Inhibition of nociceptive neurons by internal capsule stimulation.

The mechanism of pain relief by internal capsule (IC) stimulation was investigated in 32 adult cats. Nociceptive neuronal activity of the nucleus ventralis posteromedialis (VPM), responding to contralateral pulp stimulation, was suppressed by IC stimulation to a greater extent than activity in the posterior nuclear group (PO) or centre-median nucleus. On the contrary, suppression of neuronal firing by intraventricular morphine-HCl predominated in PO neurons. These results suggest that pain relief by IC stimulation may be mediated through inhibitory effects on nociceptive neurons of the thalamic sensory relay nuclei.