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 che st wall. The compound motor evoked potentials (cMEPs) recorded in resp onse 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 facil itation of the response in the diaphragm to TMS during volitional insp iration is due to increased excitability at synapses associated with t he spinal motoneurone pool, but they do not exclude a component due to increased higher centre excitability. We conclude that it is unsafe t o assign a cortical contribution to 'automatic' inspiration on the sol e basis of facilitation of the electromyographic response in the diaph ragm to TMS.