Cardiac and abdominal vagal afferent inhibition of primate T9-S1 spinothalamic cells.

Effects of electrically stimulating vagal afferents were determined on lumbosacral spinothalamic tract (STT) neurons in the T9-S1 segments. Stimulating left or right vagal afferents inhibited 20 (50%) and excited 4 (10%) of 40 STT neurons. Vagal stimulation reduced activity of the 20 inhibited cells by 71 +/- 6% and reduced the average activity of all 40 STT neurons by 28% from 11.5 +/- 1.3 to 8.3 +/- 1.4 impulses/s (P less than 0.01). Effects of activating thoracic and abdominal or just abdominal vagal afferents were also determined. Stimulating right abdominal vagal afferents inhibited 4 (11%), excited 1 (3%), and did not affect 30 (86%) of the STT neurons and overall did not significantly affect STT cell activity. In contrast, in 33 of these cells stimulation of afferents in the right cervical vagus inhibited 16 (48%), excited 2 (6%), and did not affect 15 (45%) neurons and overall significantly reduced cell activity by 29% (P less than 0.01). These data and those of Ammons et al. (J. Neurophysiol. 50: 926-940, 1983; Circ Res. 53: 603-612, 1983; J. Neurophysiol. 54: 73-89, 1985) suggest that cardiopulmonary but not abdominal vagal afferent input reduces STT cell activity in many spinal segments. This inhibitory vagal reflex may play a role in protecting the heart.