Brain Stimulation (Jan 2020)
Age-related changes in late synaptic inputs to corticospinal neurons and their functional significance: A paired-pulse TMS study
Abstract
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 (3.5–5.3 ms). Results: Peak SICF recorded with a PA current (SICFPA) was reduced in older adults (P < 0.0001), and occurred at a longer latency (P < 0.05). Furthermore, there was reduced recruitment of SICFPA in older adults (P < 0.0001), but this did not interact with the age-related shift in SICFPA (P = 0.2). In addition, reduced performance on the Purdue pegboard was predicted by increased SICFPA (P < 0.04) occurring at longer latencies (P < 0.04) in old but not young adults. For SICF recorded with an AP current (SICFAP), facilitation was again reduced at longer latencies in older adults (P < 0.0001), but recruitment was not different between groups (P = 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.
