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Cyclic Motion of the Soft Palate in Feeding
1 Department of Physical Medicine and Rehabilitation, Johns Hopkins University and Good Samaritan Hospital, Baltimore, MD 21287, USA; Correspondence: * corresponding author, jpalmer{at}jhmi.edu
The soft palate moves rhythmically during feeding, but the timing and frequency of this motion are not known. We tested the hypothesis that cyclic soft palate motion is temporally linked to cyclic jaw movement. Nine healthy, asymptomatic human subjects with normal dentition ate solid food coated with barium. Videofluorographic recordings showed that rhythmic motions of the soft palate during mastication were linked temporally to jaw motion. Soft palate motion occurred in every recording but not in every jaw cycle. The soft palate moved upward as the jaw opened, but the nasopharynx was not sealed. During swallowing, however, the soft palate invariably elevated during the intercuspal phase of jaw motion, sealing the nasopharynx. The frequency of soft palate cycles was lowest early in a feeding sequence and gradually increased as the sequence progressed from ingestion to swallowing. We conclude that cyclic movement of the soft palate in feeding is temporally linked to jaw motion.
Key Words: soft palate • feeding • CNS regulation • mastication
During swallowing, the soft palate (SP) rises to come into contact with the posterior wall of the pharynx, closing off the nasopharynx (Ardran and Kemp, 1955). The tongue propels the bolus through the fauces to enter the pharynx, and then, with the aid of the pharyngeal muscles, through the upper esophageal sphincter (UES). The SP returns to its ‘relaxed’ position after the bolus enters the esophagus. This pattern of SP behavior is controlled by the central pattern generator (CPG) for swallowing (Miller, 1999; Jean, 2001). Soft palate movements in chewing on solid foods were noted by Hiiemae and Palmer (1999). Cycles of palatal elevation occurred during mastication and oral food transport. The present study investigated cyclical SP motion by characterizing the frequency of its occurrence during feeding. The temporal relationship of SP motion cycles to jaw motion cycles was also examined. We tested the hypothesis that cyclic SP motion is temporally linked to cyclic jaw movement.
Nine healthy, asymptomatic young adults (four males, five females; median age, 24 yrs; range, 21-33 yrs) participated after giving fully informed consent as provided for in the protocol approved by our Institutional Review Boards. Subjects were seated comfortably in a chair, and complete feeding sequences, from ingestion to terminal swallow, were recorded with lateral projection videofluorography (VFG) at 30 fps. Each subject completed 2 trials each for 4 different foods: 6 g each of soft (banana), crunchy (shortbread cookie), and fibrous foods (cooked filet steak), each lightly coated with barium paste; and 1 g of fresh carrot with barium powder (except that two of the subjects completed only one trial each of meat and carrot). There were 68 recordings in total.
Data Reduction For each jaw motion cycle, the following times were noted: (a) the end of visible upward movement (end close, JEC); (b) start open (start open, JSO); (c) maximum open (maximum gape, JMG); and (d) start upward, sometimes after a pause at JMG (start close, JSC). The interval from JEC to JSO was defined as the intercuspal phase (IP) of jaw motion. The times of soft palate movement events were obtained: onset of soft palate elevation (palate start up, PSU); end of soft palate elevation (palate end upward movement, PEU); onset of soft palate descent (palate start down, PSD); and end of palate descent (palate end down, PED). The time from PEU to PSD was defined as the duration of maximal SP elevation.
Complete sequences were acquired on a computer equipped with a frame-grabber and image analysis software. Positions of the jaw and hyoid bone were plotted over time relative to the upper occlusal plane. The timing of soft palate motion was plotted on the same graphs with the use of a binary displacement scale, in which PSU and PED are 0, and PEU and PSD are 1 (Fig. 1
Data Analysis The frequency of soft palate cycles was defined as the percentage of jaw motion cycles with a concomitant palate motion cycle. The frequency of SP cycles was highly variable: We used logistic regression analysis to test for differences in the frequency of palate and jaw cycles among stages in sequence, foods, and subjects. Jaw end close (JEC) was used as the start of the jaw cycle, since palate start up (PSU) was tightly linked to JEC in every cycle. Because the timing of SP cycles was similar among subjects, data from all subjects and sequences were pooled. We used mixed-model ANOVA to test differences in the timing of soft palate and jaw movements by stages in sequence, food, and subject. Analyses were performed with SPSS 11.0 (SPSS Inc., Chicago, IL, USA). The critical value of P for rejection of the null hypothesis was 0.05.
Rhythmic SP elevation occurred in all sequences, all foods, and all subjects, but not in every jaw motion cycle (see Fig. 1  for example). The median overall frequency for SP cycles (ratio of SP to jaw cycles) was 39% (interquartile range [IQR], 25 to 58% of jaw motion cycles). The frequency of SP cycles differed significantly among stages in sequence, gradually increasing as the sequence progressed (P < 0.001): i.e., Stage I transport, 17% (IQR, 0.0 to 33%); processing, 25% (IQR, 13 to 48%); OPAT, 52% (IQR, 26 to 74%); and swallowing, 100% (invariant). The frequency of SP cycles varied significantly among foods (P = 0.028): meat, 33% (IQR, 23 to 58%); carrot, 35% (IQR, 33 to 53%); cookie, 37% (IQR, 27 to 58%); and banana, 47% (IQR, 32 to 56%). However, post hoc paired comparisons revealed that only cookie and meat differed significantly (P = 0.003) from one another. There were significant differences in the frequency of SP cycles among subjects (P < 0.001; median, 38%; IQR, 29 to 58%). There was no clustering of SP cycles within a sequence.
Jaw and SP cycles were ‘out-of-phase’ during the first 3 stages in the sequence (stage I transport, processing, and OPAT), so that the SP rose when the jaw was moving downward and fell when the jaw was moving upward (Figs. 1
The timing of SP motion in stage I transport and processing did not differ significantly for the onset of elevation (P = 0.166), duration of elevation (P = 0.611), maximum up duration (P = 0.963), or SP descent duration (P = 0.826; Table 1  , Figs. 3a, 3c). The onset of SP elevation was earlier in the jaw motion cycle in processing (0.09 ± 0.17 sec, Mean ± SD) than during OPAT (0.32 ± 0.33 sec, P < 0.001) or swallowing (0.23 ± 0.36 sec, P < 0.001). However, the duration of SP elevation (PSU-PEU) was slightly longer during processing (0.17 ± 0.10 sec) than in OPAT (0.14 ± 0.08 sec, P = 0.03; Table 1, Figs. 3a, 3c). Neither the onset of SP elevation, nor its duration, had significant inter-individual variation (P = 0.844 and P = 0.298, respectively). During OPAT, the duration of each phase of jaw motion was longer in cycles with than in cycles without stage II transport: intercuspal phase, P < 0.001; jaw opening, P < 0.001; maximum gape, P = 0.001; jaw closing, P = 0.006 (Table 2, Fig. 3b). The onset of SP elevation was later in cycles with than in cycles without stage II transport (0.54 ± 0.34 sec and 0.13 ± 0.18 sec, respectively, P < 0.001); but the duration of SP elevation (PSU - PEU) was shorter (0.11 ± 0.06 sec and 0.16 ± 0.08 sec, respectively, P < 0.001) (Table 2, Fig. 3b).
During swallowing, IP duration was prolonged. The SP remained elevated as the bolus was propelled into the hypopharynx, and the jaw began opening. The intercuspal and jaw opening phases were longer in swallowing than for any other stage in the sequence (stage I transport, P < 0.001; processing, P < 0.001; OPAT, P < 0.001), as was the duration of maximum SP elevation (stage I transport, P < 0.001; processing, P < 0.001; OPAT, P < 0.001: Table 1 , Figs. 3a, 3c, 3d).
The concept of a ‘posterior oral seal’ is deeply embedded in the clinical literature (Dantas et al., 1990), based on the finding that young subjects asked to swallow small aliquots of liquid ‘on command’ contain the liquid in the oral cavity until swallow onset. Several studies have now showed that triturated solid food may enter the pharynx well before the swallow in healthy, asymptomatic young individuals with normal dentition (Palmer et al., 1992; Dua et al., 1997). Takeda et al.(2002) showed that even liquids enter the hypopharynx well before swallow onset, when subjects are instructed to "chew" the liquids before swallowing. The cyclic elevation of the soft palate described here provides a mechanism for open communication between the pharynx and the oral cavity during chewing, facilitating food transport. The frequency of SP cycles increased as the sequence progressed from ingestion to swallow, as food was propelled from the oral cavity to the pharynx. The frequency of SP cycles varied greatly among subjects, but the timing of SP motion was quite consistent in its relationship to jaw movement across subjects. We conclude that cyclic movement of the soft palate in feeding was linked temporally to jaw motion, but the frequency and timing of soft palate cycles varied significantly as the sequence progressed from ingestion to swallow. These results raise questions about the neural mechanisms regulating orofacial movements in feeding. The central pattern generator for mastication drives the rhythmic contraction of the jaw adductor and abductor muscles (Dellow and Lund, 1971) and the associated rhythmic contraction of the facial musculature (Nakamura and Katakura, 1995). The present study shows that the SP moves cyclically in mastication and food transport. We infer that these movements are produced by contraction of the palatal musculature, since the jaw and soft palate are moving in opposite directions. This suggests that the central pattern generator for mastication controls SP motion during mastication and oral food transport, but not swallowing.
Chune Yang provided superb technical assistance. This research was supported by USPHS Award ROl DC 02123 from the National Institute on Deafness and Other Communication Disorders (to JBP and KMH). Dr. Matsuo is supported by two Health Sciences Research Grants (H 12-choujyu-21 and H 15-21 EBM-018) from the Ministry of Health, Labor and Welfare of Japan and by the Medstar Research Institute.
A supplemental appendix to this article is published electronically only at http://www.dentalresearch.org. Received for publication November 25, 2003. Revision received September 1, 2004. Accepted for publication September 28, 2004.
Journal of Dental Research, Vol. 84, No. 1,
39-42 (2005)
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