Response properties of periodontal mechanosensitive units in the cat's thalamus.

1) The response characteristics of periodontal mechanosensitive neurons in the thalamic nucleus were studied by using 15 adult cats anesthetized with alpha-chloralose (40 mg/kg, i.v.). 2) 468 periodontal mechanosensitive units (PM units) responsive to light mechanical stimulation of the teeth were recorded from a medioventral aspect of the nucleus ventralis posteromedialis (VPM). Of these neurons, 213 units (45.5%) were slowly adapting, and 255 units (54.5%) were rapidly adapting. 3) The receptive field of the PM units was represented by the number of receptive quadrants on the dental arches of both jaws. About half of the PM units (279, 59.6%) were one-quadrant units. Of these neurons, 69 (14.7%) were single tooth units. Two-quadrant, three-quadrant and four-quadrant units numbered 123 (26.3%), 3 (0.6%) and 63 (13.5%) respectively. Of the PM units detected, 246 units (52.6%), 124 units (26.4%) and 98 units (20.9%) responded to mechanical stimulation of the teeth on only the contralateral side, on only the ipsilateral side and on both sides, respectively. 4) One-quadrant units were distributed mainly in the rostral part of the PM area, and four-quadrant units were found in the caudal part of the PM area. A somatotopic organization for each tooth was not found. 5) The incidence of single tooth units in the thalamus was extremely low, compared with those in the primary afferents and in the trigeminal nuclear complex. Most of the thalamic neurons had larger receptive fields than both the primary afferents and the trigeminal nuclear complex neurons. 6) The high incidence of four-quadrant units at the caudal part of the PM area suggests that many PM fibers converge there. However, it is still unclear whether these fibers come through the intrathalamic neurons or directly from the trigeminal nuclear complex.