Ipsilateral motor cortical responses to TMS during lengthening and shortening of the contralateral wrist flexors

Abstract

Unilateral lengthening contractions provide a greater stimulus for neuromuscular adaptation than shortening contractions in the active and non-active contralateral homologous muscle, although little is known of the potential mechanism. Here we examined the possibility that corticospinal and spinal excitability vary in a contraction-specific manner in the relaxed right flexor carpi radialis (FCR) when humans perform unilateral lengthening and shortening contractions of the left wrist flexors at the same absolute force. Corticospinal excitability in the relaxed right FCR increased more during lengthening than shortening at 80 and 100% of maximum voluntary contraction (MVC). Short-interval intracortical inhibition (SICI) diminished during shortening contractions and it became nearly abolished during lengthening. Intracortical facilitation (ICF) lessened during shortening but increased during lengthening. Interhemispheric inhibition (IHI) to the "non-active" motor cortex diminished during shortening and became nearly abolished during lengthening at 90% MVC. The amplitude of the H-reflex in the relaxed right FCR decreased during and remained depressed for 20 s after lengthening and shortening of the left wrist flexors. We discuss the possibility that instead of the increased afferent input, differences in the descending motor command and activation of brain areas that link function of the motor cortices during muscle lengthening vs. shortening may cause the contraction-specific modulation of ipsilateral motor cortical output. In conclusion, ipsilateral M1 responses to TMS are contraction-specific; unilateral lengthening and shortening contractions reduced contralateral spinal excitability but uniquely modulated ipsilateral corticospinal excitability and the networks involved in intracortical and interhemispheric connections, which may have clinical implications.

New evidence of corticospinal network modulation induced by motor imagery

“…At rest, the mean amplitude of the responses was matched among each condition to avoid size effect on the modulation analyzed (static, lengthening, and shortening). Indeed, passive muscle lengthening reduces corticospinal and spinal excitability (Chye et al 2010;Howatson et al 2011), mainly due to spinal presynaptic inhibitory mechanisms (Duclay et al 2011 measuring upper-and lower-limb activity, respectively. After shaving and dry cleaning the skin with alcohol to keep low impedance (Ͻ5 k⍀), EMG signals were obtained by using two silver-chloride (AgCl) surface electrodes (8 mm diameter) placed over the belly muscle with an interelectrode center-to-center distance of 2 cm.…”

Mirror illusion reduces motor cortical inhibition in the ipsilateral primary motor cortex during forceful unilateral muscle contractions

“…During the experimental conditions, participants were frequently reminded to completely relax the left arm when performing shortening right wrist flexion movements. Trials in which the associated left FCR or left ECR activity exceeded the background noise level of 25 V were excluded from the analyses (Howatson et al 2011;Muellbacher et al 2000;Perez and Cohen 2008). Thereafter and for all variables, outliers were identified with a modified and more stringent version of the interquartile range method, marking values below Q1 Ϫ 1.5 ϫ (Q2 Ϫ Q1) and values above Q3 ϩ 1.5 ϫ (Q3 Ϫ Q2) as outliers.…”

“…We measured corticospinal excitability by a single TMS pulse delivered at a suprathreshold intensity of 120% rMT, as part of the SICI measurement. For measuring SICI a subthreshold conditioning pulse at 80% rMT, an intensity sufficient to produce intracortical inhibition (Howatson et al 2011;Perez and Cohen 2008), preceded the suprathreshold test pulse of 120% rMT with an interstimulus interval of 2 ms (Kujirai et al 1993). The 2-ms interstimulus interval was used to create a deep amount of inhibition (Kujirai et al 1993) and to avoid a mixture of the two distinct phases of inhibition (Fisher et al 2002).…”

“…There is evidence that mirror-viewing of hand and finger movements performed at a fraction of the maximal voluntary force can facilitate ipsilateral corticospinal excitability (Garry et al 2005) and corticomotor activity (Shinoura et al 2008) compared with a no-vision condition. The increased activation of the ipsilateral M1 (Newton et al 2002;Sehm et al 2010) and the increased excitability of the corticospinal path targeting the resting hand (Foltys et al 2003;Hortobagyi et al 2003;Howatson et al 2011;Hoy et al 2007;Muellbacher et al 2000;Cohen 2008, 2009) are also observed for forceful unilateral contractions without a mirror; however, it is unknown whether the visual illusion of a slowly moving, forcefully contracting wrist in the mirror can additionally affect corticospinal excitability and motor cortical activity in the hemisphere ipsilateral to the moving hand. Such information is needed as a first step to explain how mirror-viewing could augment interlimb strength transfer, a viable treatment option for patients with unilateral orthopedic and neurological impairments (Farthing and Zehr 2014).…”

Postural challenge affects motor cortical activity in young and old adults

“…It is thought that inhibitory intracortical circuits modulate the excitability of the cortical neurons that project to the spinal motor neurons of the muscles involved in the task (Reis et al, 2008). The degree of reduction in intracortical inhibition is related to contraction intensity (Rantalainen et al, 2013;Ortu et al, 2008), contraction type (Howatson et al, 2011), and whether the movement starts or ends (Sidhu et al, 2013). SICI is also modulated during postural contractions, defined as contractions with the aim to maintain a certain posture, as shown by a reduction in SICI in the soleus muscle during standing as compared with sitting (Soto et al, 2006).…”