Mastication and swallowing are the first stage of digestion involving several motor processes such as
food intake, intra-oral food transport, bolus formation and chewing and swallowing reflex. These complicated motor
functions are accomplished by the well-coordinated activities in the jaw, hyoid, tongue, facial and pharyngeal
muscles. Although the basic activity patterns of these movements are controlled by the brainstem pattern generators,
these movements generate various peripheral sensory inputs. Among the sensory inputs, it is well-known that
somatic sensory inputs play important roles in reflexively modulating the movements so that the final motor outputs
fit the environmental demand. However, little is known about the effects of chemical sensory inputs such as
taste and olfaction originating from the ingested foods by these movements. A possible reason could be raised that
cognition of the chemical sensory inputs at the higher brain also influences the movements, so it is difficult to discuss
the neural mechanisms underlying the observed effect. In this review, we focus on the effects of chemical sensory inputs on the masticatory
movements and initiation of swallowing. We first summarize chemical sensory inputs occurring during mastication and swallowing,
and their receptive mechanisms. In addition, we will introduce the effect of application of monosodium L-glutamate (MSG) solution
as an umami taste to the oropharynx on the swallow initiation which is involuntary controlled and the possible neural mechanisms underlying
this effect is discussed.
Keywords: Mastication, swallowing, chemical stimulation, modulation, taste, glutamate, food.
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