Interaction between genioglossus and diaphragm responses to transcranial magnetic stimulation in awake humans

The modulation of activity of the upper airway dilator and respiratory muscles plays a key role in the regulation of ventilation, but little is known about the link between their neuromuscular activation processes in vivo. This study investigated genioglossus and diaphragm responses to transcranial magnetic stimulation applied in different facilitatory conditions. The amplitude and latency of motor-evoked potential responses and the stimulation intensity threshold leading to a motor response (motor threshold) were recorded with stimulation applied at the vertex and anterolateral area in 13 awake normal subjects. Stimuli were applied during inspiration with and without resistance, during expiration with and without maximal tongue protrusion and during deep inspiration. In each stimulation location and condition, no diaphragmatic response was obtained without previous genioglossus activity (diaphragmatic and genioglossus responses latencies during expiration: 18.1 +/- 2.9 and 6.3 +/- 2.6 ms, respectively, mean +/- s.d., P < 0.01). Genioglossus motor-evoked potential amplitude, latency and motor threshold were significantly modified with tongue protrusion with a maximal effect observed for stimulation in the anterolateral area. Deep inspiration was associated with a significant facilitatory effect on both genioglossus and diaphragm motor responses. The facilitatory effects of respiratory and non-respiratory manoeuvres were also observed during focal stimulation where isolated genioglossus responses were observed. Genioglossus and diaphragm differed in their motor threshold both at baseline and following facilitatory manoeuvres.

(1) transcranial magnetic stimulation-induced genioglossus response systematically precedes that of diaphragm; (2) this sequence of activation is not modified by respiratory and non-respiratory manoeuvres; and (3) the genioglossus and diaphragm are differently influenced by these manoeuvres in terms of latency of the motor response and of motor threshold.