Generation of the masticatory central pattern and its modulation by sensory feedback

The basic pattern of rhythmic jaw movements produced during mastication is generated by a neuronal network located in the brainstem and referred to as the masticatory central pattern generator (CPG). This network composed of neurons mostly associated to the trigeminal system is found between the rostral borders of the trigeminal motor nucleus and facial nucleus. This review summarizes current knowledge on the anatomical organization, the development, the connectivity and the cellular properties of these trigeminal circuits in relation to mastication. Emphasis is put on a population of rhythmogenic neurons in the dorsal part of the trigeminal sensory nucleus. These neurons have intrinsic bursting capabilities, supported by a persistent Na+ current (INaP), which are enhanced when the extracellular concentration of Ca2+ diminishes. Presented evidence suggest that the Ca2+ dependency of this current combined with its voltage-dependency could provide a mechanism for cortical and sensory afferent inputs to the nucleus to interact with the rhythmogenic properties of its neurons to adjust and adapt the rhythmic output. Astrocytes are postulated to contribute to this process by modulating the extracellular Ca2+ concentration and a model is proposed to explain how functional microdomains defined by the boundaries of astrocytic syncitia may form under the influence of incoming inputs.

► A population of rhythmically bursting neurons in the main sensory nucleus could form the core of the masticatory CPG.
► Bursting in these neurons depends on a persistent Na+ conductance modulated by the extracellular [Ca2+].
► By controlling the extracellular [Ca2+], astrocytes could contribute to setting up conditions for bursting.
► Astrocytes of the main sensory nucleus respond to stimulation of afferent fibres and to NMDA application.
► Astrocytes could help synchronizing populations of neurons within functional domains defined by boundaries of their syncitia.