spinal cord.…”

Combined action observation and imagery facilitates corticospinal excitability

“…The combined action observation and imagery condition also produced MEPs of larger amplitude than passive observation alone (PO). Changes in MEP amplitude represent modulation of corticospinal excitability (Rothwell, 1997; Petersen et al, 2003; Naish et al, 2014). The results therefore indicate that combined action observation and imagery of simple human movements can facilitate corticospinal excitability, and the extent of this facilitation is greater than occurs during PO alone.…”

“…When TMS is applied to the primary motor cortex, motor evoked potentials (MEPs) are produced in the corresponding muscles; the amplitude of which provides a marker of corticospinal excitability at the time of simulation (Rothwell, 1997; Petersen et al, 2003; Naish et al, 2014). Research into action observation indicates that single-pulse TMS delivered to participants’ motor cortex during observation of human movements produces MEPs of larger amplitude than those obtained under control conditions (e.g., Fadiga et al, 1995; Strafella and Paus, 2000; Patuzzo et al, 2003; Borroni et al, 2005; Aglioti et al, 2008; Loporto et al, 2012).…”