Central Neural Mechanisms Involved in the Control of Jaw Movement during Postnatal Development

Mechanisms responsible for controlling masticatory muscle activity during suckling and mastication initiate in the brain. Premotor neurons situated in the brainstem transmit masticatory motor commands to the jaw-closing and jaw-opening motoneurons, which in turn activate the masticatory muscles. The premotor neurons receive inputs from multiple sources such as the orofacial sensory organs, the cerebral cortex, and the central pattern generator in the brainstem. These neural circuits might be important for controlling jaw movement, although their properties in this regard are likely to be altered in postnatal development as the oral motor behavior of mammals shifts significantly from suckling to chewing during the early postnatal period. This timing of jaw movement development mimics that of the orofacial musculoskeletal apparatus. The current article reviews findings on local neural circuits of trigeminal motoneurons and premotor neurons, during early development in rats. We describe findings from electrophysiological recordings, optical imaging with voltage-sensitive dyes, calcium imaging, and laser photolysis of caged glutamate. We also review research into the subtypes of premotor neurons in the supratrigeminal region （SupV） that differ in their electrophysiological properties and possess different morphological characteristics. We further describe how trigeminal motoneurons receive glutamatergic, glycinergic, and GABAergic inputs from the SupV and the reticular formation dorsal to the facial nucleus. Finally, we discuss how these synaptic and intrinsic membrane properties change in postnatal development. Taken together, the findings reviewed herein suggest that these neural circuits are important in diverse oral motor behaviors, and that their postnatal changes are involved in the transition from suckling to mastication functions.