Section 3-3. Neural transmission from teeth to brain

It was disclosed that teeth exerted decisive influences on secretion of saliva. “However, wait a minute. Teeth are located close to salivary gland but they are not directly connected each other. By what kind of mechanism do teeth affect secretion of saliva?” This question naturally comes to our mind. Teeth are located close to salivary gland but they are not connected mutually. How is the information transmitted between them? To get the answer to this question, I started survey of the existing documents related to teeth’s function how to transmit information. Please look at Figure 13.
It has been well known that teeth possess periodontal membrane mechanical receptors which act as an antenna for transmission of information. This antenna is expressed as the result of expansion of nerves from two kinds of neuronal nuclei including trigeminal nuclei and mesencephalic nucleus of trigeminal nerve and is extensively distributed as the nerve terminal within periodontal membrane of dental root. We sometimes say “Crunchy texture” when talking about food. The signals indicative of crunchy texture are transmitted from this antenna to brain where such signals are processed to feel the crunchy texture. It is known that this nerve proceeds to somatosensory area of cervical cortex via cerebral limbic system. As regards correlation between teeth and somatosensory area of cervical cortex in terms of such a transmission, “One tooth innervation system” but not “Multi-teeth innervation system” plays a major role in which the primary afferent nerves act as the identification unit. One cerebral cell corresponds to one tooth. Accordingly, this mechanism permits independent transmission of different information by individual teeth. Loss of even one tooth is accompanied by disappearance of the corresponding brain cell; as the natural consequence, information transmission caused by stimulation with chewing and warm-cold sensation on this lost tooth would become absent. The teeth assembly comprising 32 teeth is collapsed with the totality as the information transmission organ being altered and in vain.
On the other hand, how is the nervous innervation in salivary gland? Salivary gland includes three major salivary glands such as parotid gland located under the ear, submaxillary gland under jaw, and sublingual gland, together with minor salivary gland located under mucous membrane in oral cavity. When the ratio against the total volume of secreted saliva is compared, the saliva from submaxillary gland accounted for almost 70%, followed by 25% from parotid gland. To submaxillary gland and sublingual gland, as well as to parotid gland, parasympathetic nerves such as facial nerve representing parasympathetic nerve and glossopharyngeal nerve invade, respectively, with the salivary nucleus located close to the junctional region between pons and medulla being employed as the center. Simultaneously, involved are sympathetic nerve fibers of which upper center included hypothalamic area, with cerebral cortex limbic lobe being recognized as the lower center. Accordingly, these parts are under control of both sympathetic and parasympathetic nerves. Both sympathetic and parasympathetic nerves stimulate secretion of saliva but do not exert any antagonistic effects. However, their respective roles seem different; in more details, animal experiments on dogs indicated that parasympathetic nerve stimulation is mainly associated with secretion of juicy and serous saliva while sympathetic nerve stimulation mainly contributes to secretion of viscous saliva containing much of solid components. Interestingly, parasympathetic nerve stimulation induces salivary secretion from any of glands such as parotid gland and submaxillary gland whereas stimulation of sympathetic nerves causes secretion from submaxillary gland without secretion from parotid gland.

Figure 13
Information transmission pathways for chewing stimulation from teeth to the brain
Around the dental root, the antenna from the cervical nerve called as periodontal membrane mechanical receptors is set up. The stimulation induced by bites is caught by this antenna, and transmitted via trigeminal nerve to the somatosensory area of cervical cortex.(Keijiro Karita, et al: Cited from Reference 23)

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