Abstract
We analyzed F-waves of healthy subjects, obtained during passive tactile discrimination and at rest, to investigate changes in upper extremity spinal motor neuron excitability. Twenty healthy subjects (average age 25.2 ± 5.2 years, average height 172.1 ± 5.1 cm, average upper limb length 88.8 ± 3.5 cm) were evaluated during passive tactile discrimination and at rest. We recorded F-waves from the abductor pollicis brevis muscle of the upper extremity during passive tactile discrimination and at rest. The average values of F-wave persistence were 65.8 ± 22.2% at rest, and 74.6 ± 21.4% during passive tactile discrimination. The persistence of F-wave significantly increased during passive tactile discrimination compared to at rest (paired t-test, p<0.01). The average values of the F/M amplitude ratio were 4.61 ± 5.8% at rest, and 4.58 ± 5.8% during passive tactile discrimination. There was no significant change in F/M amplitude ratio between rest and passive tactile discrimination. The average values of latency were 28.2 ± 2.2 ms at rest, and 28.0 ± 2.3 ms during passive tactile discrimination. There was no significant change in latency between rest and passive tactile discrimination. These findings indicate that the firing frequency of the upper extremity motor unit increased during passive tactile discrimination, but that the excitability of each motor unit and the conduction velocity of the motor units of the upper extremity did not increase. Our findings suggest that passive tactile discrimination is modulated by descending commands from upper motor neurons in healthy subjects.
