A positive feedback loop between spinal cord nociceptive pathways and antinociceptive areas of the cat's brain stem.

Electrophysiological evidence has been obtained suggesting the presence of reciprocal excitation between descending pathways from the nucleus raphe magnus (NRM) and adjacent reticular formation (Ret.F) and spinal cord neurones projecting to these brain stem areas. In decerebrate and decerebellate cats, 40 spinal cord neurones were recorded whose recording sites were in or close to lamina VIII of the lumbar spinal cord. All 40 neurones recorded in the lumbar cord were postsynaptically excited by electrical stimulation of the NRM, the Ret.F. or most commonly, of both. The excitation was mediated by fast-conducting fibres and lasted for over 100 msec after a single shock. The shortest latency responses were obtained following stimulation of the contralateral Ret.F. These neurones had complex peripheral inputs subjected to descending controls. All the neurones could be excited by deep pressure of the ipsilateral and/or contralateral hind limbs. Peripheral inhibitory inputs were also observed. Eighteen out of the 40 neurones had axons that projected to NRM and the adjacent Ret.F. Conduction velocities ranged between 31.6 and 91 m/sec. In addition, 11 other axons were recorded in the white matter of the cervical cord from neurones projecting to NRM and Ret.F. Conduction velocities of this group of axons ranged between 13 and 70 m/sec. The majority of the axons projecting to NRM and Ret.F. were found to join pathways in the ventro-lateral quadrant of the spinal cord either ipsi- or contralaterally to their Ret.F. destination. Recordings were also made from 12 neurones whose recording sites were located in the NRM and Ret.F. Their responses to electrical stimulation of sites within lamina VIII of the lumbar spinal cord were studied. Only excitatory responses could be evoked by such stimulation. These results are discussed in relation to the mechanisms of activation of central antinociceptive systems.