Age and sex differences in human motor cortex input-output characteristics

Area of Science:

• Neuroscience
• Motor Control
• Human Physiology

Background:

• Transcranial magnetic stimulation (TMS) is a key tool for probing the human motor system.
• Understanding age-related changes in motor evoked potentials (MEPs) is crucial for assessing neural function.
• Sex differences in motor system excitability are an area of ongoing research.

Purpose of the Study:

• To investigate the influence of age and sex on stimulus-response curves of motor evoked potentials (MEPs).
• To identify age- and sex-related differences in TMS parameters and MEP variability.

Main Methods:

• Constructed stimulus-response curves for MEPs in a hand muscle using TMS in 42 healthy subjects.
• Analyzed resting motor threshold, maximal MEP amplitude (MEP(max)), maximal slope, and trial-to-trial MEP variability.
• Examined the effects of age and sex on these neurophysiological measures.

Main Results:

• No significant effect of age on resting motor threshold, MEP(max), or maximal slope.
• Older subjects required higher stimulus intensities to achieve MEP(max) and maximal slope.
• Increased trial-to-trial MEP variability was observed in older subjects, particularly near threshold.
• MEP variability decreased with increasing stimulus intensity, but less rapidly in older individuals.
• Age and threshold were stronger modulators of MEP variability than sex, though females tended to show higher variability.

Conclusions:

• Age-related changes in motor system excitability are evident in MEP input-output characteristics.
• Increased MEP variability in older adults may stem from reduced or less synchronous motoneuron activation.
• These findings highlight age as a critical factor influencing motor pathway function and variability.