Shuichi Nozaki

Feeding behavior in mammals: corticobulbar projection is reorganized during conversion from sucking to chewing

It is known that repetitive stimulation of the frontal cortex (cortical masticatory area, CMA) induces rhythmical jaw movements similar to chewing in adult mammals. In the present study we were able to induce rhythmical jaw movements similar to sucking by repetitive stimulation of the frontal cortex in neonatal guinea pigs. This area, which we named the cortical sucking area (CSA), was located rostral to the CMA which was later formed upon maturation. Neurons of the CSA were shown electrophysiologically and morphologically to project primarily to the dorsal part of the paragigantocellular reticular nucleus of the contralateral side. This was the site which the CMA neurons, later, projected to induce chewing. It is generally thought that tooth eruption triggers the conversion from sucking to chewing. However, guinea pigs are born with a complete permanent dentition and therefore devoid of this peripheral trigger for the conversion to chewing. Accordingly we propose that shift of the cortical projection area from the CSA to the CMA during the maturation causes the conversion of the mammalian feeding behavior. It is discussed that this transition involves extensive reorganization of the cortical efferent system including the pyramidal tract during early postnatal development.

Control of trigeminal motoneurons exerted from bulbar reticular formation in the cat.

Abstract The mode of control from the medial bulbar reticular formation to masseteric and anterior digastric motoneurons was studied in cats in relation to cortical control of trigeminal motoneurons. Both the early and late phases of depression of the masseteric reflex induced by orbital cortical stimulation were abolished by a localized transection of the medial part of the brain stem reticular formation at the pontobulbar junction of the encephale isole. Stimulation of the medial bulbar reticular formation produced a sequence of a monosynaptic IPSP, an EPSP, a hyperpolarizing potential in masseteric motoneurons, and a monosynaptic EPSP followed by a hyperpolarizing potential in anterior digastric motoneurons. The late hyperpolarizing potential of masseteric motoneurons had a time course similar to that of anterior digastric motoneurons. Systematic stimulation of the medulla oblongata revealed that the most prominent inhibitory effect of reticular stimulation on the masseteric reflex was obtained from the dorsolateral part of the nucleus reticularis gigantocellularis and the adjacent nucleus reticularis parvocellularis. Neither the orbital cortical nor the medial bulbar reticular stimulation evoked a spike potential in the supratrigeminal neurons. We conclude that the cortical inhibitory pathway to masseteric motoneurons was mediated via inhibitory neurons in the medial bulbar reticular formation and was basically separate from the peripheral inhibitory pathway via the supratrigeminal neurons.