Corticospinal excitability during fatiguing whole body exercise

Abstract

The corticospinal pathway is considered the primary conduit for voluntary motor control in humans. The efficacy of the corticospinal pathway to relay neural signals from higher brain areas to the locomotor muscle, i.e., corticospinal excitability, is subject to alterations during exercise. While the integrity of this motor pathway has historically been examined during single-joint contractions, a small number of investigations have recently focused on whole body exercise, such as cycling or rowing. Although differences in methodologies employed between these studies complicate the interpretation of the existing literature, it appears that the net excitability of the corticospinal pathway remains unaltered during fatiguing whole body exercise. Importantly, this lack of an apparent effect does not designate the absence of change, but a counterbalance of excitatory and inhibitory influences on the two components of the corticospinal pathway, namely the motor cortex and the spinal motoneurons. Specific emphasis is put on group III/IV afferent feedback from locomotor muscle which has been suggested to play a significant role in mediating these changes. Overall, this review aims at summarizing our limited understanding of how fatiguing whole body exercise influences the corticospinal pathway.

Keywords: Corticospinal excitability; Exercise; Fatigue; Motoneuron; Motor cortex.

FIG. 1

Changes in the excitability of the motor pathway (area of MEP normalized to M_max; A and D), the motoneuron pool (CMEP normalized to M_max; B and E), and the motor cortex (MEP/CMEP ratio; C and F) associated with an ~50% increase in EMG occurring in the absence (non-fatiguing cycling) and presence (fatiguing cycling exercise) of significant central and peripheral fatigue. *P<0.05, significant difference within trials. Figure reproduced with permission from Weavil, J.C., Sidhu, S.K., Mangum, T.S., Richardson, R.S., Amann, M., 2015. Intensity-dependent alterations in the excitability of cortical and spinal projections to the knee extensors during isometric and locomotor exercise. Am. J. Physiol. Regul. Integr. Comp. Physiol., 308, R998–1007.

FIG. 2

Steps involved in voluntary force production and factors acting at motoneuronal level. A: diagrammatic representation of the “chain” involved in voluntary contractions. A major source of feedback, that from the muscle, is shown acting at three levels in the central nervous system. Other sources of feedback that also act at these levels are not shown. B: summary of inputs to α- and γ-motoneurons for an agonist muscle. Cells with solid circles are inhibitory. Dotted curved region at premotoneuronal terminals denotes presynaptic inhibition acting selectively on the afferent paths to motoneurons. Figure reproduced with permission from Gandevia, S.C., 2001. Spinal and supraspinal factors in human muscle fatigue. Physiol. Rev., 81, 1725–1789.