Comparison of human motor cortical projections to abdominal muscles and intrinsic muscles of the hand.

Percutaneous electrical stimulation of the motor cortex was used to activate rapidly conducting corticofugal pathways to human abdominal muscles. Following cortical stimulation the response latencies for the abdominal muscles were similar to those for limb muscles which are a similar distance from the motor cortex. Cortically evoked responses recorded from the abdominal muscles had the same latency and similar amplitude during several voluntary tasks including expiration, expulsive manoeuvres and trunk flexion. Responses could also be evoked when the chemical drive to breathe was increased by rebreathing. In addition, the properties of the cortical projection to muscles of the abdominal wall were directly compared with those of the projection to the intrinsic muscles of the hand. The latencies of responses in abdominal muscles and intrinsic muscles of the hand were measured during static contractions over a range of strengths in the same subjects (0-100% maximal voluntary contraction, MVC). For both muscle groups, cortically evoked muscle responses of minimal latency occurred when background contractions reached 10-20% MVC with responses of maximal amplitude at 60% MVC. The variability in latency of fifty consecutive responses were similar for the two muscle groups. Furthermore, post-stimulus time histograms for 4 rectus abdominis motoneurones revealed a brief initial excitatory peak of 1.15 ms duration (range 0.96-1.34 ms) following cortical stimulation. The characteristics of this peak are the same as reported for motoneurones of intrinsic hand muscles. These findings demonstrate a powerful rapidly conducting pathway from the motor cortex to the human abdominal muscles. This pathway has many of the same properties as the monosynaptic corticospinal projection to the distal muscles of the upper limb.