Responses in human intercostal and truncal muscles to motor cortical and spinal stimulation

Percutaneous electrical stimulation of the human motor cortex (up to 750 V) has been used to study the cortical projections to intercostal and truncal muscles. The latencies of electromyographic (EMG) responses were measured to motor cortical stimuli and also to spinal stimulation of the appropriate nerve roots. Following single anodal stimuli at (or near) the vertex, short-latency responses were recorded in pectoralis major (mean 9.6 msec), latissimus dorsi (9.7 msec), paravertebral muscles (11.1 msec), 3rd/4th parasternal intercostals (11.1 msec), and 6th/7th intercostal muscles (12.3 msec). Responses in each muscle group were potentiated by background voluntary respiratory and truncal manoeuvres which activated the muscles. The mean estimated central conduction times from motor cortex to spinal segmental level were 4.8 msec for pectoralis major, 5.8 msec for parasternal intercostals and 6.2 msec for 6th/7th intercostal muscles. The central conduction times and properties of the cortically evoked responses are consistent with a rapidly conducting, oligosynaptic pathway from the human motor cortex to accessory respiratory muscles and to truncal muscles.