Brainstem organization of the swallowing network.

Swallowing is a complex motor sequence involving the coordinated contraction of several muscles in the mouth, pharynx, larynx and esophagus. The motor sequence of swallowing, which can be entirely performed without afferent feedback, is centrally programmed by a neuronal network. This network can be divided into three levels: an afferent level corresponding to the input arm to the network, that is mainly the solitary tract; an efferent level corresponding to the output arm of the network, that is the different pools of motoneurons involved in swallowing and localized within the trigeminal and hypoglossal nuclei and the nucleus ambiguus; an organizing level corresponding to the interneuronal network which programs the swallowing motor sequence. The 'swallowing interneurons' of the organizing level are localized in two medullary regions: a dorsal region including the nucleus of the solitary tract (NST) and the adjacent reticular formation, and a ventral region corresponding to the lateral reticular formation above the nucleus ambiguus. Neurons localized within the NST region are interneurons which largely go to make up the part of the network which initiates and programs swallowing. Originating from these neurons, the central swallowing command is relayed by the interneurons within the ventrolateral reticular formation before reaching the different groups of motoneurons which excite the swallowing reactions.