Is plasticity in the human motor cortices altered in healthy older adults?

Abstract

Neuroplasticity refers to the ability of the brain to alter its structure and function in response to the demands of its environment. Substantial evidence from studies in non-human animals suggests that neuroplasticity at the synaptic level is reduced in the aged brain. In humans, ageing is associated with cognitive and motor decline and some experimental evidence using non-invasive brain stimulation and motor training suggests that neuroplasticity is reduced in the motor cortex. It is also evident, however, that there is variability in age-related decline of motor performance and neuroplasticity across individuals. In the studies investigating neuroplasticity, little consideration has been given to changes occurring outside the targeted region, for example in homotopic regions of the contralateral hemisphere. This is an important limitation, as neuroimaging studies suggest that older adults exhibit more widespread activity both within and across the cerebral hemispheres, relative to young adults, when performing the same motor task. This finding suggests that additional neural populations may be recruited in older adults to assist in learning new motor skills. The aim of this thesis then, was to investigate whether the manifestation of neuroplasticity across the cerebral hemispheres is altered in older adults. To achieve this, three different protocols using skilled training or non-invasive brain stimulation were employed to experimentally induce neuroplasticity primarily in the primary motor cortex (M1). These protocols have been shown to induce changes with characteristics that resemble long-term potentiation (LTP) and long-term depression (LTD) in animal models, which is the candidate mechanism for synaptic plasticity. In each study neuroplasticity effects were probed by measuring changes in corticospinal excitability in bilateral coritcospinal pathways (corticospinal plasticity) using single-pulse transcranial magnetic stimulation (TMS) and motor-evoked potentials (MEPs).In the first study young and older participants were trained on a simple and complex motor task.The degree to which performance improvement transferred from the trained to the untrained hand and the extent to which this reflected changes in the distribution of corticospinal plasticity across the hemispheres were assessed. Both groups demonstrated performance improvements in the trained and untrained hand, although performance improvement in the trained hand was smaller in older adults. The percentage transfer from the trained to the untrained hand was comparable between young and older adults. Importantly, after training on the simple task, corticospinal excitability increased bilaterally in both young and older adults, a similar trend was also observed with the complex task. This result indicates that corticospinal plasticity, its manifestation in bilateral corticospinal pathways, and its relation to behavioural performance was comparable between young and older adults.iii In a second study, intermittent theta burst stimulation (iT...

Determining the Corticospinal Responses and Cross-Transfer of Ballistic Motor Performance in Young and Older Adults: A Systematic Review and Meta-Analysis

“…Overall, the motor training in the included studies consisted of 300 ballistic movements (Carroll et al, 2008;Cirillo et al, 2010;Dickins et al, 2015;Hinder et al, 2011Hinder et al, , 2013aHinder et al, , 2013bLee et al, 2010;Reissig et al, 2015;Rogasch et al, 2009) and 150 ballistic movements (Stockel et al, 2016). Motor performance testing consisted of 10 ballistic movements (Carroll et al, 2008;Hinder et al, 2011Hinder et al, , 2013aHinder et al, , 2013bReissig et al, 2015;Rogasch et al, 2009) and 15 ballistic movements (Dickins et al, 2015) performed in the absence of feedback or encouragement. A detailed description of study characteristics is provided in Table 2.…”