Identification of the medullary swallowing regions in the rat

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Abstract
The aim of the present study was to identify the central structures involved in the organization of the swallowing reflex in the rat. Using concentric bipolar electrodes, the medulla and pons were systematically explored in order to determine which central areas responded to stimulation by inducing swallowing. These areas, which were located in the dorsal medulla oblongata, were the solitary tract, the nucleus of the solitary tract (NST) and the adjacent reticular formation. Stimulation of the ventral ponto-medullary regions was ineffective with regard to the initiation of the swallowing reflex. The activity of medullary swallowing neurons was recorded using extracellular microelectrodes. These swallowing neurons responded with a burst of spikes (swallowing activity) which was closely linked to the swallowing reflex elicited by stimulation of the superior laryngeal nerve (SLN). Under SLN stimulation, the activity of some of the swallowing neurons furthermore showed an initial response consisting of 1 or 2 spikes after a brief latency. According to their location and the latency of their initial response, swallowing neurons were divided into two groups. Group I neurons were located in a dorsal area of the medulla oblongata corresponding to the NST and the adjacent reticular formation. All these neurons exhibited an initial response with a very short latency (1 to 4 ms), the swallowing activity of most of these neurons started before the onset of the swallowing motor sequence. Group II neurons were located either in a ventral area corresponding to the nucleus ambiguus and the surrounding reticular formation or in a dorsal and medial area corresponding to the hypoglossal nucleus and its vicinity. Some of these neurons also exhibited an initial response to SLN stimulation, but with a longer latency (7–12 ms). Motor paralysis of the animal (performed by curare injection) did not affect the swallowing activity of the neurons belonging to either group. Thus, the swallowing activity of the medullary neurons studied was a truly central activity. It is concluded that the swallowing neurons studied belonged to the medullary swallowing center; the group II neurons were motoneurons and interneurons forming the efferent stage of the swallowing center, and the group I neurons were the interneurons which largely belong to the center which programs the swallowing motor sequence.

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