Modulation of motor cortex excitability after upper limb immobilization

Abstract

Objective: To examine the mechanisms of disuse-induced plasticity following long-term limb immobilization. Methods: We studied 9 subjects, who underwent left upper limb immobilization for unilateral wrist fractures. All subjects were examined immediately after splint removal. Cortical motor maps, resting motor threshold (RMT), motor evoked potential (MEP) latency and MEP recruitment curves were studied from abductor pollicis brevis (APB) and flexor carpi radialis (FCR) muscles with single pulse transcranial magnetic stimulation (TMS). Paired pulse TMS was used to study intracortical inhibition and facilitation. Compound muscle action potentials (CMAPs) and F waves were obtained after median nerve stimulation. In 4/9 subjects the recording was repeated after 35 -41 days.Results: CMAP amplitude and RMT were reduced in APB muscle on the immobilized sides in comparison to the non-immobilized sides and controls after splint removal. CMAP amplitude and RMT were unchanged in FCR muscle. MEP latency and F waves were unchanged. MEP recruitment was significantly greater on the immobilized side at rest, but the asymmetry disappeared during voluntary muscle contraction. Paired pulse TMS showed an imbalance between inhibitory and excitatory networks, with a prevalence of excitation on the immobilized sides. A slight, non-significant change in the strength of corticospinal projections to the non-immobilized sides was found. TMS parameters were not correlated with hand dexterity. These abnormalities were largely normalized at the time of retesting in the four patients who were followed-up.Conclusions: Hyperexcitability occurs within the representation of single muscles, associated with changes in RMT and with an imbalance between intracortical inhibition and facilitation. These findings may be related to changes in the sensory input from the immobilized upper limb and/or in the discharge properties of the motor units.Significance: Different mechanisms may contribute to the reversible neuroplastic changes, which occur in response to long-term immobilization of the upper-limbs.

Assessment of corticospinal excitability after traumatic spinal cord injury using MEP recruitment curves: a preliminary TMS study

“…Although at the level of SCI lesion, a regenerative sprouting of interneurons may occur, above the lesion descending corticospinal axons extend new collaterals, and in the motor cortex new neurons are recruited along with the redistribution of projection neurons. The activation of functionally intact corticospinal neurons may depolarize a larger fraction of the 40 A recent study reported significantly increased active motor threshold in patients with incomplete cervical SCI. 18 This result was explained by the low density of motoneurons that innervate the hand muscles and seems to be in contrast with our findings.…”

Use-Dependent Hemispheric Balance

“…TMS of the motor cortex has been used to study plastic changes in cortical excitability associated with immobilization (Liepert et al, 1995;Zanette et al, 1997Zanette et al, , 2004Facchini et al, 2002;Huber et al, 2006). Although these studies yielded conflicting findings, a likely result was that even a short-term non-use is able to induce a decrease of cortical excitability in the motor area related to the restricted muscles (Facchini et al, 2002;Huber et al, 2006), probably because of the induction of local synaptic depression (Huber et al, 2006).…”

Extensive training of elementary finger tapping movements changes the pattern of motor cortex excitability

“…The injury might have altered corticospinal excitability and plasticity (Facchini et al 2002;Zanette et al 2004). …”