Activation of neck muscles from the human motor cortex.

Percutaneous stimulation of the motor cortex has been used to assess directly the supranuclear projection to the sternomastoid, trapezius and splenius capitis muscles. The projection to sternomastoid had a mean latency of 6.5 ms for the contralateral electromyographic response. A smaller and more variable response, usually with a longer latency (mean 9.5 ms), occurred in the ipsilateral sternomastoid. Electromyographic responses on both sides were potentiated by voluntary contraction or strong inspiratory efforts. They were evoked at lower stimulus intensities in the contralateral sternomastoid. Short-latency responses were recorded from the contralateral but not the ipsilateral trapezius and splenius capitis muscles. These results indicate that weakness of head rotation towards the hemiplegic limb following a supranuclear lesion may reflect reduced power of dorsal neck muscles rather than of sternomastoid.