Inhibitory effects of phrenic afferent fibers on primate lumbosacral spinothalamic tract neurons

Studies were conducted to determine if electrical or mechanical stimulation of phrenic afferent fibers (PHR) would inhibit the activity of lumbosacral spinothalamic tract (STT) neurons. Twelve monkeys were anesthetized, paralyzed, and artificially ventilated. Extracellular action potentials were recorded from 78 STT neurons located in L2-S3 spinal segments. Electrical stimulation of PHR reduced the activity of 65%, did not affect 33%, and excited 1% of STT neurons. Mechanical stimulation of the diaphragm reduced the activity of 63%, did not effect 34%, and excited 1% of lumbosacral STT neurons. Distention of the urinary bladder (UBD) inhibited 52%, did not affect 23%, excited 23%, and elicited a biphasic response in 1% of STT neurons. However, there was no correlation between the effect of PHR and UBD or somatic classification of the neurons. We conclude that electrical or mechanical stimulation of PHR can produce a generalized inhibition of lumbosacral STT neurons. This inhibitory effect of PHR is similar to inhibitory effects reported for a variety of other afferent systems.