Swallowing during ongoing fluid ingestion in the rat.

In order to identify fixed versus flexible aspects of swallowing and oromotor control, we developed a method for the concurrent analysis of swallow frequency, swallow volume, and the temporal correlation between swallowing and rhythmic orolingual movements during prolonged bouts of fluid ingestion in unanesthetized, unrestrained rats. Rats actively ingested 0.1 M sucrose solutions infused intraorally: (a) at a constant rate within sessions; and (b) during separate sessions across which the rate of intraoral infusion (= ingestion rate) was varied. Salient characteristics of 'intraoral intake' included: (1) covariation of swallow frequency and swallow volume to mediate adjustments in the rate of ingestion; (2) emission of rhythmic coupled movements (5-7 Hz) of the jaw and tongue that serve to transport fluid through the mouth; (3) a 20-40 ms increase in the duration of the rhythmic movement cycle associated with swallow onset; and (4) emergence of pauses between bursts of rhythmic oromotor behavior with increases in cumulative intake. That intraoral intake and fluid ingestion via spout-licking (Weijnen et al., Brain Behav. Evol., 25, 1985) share all of these features suggests strongly that common neural processes, relatively uninfluenced by sources of sensory feedback attending contact with an environmental fluid source, underlie these aspects of normal fluid ingestion. The dynamic adjustment of swallow frequency and swallow volume presents a challenge to traditional models of reflexive swallow initiation derived primarily from the neurophysiological study of acute, non-behaving preparations.