Impaired response of human motoneurones to corticospinal stimulation aftervoluntary exercise

1. Activation of descending corticospinal tracts with transmastoid electric al stimuli has been used to assess changes in the behaviour of motoneurones after voluntary contractions. Stimuli were delivered before and after maxi mal voluntary isometric contractions (MVCs) of the elbow flexor muscles. 2. Following a sustained MVC of the elbow flexors lasting 5-120 s there was an immediate reduction of the response to transmastoid stimulation to abou t half of the control value. The response recovered to control levels after about 2 min. This was evident even when the size of the responses was adju sted to accommodate changes in the maximal muscle action potential (assesse d with supramaximal stimuli at the brachial plexus). 3. To determine whether the post-contraction depression required activity i n descending motor paths, motoneurones were activated by supramaximal tetan ic stimulation of the musculocutaneous nerve for 10 s. This did not depress the response to transmastoid stimulation. 4. Following a sustained MVC of 120 s duration, the response to transcrania l magnetic stimulation of the motor cortex gradually declined to a minimal level by about 2 min and remained depressed for more than 10 min. 5. Additional studies were performed to check that the activation of descen ding tracts by transmastoid stimulation was likely to involve excitation of direct corticospinal paths. When magnetic cortical stimuli and transmastoi d stimuli were timed appropriately, the response to magnetic cortical stimu lation could be largely occluded. 6. This study describes a novel depression of effectiveness of corticospina l actions on human motoneurones. This depression may involve the corticomot oneuronal synapse.