Site of facilitation of diaphragm EMG to corticospinal stimulation during inspiration

The electromyographic response of the diaphragm to (a) transcutaneous electrical stimulation (TCES) of the spinal cord at C5 and above, (b) transcranial magnetic stimulation (TMS) over the motor cortex and (c) transcutaneous electrical stimulation of the phrenic nerve in the neck (TPNS), was recorded in six normal subjects at the antero-lateral chest wall. The compound motor evoked potentials (cMEPs) recorded in response to both TMS and TCES were facilitated to a similar extent during volitional inspiration; this facilitation was greater than any change seen in response to TPNS with inspiration. The results show that facilitation of the response in the diaphragm to TMS during volitional inspiration is due to increased excitability at synapses associated with the spinal motoneurone pool, but they do not exclude a component due to increased higher centre excitability. We conclude that it is unsafe to assign a cortical contribution to 'automatic' inspiration on the sole basis of facilitation of the electromyographic response in the diaphragm to TMS.