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

Abstract

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.

Discharge patterns of neurons in the medial pontobulbar reticular formation during fictive mastication in the rabbit

“…As described in the introduction, the masticatory CPG model also includes a relay between the medial components of the CPG and trigeminal motoneurons, because the former have few direct connections with the trigeminal motor nucleus (Mizuno et al ., 1978; Nozaki et al ., 1983; Landgren et al ., 1986; Fort et al ., 1994; Fay & Norgren, 1997; Kolta et al ., 2000). However, medial brainstem nuclei (Rgc, nParv and nPontc) do project strongly to adjacent lateral brainstem areas (the reticular formation surrounding the trigeminal motor nucleus and nParv‐α/subnucleus γ of the oral nucleus of the spinal trigeminal tract (nVspo‐γ); Fort et al ., 1994; Kolta et al ., 2000).…”

“…Only one third of the neurons in our study responded to stimulation of the inferior alveolar nerve, which contains primary afferents from the lower jaw and lip. The relatively long latencies of these responses confirm earlier work in the cat (Nozaki et al ., 1983; Sahara et al ., 1996), and suggest that the pathways are at least disynaptic. In contrast, almost all neurons (> 90%) located in the reticular nuclei around nVmot and in nVspo‐γ are excited by orofacial afferents at monosynaptic latencies (Eisenman et al ., 1963; Dallel et al ., 1990; Donga & Lund, 1991; Westberg & Olsson, 1991; Inoue et al ., 1992; Westberg et al ., 1998).…”

Mastication and its Control by the Brain Stem

“…Stimulation of medial nuclei excites digastric motonuerons and inhibits masseteric motoneurons at very short latency, 145 and medial RF neurons can be antidromically activated by stimulation of n. V mot. 137 On the other hand, anatomical studies have not provided much evidence that medial reticular areas have a strong monosynaptic projection to V motoneurons, suggesting that the rhythm generators act mainly via other groups of premotor neurons close to n. V mot. This point of view has been expressed in several papers from the group at UCLA.…”

Two major types of premotoneurons in the feline trigeminal nucleus oralis as demonstrated by intracellular staining with horseradish peroxidase

“…Yasui et al (19851, who made cortical injections of wheat germ agglutinin (WGA)-HRP into the rostra1 part of the orbital gyrus, described anterogradely labeled fibers in the medial aspect of Vo.r and its adjacent reticular formation, and Qlsson and and Westberg and Qlss,on (1991) also reported that Vo.r interneurons responding to inferior alveolar or lingual nerve inputs also receive monosynaptic inputs from corticobulbar tract cells in the coronal gyrus. In addition, disynaptic excitation of digastric motoneurons by orbital gyrus stimulation has been described (Nakamura et al, 19751, although the location of the relevant premotoneurons has been reported to be the gigantocellular tegmental field (Takatori et al, 1981, Nozaki et al, 1983.…”