Identification and input-output properties of bulbar reticular neurons involved in the cerebral cortical control of trigeminal motoneurons in cats.

Neurons found in the medial bulbar reticular formation were activated by stimulation of the orbital gyrus and responded with antidromic spike potentials to selective stimulation of either the masseter or anterior digastric motoneuron pool in the trigeminal motor nucleus in cats anesthetized with alpha-chloralose. These two kinds of reticular neurons were assumed to be inhibitory neurons projecting to masseter motoneurons (IM neurons) and excitatory neurons projecting to anterior digastric motoneurons (ED neurons), involved in the effects of stimulation of the orbital gyrus on trigeminal motoneurons: inhibition of masseter motoneurons and excitation of anterior digastric motoneurons. Input-output properties of IM and ED neurons were studied intracellularly with the following results: (1) stimulation of the orbital gyrus evoked EPSPs in IM and ED neurons with mono- and polysynaptic latencies; and (2) stimulation of the lingual nerve evoked a spike potential in a few IM and ED neurons after a rather long latency, indicating that the pathways involved in the cortical control of trigeminal motoneurons via IM and ED neurons were basically separate from those responsible for the reflex control by the peripheral inputs. Intracellular injection of horseradish peroxidase revealed that both IM and ED neurons were small or medium in size and the former were smaller than the latter, while none of the large reticular neurons directly projected to the trigeminal motor nucleus. This suggests a possible functional differentiation among bulbar reticular neurons according to cell size.