EXCITABILITY OF HUMAN MOTOR CORTEX DURING HYPERVENTILATION AND HYPERCAPNIA

We tested the hypothesis that the excitability of corticospinal neuron s was altered by changes in PCO2. Magnetic stimulation was used to exc ite the neurons in the human motor cortex that give rise to the fast-c onducting corticospinal pathway. The characteristics of the composite excitatory postsynaptic potentials (EPSPs) produced in individual spin al motoneurons by cortical stimulation were derived from changes in th e firing probability of voluntarily activated motor units. The amplitu des of these composite EPSPs in response to a constant cortical stimul us were assumed to reflect the excitability of cortical neurons. In 10 healthy subjects, we found no statistically significant changes in th e excitability of the cortical neurons during normocapnic conditions ( mean end-tidal PCO2 5.1 kPa), during hyperventilation-induced hypocapn ia (mean end-tidal PCO2 2.9 kPa), and during hyperoxic hypercapnia ind uced by a rebreathing technique (mean end-tidal PCO2 6.9 kPa). We conc lude that the excitability of corticospinal neurons activated by magne tic stimulation is not significantly affected by changes in PCO2.