Age-associated cognitive deficit together with dementia have become an increasingly important health and socioeconomic issues. Recent research studies have focused on the oral health conditions, especially mastication (chewing), as well as on the number of teeth, in the context of cognitive deficits. Both animal and human studies suggested a possible causal relationship between mastication and cognitive function. Mastication is important not only for food intake, but also for preserving and promoting the general health and for maintaining the hippocampus-dependent cognitive performance, which deteriorates with aging. There are multiple neural circuits connecting the masticatory organs and the hippocampus. Both animal and human studies indicate that impaired masticatory function causes morphological and functional alterations of the hippocampus and the hippocampus-dependent spatial memory deficits, especially in elderly. Proper chewing helps to maintain the hippocampal function and sustain cognitive functioning, Thus, chewing may represent a useful approach in preserving and promoting the hippocampus-dependent cognitive function in older people.
Mastication is the first stage of the digestion and involves the intermittent rhythmic act in which the tongue, facial and jaw muscles act in coordination to position the food between the teeth, cut and soften it, and prepare it for swallowing. Mastication decreases the particle-size distribution in the food bolus and forms a cohesive bolus with the saliva in order to facilitate swallowing. Teeth provide a uniquely discriminating sense of touch and directional specificity for intra-oral contact for management of a food bolus and together with the pulp-dentine-enamel complex, plays a critical role in the generation and control of jaw movements. The sensory information from the oral cavity including teeth is transmitted through the trigeminal sensory nerve to the trigeminal sensory nuclei, cerebellum, hypoglossal motor nuclei and the brainstem reticular formation all of which are involved are in attention, perception and conscious learning. Thus, the sensory information from the masticatory organs may affect hippocampus through multiple neural pathways via the thalamus and cerebral cortex.
Using animal models, researchers have shown that improper chewing or masticatory dysfunction triggers structural and functional alterations in the memory centers of the brain. Reduced mastication triggers cell atrophy in the hippocampal area leading to spatial learning and memory deficits. In contrast, chewing was shown to be associated with better cognitive performance, improved word recall and spatial working-memory task, and increased the cerebral blood flow. Therefore, chewing might be an effective approach in maintaining the hippocampus-related spatial learning and memory and is a useful approach in preserving and promoting the hippocampus-dependent cognitive function in older people.
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Kagaku Azuma, Qian Zhou, Masami Niwa, and Kin-ya Kubo. Association between Mastication, the Hippocampus, and the HPA Axis: A Comprehensive Review International Journal of Molecular Sciences. 2017, 18, 1687.