The excitability of human corticospinal neurons is depressed by thiopental.

BACKGROUND We tested the effect of thiopental on the excitability of the corticospinal-motoneuron axis in normal human subjects. METHODS Magnetic stimulation was used to excite the neurons in the motor cortex which give rise to the fast conducting corticospinal pathway. The characteristics of the composite excitatory post-synaptic potentials (EPSPs) produced in individual spinal motoneurons by cortical stimulation were derived from changes in the firing probability of voluntarily activated motor units of the first dorsal interosseous muscle. RESULTS In 5 normal subjects, we found that thiopental, in incremental doses sufficient to sustain drowsiness (total dose 75 to 175 mg), significantly reduced the amplitude of these composite EPSPs. CONCLUSIONS Thiopental reduced the facilitation of motoneurons from the cortex most likely by depressing cortical neurons.