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===Mastication and Cognitive Processes===
===Brainstem and Cognitive Processes===
In recent years, mastication has been a topic of discussion about the maintenance and support effects of cognitive performance. An elegant study performed through <sub>f</sub>MR and positron emission tomography (PET) has shown that mastication leads to an increase in cortical blood flow and activates the additional somatosensory cortex, motor motor and insular, as well as the striatum, the thalamus, and the cerebellum. Mastication right before performing a cognitive task increases oxygen levels in the blood (BOLD of the fMRI signal) in the prefrontal cortex and the hippocampus, important structures involved in learning and memory, thereby improving the performance task..<ref>Yamada K, Park H, Sato S, Onozuka M, Kubo K, Yamamoto T., Dynorphin-A immunoreactive terminals on the neuronal somata of rat mesencephalic trigeminalnucleus. Neurosci Lett. 2008 Jun 20;438(2):150-4. doi: 10.1016/j.neulet.2008.04.030. Epub 2008 Apr 15. https://www.ncbi.nlm.nih.gov/pubmed/?term=Dynorphin-A+immunoreactive+terminals+on+the+neuronal+somata+of+rat+mesencephalic+trigeminal+nucleus</ref> Previous epidemiological studies have shown that a reduced number of residual teeth, incongruous use of prosthetics, and a limited development of the mandibular force are directly related to the development of dementia, further supporting the notion that mastication contributes to maintaining cognitive functions.<ref>Kondo K, Niino M, Shido KDementia. A case-control study of Alzheimer's disease in Japan--significance of life-styles. 1994 Nov-Dec;5(6):314-26.https://www.ncbi.nlm.nih.gov/pubmed/?term=A+case-control+study+of+Alzheimer%27s+disease+in+Japan--significance+of+life-styles</ref>.
In recent years, mastication has been a topic of discussion about the maintenance and support effects of cognitive performance. An elegant study performed through <sub>f</sub>MR and positron emission tomography (PET) has shown that mastication leads to an increase in cortical blood flow and activates the additional somatosensory cortex, motor motor and insular, as well as the striatum, the thalamus, and the cerebellum. Mastication right before performing a cognitive task increases oxygen levels in the blood (BOLD of the fMRI signal) in the prefrontal cortex and the hippocampus, important structures involved in learning and memory, thereby improving the performance task..<ref>Yamada K, Park H, Sato S, Onozuka M, Kubo K, Yamamoto T., Dynorphin-A immunoreactive terminals on the neuronal somata of rat mesencephalic trigeminalnucleus. Neurosci Lett. 2008 Jun 20;438(2):150-4. doi: 10.1016/j.neulet.2008.04.030. Epub 2008 Apr 15. https://www.ncbi.nlm.nih.gov/pubmed/?term=Dynorphin-A+immunoreactive+terminals+on+the+neuronal+somata+of+rat+mesencephalic+trigeminal+nucleus</ref> Previous epidemiological studies have shown that a reduced number of residual teeth, incongruous use of prosthetics, and a limited development of the mandibular force are directly related to the development of dementia, further supporting the notion that mastication contributes to maintaining cognitive functions.<ref>Kondo K, Niino M, Shido KDementia. A case-control study of Alzheimer's disease in Japan--significance of life-styles. 1994 Nov-Dec;5(6):314-26.https://www.ncbi.nlm.nih.gov/pubmed/?term=A+case-control+study+of+Alzheimer%27s+disease+in+Japan--significance+of+life-styles</ref>.
In support of this notion, SAMP8 mice with learning deficits show a marked increase in the plasma levels of corticosterone <ref name=":2" /> and subregulation of GR and GRmRNA of the hippocampus. The occlusal disharmony also affects catecholaminergic activity. Alternating the closure of the bite by inserting an acrylic bite-plane on the lower incisors leads to an increase in levels of dopamine and noradrenaline in the hypothalamus and the frontal cortex<ref name=":3" /><ref>Gomez, F.M., et al., ''Effects of dopaminergic drugs, occlusal disharmonies, and chronic stress on non-functional masticatory activity in the rat, assessed by incisal attrition.'' J Dent Res, 1998. '''77'''(6): p. 1454-64. https://www.ncbi.nlm.nih.gov/pubmed/?term=Effects+of+dopaminergic+drugs%2C+occlusal+disharmonies%2C+and+chronic+stress+on+non-functional+masticatory+activity+in+the+rat%2C+assessed+by+incisal+attrition.+J+Dent+Res%2C+1998</ref>,and decreases in thyroxinaydroxylase, GTP cyclohydrochloride, and immunoreactive serotonin in the cerebral cortex and the caudate nucleus, in the nigra substance, in the locus ceruleus, and in the dorsal raphe nucleus, which are similar to chronic stress-induced changes..<ref>Feldman, S. and J. Weidenfeld, ''Glucocorticoid receptor antagonists in the hippocampus modify the negative feedback following neural stimuli.'' Brain Res, 1999. 821(1): p. 33-7. https://www.ncbi.nlm.nih.gov/pubmed/?term=Effects+of+dopaminergic+drugs%2C+occlusal+disharmonies%2C+and+chronic+stress+on+non-functional+masticatory+activity+in+the+rat%2C+assessed+by+incisal+attrition.+J+Dent+Res%2C+1998</ref> hese changes in the catecolaminergic and serotonergic systems induced by occlusal disharmonies clearly affect the innervation of the hippocampus. The conditions of increasing the vertical dimension alter neurogenesis and lead to apoptosis in the ippocampal gyrus by decreasing the expression of the ippocampal brain derived from neurotrophic factors: all this could contribute to the changes in observed learning in animals with occlusal disharmony.<ref name=":1" />
In support of this notion, SAMP8 mice with learning deficits show a marked increase in the plasma levels of corticosterone <ref name=":2" /> and subregulation of GR and GRmRNA of the hippocampus. The occlusal disharmony also affects catecholaminergic activity. Alternating the closure of the bite by inserting an acrylic bite-plane on the lower incisors leads to an increase in levels of dopamine and noradrenaline in the hypothalamus and the frontal cortex<ref name=":3" /><ref>Gomez, F.M., et al., ''Effects of dopaminergic drugs, occlusal disharmonies, and chronic stress on non-functional masticatory activity in the rat, assessed by incisal attrition.'' J Dent Res, 1998. '''77'''(6): p. 1454-64. https://www.ncbi.nlm.nih.gov/pubmed/?term=Effects+of+dopaminergic+drugs%2C+occlusal+disharmonies%2C+and+chronic+stress+on+non-functional+masticatory+activity+in+the+rat%2C+assessed+by+incisal+attrition.+J+Dent+Res%2C+1998</ref>,and decreases in thyroxinaydroxylase, GTP cyclohydrochloride, and immunoreactive serotonin in the cerebral cortex and the caudate nucleus, in the nigra substance, in the locus ceruleus, and in the dorsal raphe nucleus, which are similar to chronic stress-induced changes..<ref>Feldman, S. and J. Weidenfeld, ''Glucocorticoid receptor antagonists in the hippocampus modify the negative feedback following neural stimuli.'' Brain Res, 1999. 821(1): p. 33-7. https://www.ncbi.nlm.nih.gov/pubmed/?term=Effects+of+dopaminergic+drugs%2C+occlusal+disharmonies%2C+and+chronic+stress+on+non-functional+masticatory+activity+in+the+rat%2C+assessed+by+incisal+attrition.+J+Dent+Res%2C+1998</ref> hese changes in the catecolaminergic and serotonergic systems induced by occlusal disharmonies clearly affect the innervation of the hippocampus. The conditions of increasing the vertical dimension alter neurogenesis and lead to apoptosis in the ippocampal gyrus by decreasing the expression of the ippocampal brain derived from neurotrophic factors: all this could contribute to the changes in observed learning in animals with occlusal disharmony.<ref name=":1" />
===Mesencefalo and Mastication===
===Brainstem and Mastication===
The brainstem district is a relay area that connects the upper centres of the brain, the cerebellum, and the spinal cord, and provides the main sensory and motor innervation of the face, head, and neck through the cranial nerves. This plays a determining role in regulation of respiration, locomotion, posture, balance, excitement (including intestinal control, bladder, blood pressure, and heart rate). It is responsible for regulating numerous reflexes, including swallowing, coughing, and vomiting. The brainstem is controlled by higher Cerebral Centers from cortical and subcortical regions, including the Basal Ganglia Nuclei and Diencephal, as well as feedback loops from the cerebellum and spinal cord. Neuromodulation can be achieved by the ‘classical’ mode of glutammatergic neurotransmitters and GABA (gamma-amino butyric acid) through a primary excitation and inhibition of the ‘anatomical network’, but can also be achieved through the use of transmitters acting on G-proteins. These neuromodulators include the monoamine (serotonine, noradrenaline, and dopamine) acetylcholine, as also glutamate and GABA. In addition, not only do neuropeptides and purines act as neuromodulators, so do other chemical mediators like Growth Factors which, too can have similar actions..<ref>Mascaro, M.B., et al., ''Forebrain projections to brainstem nuclei involved in the control of mandibular movements in rats.'' Eur J Oral Sci, 2009. 117(6): p. 676-84. https://www.ncbi.nlm.nih.gov/pubmed/?term=Forebrain+projections+to+brainstem+nuclei+involved+in+the+control+of+mandibular+movements+in+rats.+Eur+J+Oral+Sci%2C+2009</ref>
The neural network described above does not end with the only correlation between trigeminal somatosensory centres and other motor areas but also strays into the amigdaloidei processes through a correlation with the trigeminal brainstem area. The amygdala becomes active from fear, playing an important role in the emotional response to life-threatening situations. When lab rats feel threatened, they respond by biting ferociously. The force of the bite is regulated by the motor nuclei of the trigeminal system and trigeminal brainstem Me5. The Me5 transmits proprioceptive signals from the Masticatory muscles and parodontal ligaments to trigeminal nuclei and motors. Central Amygdaloid Nucleus (ACe) projections send connections to the trigeminal motor nucleus and reticular premotor formation and directly to the Me5.
The neural network described above does not end with the only correlation between trigeminal somatosensory centres and other motor areas but also strays into the amigdaloidei processes through a correlation with the trigeminal brainstem area. The amygdala becomes active from fear, playing an important role in the emotional response to life-threatening situations. When lab rats feel threatened, they respond by biting ferociously. The force of the bite is regulated by the motor nuclei of the trigeminal system and trigeminal brainstem Me5. The Me5 transmits proprioceptive signals from the Masticatory muscles and parodontal ligaments to trigeminal nuclei and motors. Central Amygdaloid Nucleus (ACe) projections send connections to the trigeminal motor nucleus and reticular premotor formation and directly to the Me5.