Impaired response of human motoneurones to corticospinal stimulation after voluntary exercise

Abstract

Activation of descending corticospinal tracts with transmastoid electrical stimuli has been used to assess changes in the behaviour of motoneurones after voluntary contractions. Stimuli were delivered before and after maximal voluntary isometric contractions (MVCs) of the elbow flexor muscles. Following a sustained MVC of the elbow flexors lasting 5–120 s there was an immediate reduction of the response to transmastoid stimulation to about half of the control value. The response recovered to control levels after about 2 min. This was evident even when the size of the responses was adjusted to accommodate changes in the maximal muscle action potential (assessed with supramaximal stimuli at the brachial plexus). To determine whether the post‐contraction depression required activity in descending motor paths, motoneurones were activated by supramaximal tetanic stimulation of the musculocutaneous nerve for 10 s. This did not depress the response to transmastoid stimulation. Following a sustained MVC of 120 s duration, the response to transcranial magnetic stimulation of the motor cortex gradually declined to a minimal level by about 2 min and remained depressed for more than 10 min. Additional studies were performed to check that the activation of descending tracts by transmastoid stimulation was likely to involve excitation of direct corticospinal paths. When magnetic cortical stimuli and transmastoid stimuli were timed appropriately, the response to magnetic cortical stimulation could be largely occluded. This study describes a novel depression of effectiveness of corticospinal actions on human motoneurones. This depression may involve the corticomotoneuronal synapse.

Voluntary Motor Output Is Altered by Spike-Timing-Dependent Changes in the Human Corticospinal Pathway

“…Although the excitability of motoneurons can be altered through repetitive activation (Spielmann et al, 1993;Gandevia et al, 1999) or through ongoing changes in afferent input (Martin et al, 2008), neither of these mechanisms is likely to produce the differential effects observed here, as both afferents and motoneurons experienced the same total extra activity in each conditioning protocol. Furthermore, this extra activity was very little compared with normal motoneuron firing in voluntary contractions (Ͼ15 Hz) (Bellemare et al, 1983).…”

“…Corticospinal axons were stimulated by passing an electrical current (100 s duration; Digitimer DS7AH) between electrodes placed over the mastoids (1-2 cm posterior and superior to the tip of the mastoid pro- (Ugawa et al, 1991;Gandevia et al, 1999;Taylor and Gandevia, 2004). Activation occurs at the cervicomedullary junction and evokes large short-latency responses [cervicomedullary motor-evoked potential (CMEPs)] in arm muscles (Fig.…”

Effects of muscle action type on corticospinal excitability and triceps surae muscle-tendon mechanics

“…Abbruzzese et al 1994;Duclay et al 2011;Gandevia et al 1999;Gruber et al 2009;Sacco et al 1997;Taylor & Gandevia 2004). …”

Utilizing Transcranial Magnetic Stimulation to Study the Human Neuromuscular System

“…When TMS is applied to the motor cortex at an intensity above MT, high-frequency indirect waves (I waves) are elicited in the corticospinal tract 27 , which are modifiable by many mechanisms including neurotransmitters (i.e., glutatmate, GABA), modulators of neurotransmission (i.e., acetylcholine, norepinephrine, and dopamine) 25 , and interneurones contacted by corticospinal tract cells 28 with the actual efficacy of the corticomotoneuronal synapse itself demonstrating some activity-dependent changes 29 all functioning to influence the amplitude of the MEP. As such, the amplitude of the signal can be modulated at both the cortical and spinal levels it is difficult to parse out specifically where, spatially within the nervous system, a change has occurred or a difference exists.…”