Effects of cutaneous stimulation on ipsilateral and contralateral motoneuron excitability: an analysis using H reflexes and F waves.

F waves are altered by conditioning stimulation of digital nerves. We investigated this by studying series of median nerve F waves recorded from abductor pollicis brevis (APB) 20, 50, 80, 100, 150, 200, and 500 milliseconds (ms) after conditioning stimuli applied to the ipsilateral or contralateral second digit in six normal subjects. The persistence and mean amplitude of the F waves were determined at each delay and compared with control F waves without conditioning stimuli. Ipsilateral afferent stimulation produced inhibition of F wave amplitude and persistence when applied 50 to 100 ms before the test stimulus, while contralateral conditioning stimuli facilitated the F wave when applied 80 to 200 ms before the test stimulus. Similar results were obtained in H reflex studies in one neurologically normal subject with a reproducible H reflex recording from the APB. These findings illustrate definable ipsilateral and contralateral postsynaptic influences of afferent fibers on motoneuron excitability in the arms.