Purpose: Threshold tracking is a new noninvasive approach for detecting axonal excitability changes in vivo. In this study, the authors compared the excitability indices of motor and sensory axons of median and ulnar nerves to determine whether the two nerves behave in a similar or a noninterchangeable way. They also examined whether age affects these indices.
Methods: Seventy normal subjects aged 22-70 years (mean, 36.7 ± 12.5 years) were recruited. Multiple excitability indices were measured in both motor and sensory axons from median and ulnar nerves.
Results: The threshold and rheobase were significantly higher for the ulnar motor axons recorded at the first dorsal interosseous muscle than for the median motor axons at the abductor pollicis brevis muscle. In contrast, the strength-duration time constant was decreased, threshold electrotonus reduction (in both depolarizing and hyperpolarizing directions) was significantly smaller, I/V slope was decreased, and subexcitability was reduced for the ulnar motor axons. Excitability indices measured in the sensory axons of both nerves were not overtly different. In R-square analysis, age had a homogeneous influence on sensory axon excitability but heterogeneous influence on motor axon excitability.
Conclusions: The excitability indices may be interchangeable for sensory axons but not motor axons. The authors therefore recommend recording motor axonal excitability in various muscle groups rather than a single muscle group.