This paper is a mini-review of a neural mechanism of salivary secretion during chewing, which is mainly based on our behavioral, electrophysiological, and neuroanatomical studies. Salivation during chewing is not a simple brainstem reflex induced by oral sensation, but affected by descending control from the higher centers. The descending control reaches the sympathetic and parasympathetic salivary centers in the lower center, and control both amount and quality of saliva. The higher centers of salivation are composed of the cerebral cortex, hypothalamus, and limbic systems. These centers are not specific for salivation, but control feeding and chewing behavior, thermoregulation, or water balance of the body. This implies that salivation is one of apparatus for the above-mentioned regulatory systems. As for the feeding behavior, the oral sensation is processed in the sensory and prefrontal association cerebral cortices and amygdala, which are related to preference of food and judgment of food texture. These higher centers, with excitatory and inhibitory neurons, may ingeniously control salivation during chewing.

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As the regulation mechanism of appetite, the satiety center and feeding center regulate eating actions. It was proved by Zucker's rat experiments of genic obese animals that secretion of histamine among food-intake control substances depends on mastication, namely on the stimulus by chewing. On the other hand, our previous report suggested the effects of mastication on the centers. Based on these backgrounds, effects of mastication on insulin secretion by masticating chewing gum before meal were examined.
Nineteen healthy female volunteers were enrolled in the study. They chewed gum containing sugar for 15 minutes and then received a glucose tolerance test. Blood glucose and plasma insulin levels were measured at 5 points (during fasting and 15, 30, 60 and 120 minutes after a glucose load). The same measurements were repeated under two varying conditions: (1) without chewing the gum and (2) chewing the gum base for 15 minutes. Each parameter was compared.
As a result, following mastication of gum containing sugar, blood glucose levels rose and then fell more rapidly than in the control group. The same trend was also seen in plasma insulin levels. The total insulin release did not differ. On the other hand, none of these parameters differed significantly between mastication of gum containing sugar and mastication of gum base.
From these results, the effects of mastication on the centers are suggested, and it can be considered that mastication contributes to the acceleration of sugar metabolism.

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In order to clarify whether or not masticatory exercise (= chewing) can induce arousal through sympathetic activation or relaxation through parasympathetic activation, we measured heart rate and its variability with analysis of power spectrum (low and high frequency; LF, HF), blood pressure (BP), saliva (flow rate, constituents) and electroencephalogram (α and βA waves) of nine healthy male subjects aged 20 to 42 years during chewing . The subjects chewed gums consisting of gum base only (C) and gum base plus theanine as the substance to evoke relaxation (RX), and two consistencies (soft, hard) respectively . We discussed changes that occurred during chewing, and also the influence of theanine in RX gum.
During gum chewing of both RX and C, Heart rate, BP, salivary flow rate increased, while wave height of plethysmogram decreased. Salivary flow rate, concentrations of total protein, α-amylase and electrolytes (Na, Cl, K) of saliva increased during chewing of RX gum much more than during C. Immunoglobulin (sIgA) concentration and LF/HF power rate decreased during chewing of C gum; in contrast, it didn't change during chewing of RX gum . Cortisol concentration didn't change during chewing of both gums. The α wave remarkably decreased during chewing of C gum.
In conclusion, it can be concluded that chewing remarkably increased the activities of the sympathetic nerve, without regard to taste of gum, especially during chewing. However, focused on the changes in the buccal cavity, the parasympathetic nerve was also slightly activated during chewing. Chewing of the RX gum influenced the increase of salivary secretion much more than chewing C gum. Once chewing of both RX and C gums stopped, the activated parasympathetic nerve appeared to become dominant over the sympathetic nerves, judging from cardiovascular markers. According to the changes in sIgA and a wave, relaxation as an effect of chewing might result from digesting an ingredient.

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Easiness of ingestion and palatability of rice crackers for aged people were studied using compression test, electromyographic (EMG) recordings of masticatory muscles and sensory evaluation. Five types of rice crackers with different hardness were evaluated, which were Kaihatsu-type newly developed for aged people, Souka-type, Okaki-type, Niigata-type and Baby-type.
The results obtained are as follows.
1. Hardness of rice crackers was a most important factor decides whether aged subjects can ingest them.
2. Hardness (Young's modulus) of rice crackers was as follows, Souka-tupe> Okaki-type> Niigata-type> Kaihatsu-type = Baby-type. These results corresponded to the results of sensory evaluation in hardness during mastication of rice crackers by young subjects (n = 66).
3. Aged subjects were able to ingest Kaihatsu-type and Baby-type. Those Young's modulus were less than 8.56 x 10⁶N/m.
4. Young's modulus (x) of rice crackers was significantly related to sum of masseter muscle activity (y), y = 0.15x +1.52 (R² = 0.83, p <0.05). The harder rice crackers were, the more sum of masseter muscle activity increased.
5. The harder rice crackers were, the more those chewing time and sum of masseter muscle activity increased. Because those number of chewing strokes increased.
6. Hardness of rice crackers caused trouble for aged subjects at the stage of intake. Harder rice crackers tended to increase first activity of masseter muscle and burst duration per chewing.
7. Texture was the most important factor in palatability for rice crackers.
8. Both young and aged subjects evaluated that Kaihatsu-type were better than Baby-type in all items of palatability.
These results showed that Kaihatsu-type had palatability for aged people and texture properties which were easy to ingest.

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