EVIDENCE THAT TRIGEMINAL BRAIN-STEM INTERNEURONS FORM SUBPOPULATIONS TO PRODUCE DIFFERENT FORMS OF MASTICATION IN THE RABBIT

To determine how trigeminal brainstem interneurons pattern different f orms of rhythmical jaw movements, four types of motor patterns were in duced by electrical stimulation within the cortical masticatory areas of rabbits. After these were recorded, animals were paralyzed and fict ive motor output was recorded with an extracellular microelectrode in the trigeminal motor nucleus. A second electrode was used to record fr om interneurons within the lateral part of the parvocellular reticular formation (Rpc-alpha, n = 28) and gamma-subnucleus of the oral nucleu s of the spinal trigeminal tract (NVspo-gamma, n = 68). Both of these areas contain many interneurons projecting to the trigeminal motor nuc leus. The basic characteristics of the four movement types evoked befo re paralysis were similar to those seen after the neuromuscular blocka de, although cycle duration was significantly decreased for all patter ns. Interneurons showed three types of firing pattern: 54% were inacti ve, 42% were rhythmically active, and 4% had a tonic firing pattern. N eurons within the first two categories were intermingled in Rpc-alpha and NVspo-gamma: 48% of rhythmic neurons were active during one moveme nt type, 35% were active during two, and 13% were active during three or four patterns. Most units fired during either the middle of the mas seter burst or interburst phases during fictive movements evoked from the left caudal cortex. In contrast, there were no tendencies toward a preferred coupling of interneuron activity to any particular phase of the cycle during stimulation of other cortical sites. It was conclude d that the premotoneurons that form the final commands to trigeminal m otoneurons are organized into subpopulations according to movement pat tern.