Abstract
Background: Although recent studies point to the involvement of the primary motor cortex in postural control, it is unknown if age-related deterioration of postural control is associated with changes in motor cortical circuits. We examined the interaction between age and sensory condition in the excitability of intracortical motor pathways as indexed by short-interval intracortical inhibition (SICI) and intracortical facilitation (ICF) during standing.Methods: We used magnetic brain stimulation to evoke SICI and ICF in 11 young (range 21–25 years) and 12 healthy old adults (range 60-74 years) while they stood on a rigid platform or foam, with the eyes open or closed.Results: There was an overall age-related 43% reduction in SICI (p = 0.001). SICI lessened when standing on foam in old (31%) but not in young (1%) adults (condition × group interaction, p = 0.049). This reduction was associated with increases in center of pressure velocity (r = -0.648, p = 0.043). Age (p = 0.527) and sensory conditions (p = 0.325) did not affect ICF.Conclusion: Motor cortical circuits controlling leg muscles are modulated differently in healthy old vs. young adults during upright posture. Future experiments will clarify whether this difference mediates impaired postural control or serves as a compensatory mechanism to counteract postural instability.
Highlights
Accumulating evidence points to the involvement of the primary motor cortex (M1) in postural control (Beloozerova et al, 2003; Taube et al, 2006, 2008; Tokuno et al, 2009)
shortinterval intracortical inhibition (SICI) lessened when standing on foam in old (31%) but not in young (1%) adults
The data confirmed the overriding hypothesis of an interaction between age and sensory condition in M1 excitability: SICI lessened when standing on foam compared with standing on a rigid surface in old but not in young adults
Summary
Accumulating evidence points to the involvement of the primary motor cortex (M1) in postural control (Beloozerova et al, 2003; Taube et al, 2006, 2008; Tokuno et al, 2009). The excitability of the corticospinal pathway, including the direct projections (monosynaptic) from cortical neurons to spinal motor neurons, is greater in old compared with young adults during upright standing (Baudry et al, 2014a,b). It is unknown if, in addition to age-related changes in the corticospinal pathway, aging influences intracortical circuits during upright standing. The changes in background EMG between the rigid and the foam conditions did not correlate significantly with the changes in test MEP amplitude (r = 0.27, p = 0.244), SICI (r = −0.29, p = 0.202) or ICF (r = −0.25, p = 0.275).
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