Latent addition in human motor and sensory axons: different site-dependent changes across the carpal tunnel related to persistent Na+ currents.
OBJECTIVE
To compare site-dependent changes across the carpal tunnel in axonal persistent Na+ conductances in motor and sensory axons. Positive sensory symptoms are prominent features in carpal tunnel syndrome, and a persistent Na+ current is a major determinant of axonal excitability.


METHODS
The technique of latent addition was used to estimate persistent Na+ currents in median motor and sensory axons at the wrist and palm of 10 normal subjects. Brief hyperpolarizing conditioning current pulses were delivered, and threshold change at the conditioning-test interval of 0.2 ms was measured as an indicator of persistent Na+ currents.


RESULTS
Threshold changes at 0.2 ms were greater in sensory than in motor axons at both the wrist and palm. In motor axons, the threshold changes were significantly smaller at the palm (mean, 4.9%) than at the wrist (10.0%). By contrast, the threshold changes were similar at the two sites of sensory axons (12.6 and 13.1%). The passive membrane time constant was similar for motor and sensory axons at the palm and wrist.


CONCLUSIONS
Nodal persistent Na+ conductances have substantial site-dependent changes decreasing distally across the carpal tunnel in median motor axons, but not in sensory axons.


CONCLUSIONS
Whereas sensory axons generally have higher excitability than motor axons, the sensory-motor differences become more prominent across, and possibly at the carpal tunnel than the nerve trunk, and it is suggested that this contributes to the predominance of positive sensory symptoms in carpal tunnel syndrome.
Kuwabara
Satoshi
S
Department of Neurology, Chiba University School of Medicine, 1-8-1 Inohana, Chuo-ku, Chiba 260-8670, Japan. kuwabara-s@faculty.chiba-u.jp
Misawa
Sonoko
S
Tamura
Noriko
N
Nakata
Miho
M
Kanai
Kazuaki
K
Sawai
Setsu
S
Ogawara
Kazue
K
Hattori
Takamichi
T
D000200:Action Potentials / Q000502:physiology; D000328:Adult; D001369:Axons / Q000502:physiology*; D002349:Carpal Tunnel Syndrome / Q000175:diagnosis; D004558:Electric Stimulation; D004568:Electrodiagnosis / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008475:Median Nerve / Q000502:physiology*; D009046:Motor Neurons / Q000502:physiology*; D009431:Neural Conduction / Q000502:physiology; D009475:Neurons, Afferent / Q000502:physiology*; D011930:Reaction Time / Q000502:physiology; D012684:Sensory Thresholds / Q000502:physiology; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000502:physiology*; D014953:Wrist / Q000294:innervation*
D015222:Sodium Channels; D012964:Sodium