Changes in human alpha-motoneurone excitability following mechanical muscle stimuli.

Short mechanical stretches given to partially activated human abductor digiti minimi muscle (ADM) evoke early (M1) and late (M2) reflex responses. Transcranial magnetic brain stimuli were used to evoke compound muscle action potentials in ADM and hence to estimate motoneuronal excitability at various times after mechanical stimuli. There was no evidence that Ia volleys produce additional facilitation in motoneurones of muscles which are already voluntarily activated. However, the inhibitory phase between M1 and M2 was associated with a reduction in size of muscle responses from brain stimuli. This may reflect reduced Ia input, polysynaptic Ia inhibition or Renshaw inhibition.