Age-related strengthening of cerebello-cortical motor circuits

Abstract

Effective connectivity between the cerebellum and primary motor cortex (M1) is critical for motor learning and motor control. Despite evidence of cerebellar atrophy and declines in motor learning and motor control with advanced age, recent behavioral studies indicate that cerebellar-dependent motor learning processes are preserved or even enhanced in older adults. However, physiological evidence of heightened cerebellar excitability leading to strengthened cerebellar-M1 connectivity with advanced age is lacking. Here, we used transcranial magnetic stimulation to assess age-related effects on cerebellar inhibition, a measure of cerebellar-M1 connectivity, in 20 young and 19 older adults. We observed stronger cerebellar inhibition in older compared with young adults. The behavioral implications of strengthened cerebellar inhibition with advanced age found in this study remain to be determined.

Keywords: Aging, Cerebellum; Cerebellar inhibition; Primary motor cortex; Transcranial magnetic stimulation.

Fig 1.. Cerebellar inhibition (CBI).

(A) Example electromyography traces depict motor evoked potentials from the right first dorsal interosseous muscle of a young participant. Arrows indicate the stimulus artifact from the conditioning stimulus (CS) and test stimulus (TS). Each trace is the waveform average of 15 trials for the nonconditioned (NC; gray), conditioned at 5 ms (red) and conditioned at 7 ms (blue) responses. (B) The y-axis represents the ratio of the conditioned (C) and nonconditioned (NC) motor evoked potential (MEP) amplitude. The bar graph shows group average, with individual data from Johns Hopkins University (circles), The University of Tokyo (triangles) and University Health Network Toronto (squares) represented by the open symbols. In young adults, CBI was greater at 5 ms compared with 7 ms. CBI at 7 ms was stronger in older compared with young adults. (C) The y-axis represents MEP amplitude. The bar graph shows group average, with individual data represented by the open symbols. The NC MEP amplitude was lower in older compared with young adults. Data are mean ± SEM. *p < 0.05.