Single motor axon conduction velocities of human upper and lower limb motor units. A study with transcranial electrical stimulation.
OBJECTIVE
To calculate conduction velocities (CV) of single motor axons innervating hand, forearm and leg muscles, weak anodal electrical transcranial stimuli were used and single motor unit potentials were recorded in 17 normal subjects.


METHODS
The central motor conduction time and neuromuscular transmission delay were subtracted from the latency of unit response to cortical stimulation and single motor axon CV were calculated.


RESULTS
In extensor indicis proprius (EIP) units, CV ranged from 30.3 to 76.1m/s (mean: 51.3 +/- 7.1m/s, 139 units). In first dorsal interosseous (FDI), they ranged from 45.1 to 66.2m/s (mean: 54.6 +/- 2.6m/s, 88 units). In tibialis anterior (TA), velocities ranged from 27.8 to 55.9m/s (mean: 41.3 +/- 7.5m/s, 123 units). In FDI units, velocities were compared with those obtained with the F-wave method (range: 50.3-64.5m/s, mean: 58.1 +/- 2.0m/s).


CONCLUSIONS
Compared with previously published values, the present method gives better access to slow-conducting units, first recruited by transcranial stimulation and voluntary effort. The spectrum of individual CV was much broader for EIP and TA than for FDI. A linear decline of maximal CV with age was observed, while minimal CV were not affected, suggesting that aging causes a selective loss of the fastest-conducting units.
Dalpozzo
Francesca
F
University Department of Neurology, Hôpital de la Citadelle, Boulevard du Xlleme de Ligne, 1, 4000, Liège, Belgium.
Gérard
Pascale
P
De Pasqua
Victor
V
Wang
François
F
Maertens de Noordhout
Alain
A
D000328:Adult; D000367:Age Factors; D001132:Arm; D001369:Axons / Q000502:physiology; D004558:Electric Stimulation; D005260:Female; D006801:Humans; D007866:Leg; D008280:Magnetics; D008297:Male; D008875:Middle Aged; D009044:Motor Cortex / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D018482:Muscle, Skeletal / Q000294:innervation; D009431:Neural Conduction / Q000502:physiology*; D010543:Peroneal Nerve / Q000502:physiology; D011826:Radial Nerve / Q000502:physiology; D014459:Ulnar Nerve / Q000502:physiology