Abstract
We have suggested that lamina VII interneurons receiving inhibition from flexor reflex afferents (FRA) mediate muscle tone suppression in cats. Here we examined whether interneurons having above characteristics contributed to the suppression and restoration of muscle tone, which were respectively induced by injections of carbachol and serotonin into the PRF. Activity of interneurons was extracellularly recorded from the lower lumbar segments (L6-7) of acute decerebrate cats. Lamina VII cells (n=19), which satisfied the characteristics, had a firing rates of 2.1 + 1.3 Hz. Their firing rate was increased (34.6 + 8.7 Hz) after carbachol injections, while it was reduced (4.3 + 2.7 Hz) after serotonin injections. However, cells receiving excitatory FRA inputs (n=15), including those mediating reciprocal Ia inhibition and non-reciprocal group I inhibition reduced their firing rate by carbachol and was re-established after serotonin. Changes in firing rate of 157 cells were also examined with respect to their location. Before carbachol, cells in the intermediate region and dorsal horn had higher firing rates than those in the ventral horn (lamina VII). However the latter had higher firing rates after carbachol.
These results suggest lamina VII interneurons that receive inhibition from volleys in FRAs are possibly involved in the carbachol-induced motor inhibition. Organization of spinal interneurons can be under the control of cholinergic-serotonergic reciprocity in the PRF. [Jpn J Physiol 54 Suppl:S184 (2004)]
