Plasticity in corticomotor control of the human tongue musculature induced by tongue-task training

Abstract

Transcranial magnetic stimulation (TMS) has been used to assess characteristics of the corticomotor control of the jaw muscles, but less is known about the cortical control of the human tongue and its modification by training. The aim of the present study was to determine the effect of training humans in a novel tongue-protrusion task for 1 week on corticomotor excitability as assessed by changes in electromyographic activity elicited in the tongue musculature by TMS, and in the tongue cortical motor map revealed by TMS. Eleven healthy subjects participated. Stimulus-response curves were generated from the motor evoked potentials (MEPs) recorded in the tongue musculature and, from the first dorsal interosseos (FDI) muscle as a control, at three time periods: at baseline, immediately after the 1-week training period, and at 2-weeks follow-up. In addition, the corticomotor representations of the tongue and FDI muscles were mapped on a 1 x 1 cm scalp grid. The tongue-training task required each subject to protrude the tongue onto a force transducer placed in front of the subject, and consisted of a relax-protrude-hold-relax cycle lasting 12.5 s with 1 N as the target at the hold phase. The subjects repeated this task for 60 min every day for 1 week. All subjects reported moderate levels of fatigue in the tongue during the first training day; however, these subjective reports decreased during the week (ANOVA P<0.001), and the subjects showed a progressive increase in their ability to perform the task successfully ( P<0.001). The threshold for evoking MEPs by TMS in the tongue musculature was significantly decreased after the last training day compared with baseline and the 2-weeks follow-up ( P<0.001). The amplitude of the MEPs in the tongue musculature was significantly increased at higher intensities of TMS after the last training day but returned to baseline values at the 2-weeks follow-up (P = 0.005). No significant effect of the training on MEPs in the FDI was observed (P = 0.493). Analysis of the corticomotor topographic maps revealed a significant ( P<0.05) increase in excitability and, hence, the cortical area from which TMS could evoke MEPs in the tongue, although the center of gravity representation for the tongue or FDI muscles remained stable. The present findings suggest that a specific and reversible plasticity of the corticomotor excitability related to tongue muscle control can be induced when humans learn to perform successfully a novel tongue task.

Differential Modulation of Motor Cortical Plasticity and Excitability in Early and Late Phases of Human Motor Learning

“…However, only a few studies have addressed the effect of longer-term motor practice. These described an expansion in cortical output maps to muscles involved in the trained task (Pascual-Leone et al, 1995;Svensson et al, 2003), but did not examine LTP/LTD-like effects. Interestingly, professional musicians who undergo years of motor practice have increased recruitment of corticospinal and intracortical connections as well as enhanced synaptic plasticity, possibly resulting from an increase in the number of synapses through synaptogenesis (Rosenkranz et al, 2007).…”

Effect of capsaicin-evoked jaw-muscle pain on intramuscular blood-flow

“…One way to compromise the BF has been speculated to be due to low levels of continuous muscle contraction (5,6). This hypothesis has gained some support in limb muscles (7)(8)(9) and in jaw muscles (10)(11)(12)(13). Indeed, intramuscular BF during controlled isometric contraction in normal jaw muscles has been described (5,6).…”

Tongue dysfunction screening: assessment protocol for prescribers

“…duced neuroplastic modifications in the primary motor cortex (M1) of the lingual muscles, increasing corticobulbar motoneuron excitability and the size of the lingual muscle motor area ( fig. 9) 7, 34,35 . (The corticobulbar motoneurons of the cranial nerve motor system constitute the corticobulbar bundle, previously known as the geniculate bundle, which is the counterpart of the pyramidal bundle of the spinal motor system.…”