Excitability of human motor cortex during hyperventilation and hypercapnia.

We tested the hypothesis that the excitability of corticospinal neurons was altered by changes in PCO2. Magnetic stimulation was used to excite the neurons in the human motor cortex that give rise to the fast-conducting corticospinal pathway. The characteristics of the composite excitatory postsynaptic potentials (EPSPs) produced in individual spinal motoneurons by cortical stimulation were derived from changes in the firing probability of voluntarily activated motor units. The amplitudes of these composite EPSPs in response to a constant cortical stimulus were assumed to reflect the excitability of cortical neurons. In 10 healthy subjects, we found no statistically significant changes in the excitability of the cortical neurons during normocapnic conditions (mean end-tidal PCO2 5.1 kPa), during hyperventilation-induced hypocapnia (mean end-tidal PCO2 2.9 kPa), and during hyperoxic hypercapnia induced by a rebreathing technique (mean end-tidal PCO2 6.9 kPa). We conclude that the excitability of corticospinal neurons activated by magnetic stimulation is not significantly affected by changes in PCO2.