Descending excitation and inhibition of spinal cord lamina I projection neurons.

1. Lamina I cells were recorded in the lumbar dorsal horn of decerebrate rats. Their projecting axons were mainly located in the contralateral dorsolateral funiculus (DLF) in the upper cervical cord. 2. The effect on these cells of short and long trains of stimuli applied to the upper cervical DLF was examined by measuring the ongoing activity of the cells, their response to peripheral stimuli, and the size of their receptive fields. 3. The presence of tonic descending influences from brain stem to spinal cord was investigated by measuring the properties of the lamina I cells before and during block of descending impulses. 4. The results of DLF stimulation and of cord block show that substantial and prolonged excitation affected many cells, whereas some were inhibited for shorter periods of time. 5. The experiments were repeated with stimulation of the DLF caudal to chronic section to eliminate descending fibers. The results suggest that the changes of excitability in intact animals were partly produced by stimulation of descending fibers and partly by the invasion of collaterals activated by the antidromic stimulation of the axons projecting from the lamina I cells. 6. Although long trains of DLF stimuli generally excited lamina I cells, only inhibitions were seen in the deep dorsal horn. Moreover, stimulation rostral to an acute unilateral DLF lesion was without effect on lamina I cells but inhibited deep cells. 7. It is proposed that the lamina I cells might activate brain stem circuits, which in turn influence deep dorsal horn cells.