Development of corticospinal motor excitability and cortical silent period from mid-childhood to adulthood – a navigated TMS study - 21/12/17

Doi : 10.1016/j.neucli.2017.11.004

Laura Säisänen ^a,^b,^⁎ , Petro Julkunen ^b,^c, Timo Lakka ^d,^e,^f, Virpi Lindi ^c, Mervi Könönen ^b,^g, Sara Määttä ^a,^b
^a Department of Clinical Neurophysiology, Institute of Clinical Medicine, Faculty of Health Sciences, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland
^b Department of Clinical Neurophysiology, Kuopio University hospital, Kuopio, Finland
^c Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
^d Department of Physiology, Institute of Biomedicine, Faculty of Health Sciences, University of Eastern Finland
^e Department of Clinical Physiology and Nuclear Medicine, Kuopio University Hospital, Kuopio, Finland
^f Kuopio Research Institute of Exercise Medicine, Kuopio, Finland
^g Department of Clinical Radiology, Kuopio University Hospital, Kuopio, Finland

^⁎Corresponding author at: Corresponding author. Department of Clinical Neurophysiology, Institute of Clinical Medicine, Faculty of Health Sciences, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland.

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Summary

Objectives

We characterized the maturation of the excitability of the motor cortex and corticospinal tract from childhood to adulthood using electric field (EF) navigated TMS and correlated the results with manual dexterity.

Methods

Both hemispheres of healthy right-handed children (6–9 years, n=10), preadolescents (10–12 years, n=13), adolescents (14–17 years, n=12) and young adults (22–34 years, n=12) of both genders were examined. The optimal cortical representation site and resting motor threshold (rMT) were determined for the abductor pollicis brevis muscle. Motor-evoked potential (MEP) latencies and amplitudes in relaxed and active states, input-output curves and silent period (SP) durations were determined. Manual dexterity was assessed with the Box and Block Test.

Results

rMT (in terms of maximal stimulator output or EF strength) decreased with age (P<0.001) and stabilized when reaching adolescence. The MEP amplitude (P=0.037) and latency increased (P<0.001) with age. Input-output curves showed age-dependent changes in several parameters. SP duration decreased with age (P<0.001), and demonstrated hemispheric asymmetry in the children (P=0.030). Manual dexterity correlated negatively with rMT (P<0.001).

Discussion

The excitation/inhibition balance develops with age and correlates with manual dexterity. Strong corticospinal inhibition was observed in the children and this was found to decrease with age. Interhemispheric asymmetry was only observed for SP duration in the children. Knowledge of normal development is crucial for the understanding of developmental disabilities and using estimates of effective EF may be advantageous in future pediatric studies.

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Keywords : Transcranial magnetic stimulation, Human maturation, Development, Motor cortex, Excitability, Silent period