Background: Recent work suggests that the function of intracortical interneurons activated by transcranial magnetic stimulation (TMS) is modified in older adults, with the circuits generating short-interval intracortical facilitation (SICF) at longer intervals appearing to be particularly affected. Objective: To use SICF to quantify age-related changes in the excitability and recruitment of late synaptic inputs to corticospinal neurons, and investigate if changes within these circuits contribute to altered motor performance in older adults. Methods: SICF was recorded with 3 different conditioning intensities in 23 young (23.0 ± 4.2 years) and 21 older (67.1 ± 1.1 years) adults. These measures were performed with conventional (posterior-anterior, PA) and reverse (anterior-posterior, AP) current directions using interstimulus intervals targeting late synaptic inputs to corticospinal neurons (3.5e5.3 ms). Results: Peak SICF recorded with a PA current (SICF PA ) was reduced in older adults (P < 0.0001), and occurred at a longer latency (P < 0.05). Furthermore, there was reduced recruitment of SICF PA in older adults (P < 0.0001), but this did not interact with the age-related shift in SICF PA (P ¼ 0.2). In addition, reduced performance on the Purdue pegboard was predicted by increased SICF PA (P < 0.04) occurring at longer latencies (P < 0.04) in old but not young adults. For SICF recorded with an AP current (SICF AP ), facilitation was again reduced at longer latencies in older adults (P < 0.0001), but recruitment was not different between groups (P ¼ 0.7) and was unrelated to motor function. Conclusion:These results suggest that there are age-related changes in late synaptic inputs to corticospinal neurons and that these changes influence fine motor performance.

The late indirect (I)-waves recruited by transcranial magnetic stimulation (TMS) over primary motor cortex (M1) can be modulated using I-wave periodicity repetitive TMS (iTMS). The purpose of this study was to determine if the response to iTMS is influenced by different interstimulus intervals (ISIs) targeting late I-waves, and whether these responses were associated with individual variations in intracortical excitability. Seventeen young (27.2 ± 6.4 years, 12 females) healthy adults received iTMS at late I-wave intervals (4.0, 4.5, and 5.0 ms) in three separate sessions. Changes due to each intervention were examined with motor evoked potential (MEP) amplitudes and short-interval intracortical facilitation (SICF) using both posterior-anterior (PA) and anterior-posterior (AP) TMS current directions. Changes in MEP amplitude and SICF were influenced by iTMS ISI, with the greatest facilitation for ISIs at 4 and 5 ms with PA TMS, and 4 ms with AP TMS. Maximum SICF at baseline (irrespective of ISI) was associated with increased iTMS response, but only for PA stimulation. These results suggest that modifying iTMS parameters targeting late I-waves can influence M1 plasticity. They also suggest that maximum SICF may be a means by which responders to iTMS targeting the late I-waves could be identified.

No abstract

No abstract

No abstract