Increased corticospinal excitability prior to arm cycling is due to enhanced supraspinal but not spinal motoneurone excitability

Abstract

Human studies have not assessed supraspinal or spinal motoneurone excitability in the quiescent state prior to a rhythmic and alternating cyclical motor output. The purpose of the current study was to determine whether supraspinal and (or) spinal motoneurone excitability was modulated in humans prior to arm cycling when compared with rest with no intention to move. We hypothesized that corticospinal excitability would be enhanced prior to arm cycling due, in part, to increased spinal motoneurone excitability. Supraspinal and spinal motoneurone excitability were assessed via transcranial magnetic stimulation (TMS) of the motor cortex and transmastoid stimulation of the corticospinal tract, respectively. Surface electromyography recordings of TMS motor evoked potentials (MEPs) and cervicomedullary MEPs (CMEPs) were made from the relaxed biceps brachii muscle prior to rhythmic arm cycling and at rest with no intention to move. The amplitude of the MEPs was greater (mean increase: +9.8% of maximal M wave; p = 0.006) and their onset latencies were shorter (mean decrease: -1.5 ms; p < 0.05) prior to cycling when compared with rest. The amplitudes of the CMEPs at any of 3 stimulation intensities were not different between conditions. We conclude that premovement enhancement of corticospinal excitability is greater prior to arm cycling than at rest because of increases in supraspinal but not spinal motoneurone excitability.

Changes in supraspinal and spinal excitability of the biceps brachii following brief, non-fatiguing submaximal contractions of the elbow flexors in resistance-trained males

“…Because TMS-evoked MEPs do not distinguish between changes in corticospinal excitability at the supraspinal or spinal levels, we used TMES to assess spinal excitability [4,13,17,20,24]. Collision experiments have shown that TMES directly activates the corticospinal axons [5] producing CMEPs that are independent of changes in supraspinal excitability [25], have a large monosynaptic component in biceps brachii motoneurone pool [19] and are free from classical afferent-mediated presynaptic inhibition [15].…”

Differences in corticospinal excitability to the biceps brachii between arm cycling and tonic contraction are not evident at the immediate onset of movement

“…Both CMEP amplitude and the CMEP onset latency were not different between experimental conditions. These results suggest that the enhancement in corticospinal excitability prior to arm cycling is likely supraspinally It was proposed that a possible reason for the lack of modulation in spinal excitability in the aforementioned study (Power and Copithorne, 2013) was that the measurements were not taken close enough to the onset of muscle activity. The case may have been that spinal changes were indeed occurring before movement, but simply not 50 ms beforehand.…”

“…Power and colleagues (Power and Copithorne, 2013) demonstrated that corticospinal excitability to the biceps brachii was enhanced 50 ms prior to the onset of voluntary EMG from the initiation of an arm cycling task. Corticospinal and spinal excitability were assessed through the use of single pulse TMS and TMES, respectively.…”

“…Vertex was determined by measuring the mid-point between the participant's nasion and inion, and the mid-point between the participant's tragi. The intersection of these two points was measured, marked and defined as vertex (Power and Copithorne, 2013 …”

Neuromuscular electrical stimulation of the median nerve facilitates low motor cortex excitability in patients with spinocerebellar ataxia

“…Studies showed that, during a reaction-time task in healthy humans, central motor excitability increased progressively as the onset of the movement approached (Klein et al, 2012;Power and Copithorne, 2013). For movement preparation, the low motor excitability in SCA was compensated via an early onset with prolonged activation in the motor cortex.…”