Mechanical response properties of ventroposterior medial thalamic neurons in the alert monkey

The properties of trigeminal cutaneous thalamic neurons were explored in alert cynomolgous monkeys to determine receptive field and response characteristics. Two monkeys received juice reward for sitting quietly while an investigator probed the monkey's face with mechanical stimuli. Extracellular single unit recordings were made from the ventroposterior medial thalamic nucleus (VPM), and mechanical response properties were evaluated for each cell having an extraoral cutaneous receptive field. Of 89 cells examined, 90% responded best to innocuous tactile stimulation, and were classified as low threshold. The other 10%, classified as wide dynamic range, showed a graded response to increasingly intense stimuli, with a maximum discharge to noxious pinch. Of the low threshold neurons, most exhibited excitatory responses, with about half being rapidly adapting and the others slowly adapting. The spontaneous activity of 11% of the low threshold neurons was inhibited by stimulation of the neuron's receptive field. There was no systematic difference in receptive field size for the various types of neurons, but the receptive fields of wide dynamic range cells were smaller than those previously observed in trigeminopthalamic neurons of the medullary dorsal horn. The wide dynamic range and inhibitory low threshold neurons were located primarily in the caudal third of VPM, while the excitatory low threshold neurons were located throughout. In summary, response characteristics of VPM neurons show more diversity in the alert monkey than has been reported in paralyzed and/or anesthetized animals.