Maturation of corticospinal tracts assessed by electromagnetic stimulation of the motor cortex.

Abstract

SUMMARY The motor cortex can be excited in adults using electromagnetic stimulation, and the latency to the evoked muscle action potential allows an assessment of the integrity of corticospinal tracts. We applied this technique in children to describe the maturation of corticospinal tracts. The latency from cortical stimulation to the onset of the evoked muscle action potentials and the subject's height were recorded. The subject's height was divided by the latency to the onset of the evoked muscle action potential to provide an index of the conduction velocity within descending motor pathways (VI). It is possible to evoke muscle action potentials after electromagnetic stimulation of the motor cortex in children including preterm babies and there is a stepwise increase in the sensitivity to stimulation between 8 and 11 years of age. In addition there is a progressive increase in VI with age; adult values are attained at about 11 years.The successful application of this technique in children suggests that electromagnetic stimulation of the motor cortex has the potential to allow detection of abnormality in motor pathways in newborn babies and young children.In young children it is difficult to assess the integrity of central motor pathways. Clinical examination is unreliable particularly in the very young child or in the newborn infant.' The recent development of the technique of electromagnetic stimulation of the motor cortex provides, for the first time, the opportunity to assess objectively the integrity of corticospinal tracts in children.2 This method uses a flat circular stimulating coil through which a brief current is passed.2 The pulsed magnetic field so generated induces electrical currents in the brain, which are assumed to excite the superficial layers of the cortex. When the coil is held over the motor cortex, activity is evoked in the corticospinal tracts and results in muscle action potentials, measureable by surface electromyogram electrodes.2 3 The method is simple, painless, and quick and is therefore particularly suitable for use in children. Before this technique can be used to investigate abnormalities in motor pathway function in children, however, the normal developmental changes in the conduction velocity within the motor pathways need to be defined.

Postnatal Development of Corticospinal Postsynaptic Action

“…5A) and the increase in CS tract conduction velocity are both likely to be due to increased myelination. These changes are similar to those reported, using TMS, in monkey (Eyre et al 2000;Flament et al 1992;Olivier et al 1997) and human (Eyre et al 2000;Koh and Eyre 1988).…”

“…Interestingly, data in the human also suggest that CS terminals can activate spinal neurons before development of the sorts of distal skills that are generally attributed to CS control. colleagues (2000, 2001) have recently reported that TMS evoked motor responses in preterm and term infants before the usual 1-2 yr of age (e.g., Koh and Eyre 1988;Nezu et al 1997). Surprisingly, they found an early time window, Ͻ3 mo of age, during which TMS thresholds were slightly lower than in older neonates (Eyre et al 2001).…”

Transcranial magnetic stimulation in children

“…In children, as in adults, the MEP threshold is higher when the target muscle is at rest (resting motor threshold) than when there is background muscle activation (active motor threshold) (Garvey et al, 2003). It may not be possible to elicit, even using maximal stimulator output, reliable MEP responses when muscles are at rest in children younger than 6 years of age (Koh and Eyre, 1988). When the target muscle is active, MEP responses can be elicited even in neonates (Eyre et al, 2001).…”

“…The developmental pattern of CMCT when muscles are at rest (resting-CMCT) is distinct from the pattern when muscles are active (active-CMCT). Active-CMCT reaches maturity in children within the first 3 to 5 years of post-natal life; in contrast, resting-CMCT does not reach maturity until early adolescence (Eyre et al, 1991;Fietzek et al, 2000;Koh and Eyre, 1988;Müller et al, 1994). In adults, the "latency jump" (i.e.…”

Activity- and use-dependent plasticity of the developing corticospinal system☆

“…In the monkey, late outgrowth continues for several years postnatally ). This late CS axon terminal growth, together with late maturation of conduction along the CS tract (Koh and Eyre, 1988) and elaboration of the cortical motor map (Chakrabarty and Martin, 2000), underlies the highly protracted period of CS system development.…”