Salivation

SALIVATION. Due to the continuous activity of the small salivary glands located in the mucous membrane of the oral cavity, the latter remains constantly moist. The major glands do not participate in this constant supply of saliva; their activity is only detected in the presence of special irritants that act not only on the specially adapted receptive surface, which is the oral cavity with its taste apparatus, but under certain conditions also on some of the sense organs: sight, hearing, and smell. In the latter case, the reaction occurs if these irritations are associated with representations of food, as for example, pictures of table settings, prepared dishes, noises during food preparation procedures, as well as its smell. Since the main data on the study of salivary gland reactions to various irritations were obtained from experiments on animals and mainly on dogs, it would be imprudent to transfer them entirely to humans, although in those cases where this proved possible, corresponding observations on people have confirmed the similarity of the picture of these reactions, despite some difference in quantitative and qualitative character. Science is indebted to the laboratory of Academician I. P. Pavlov for the detailed study of the work of the digestive glands and in particular of the salivary apparatus. From experiments mainly of this laboratory, it was clarified that not every mechanical irritation of the oral mucosa causes S, for example, large stones placed in the oral cavity do not cause S, while sand, cracker or meat powder give a clear reaction. Carbonated drinks also cause saliva secretion, which is explained by the release of numerous gas bubbles acting like small solid particles. The reaction proceeds quantitatively more strongly, the smaller the particles, and consequently the more of them fit on a given area of the receptive surface. These data allow us to assume the presence in the oral mucosa of special point apparatuses that perceive these irritations. The excitability to the same mechanical irritant is not the same in different areas of the mucous membrane: while a slight effect on the root of the tongue is sufficient to manifest the reaction, the mucous membrane of the cheeks and especially of the lower lip turns out to be very insensitive. Excitability to chemical irritations also depends both on the chemical nature of the agent and on the area of the mucous membrane on which it acts. Temperature effects on the oral mucosa were found effective in dogs starting from 55°, in the area of the root of the tongue, lower lip, and lateral parts of the upper, and only a temperature of 60° was an effective irritant on the entire oral surface (area of the cheeks not always). 81» The irritants just mentioned can act directly only on the small salivary glands embedded in the receptive surface itself, but, as is known, the role of these glands is so small that the secretion of salivary glands as a factor of digestion is associated with the concept of the activity of only the major glands. The latter are so far from the receptive surface that for their irritation they need a special transmission mechanism. The role of such a transmitter, as in other physiological processes, is taken over by the nervous system; if in this case there is a simple reflex act, then when receiving impulses from the sense organs, the matter is more complicated, since the cerebral cortex is also included in this transmission mechanism. In the transmission of centripetal irritations, the nerves of the parasympathetic system participate: the lingual nerve (n. lingualis), the glossopharyngeal (n. glosso-pharyngeus), the superior pharyngeal branch of the n. vagus, as well as the sensory fibers of the trigeminal nerve coming from the Gasserian ganglion. It should be noted that artificial irritation of other centripetal nerves of the body is accompanied by saliva secretion (n. ulnaris, ischiadicus, etc.), therefore the cutting of fibers transmitting impulses from the oral cavity does not yet guarantee the complete cessation of S, which may manifest itself when other sensory nerves are irritated. Otherwise, if the centrifugal paths to the major salivary glands are cut. In this case, no irritation will cause S, of course, except for artificial irritation of the peripheral ends of the cut nerves. Each salivary gland is supplied with centrifugal fibers of the cranial and sympathetic nerves. For the sublingual and submandibular glands, the cranial nerve is the chorda tympani, which contains not only centrifugal secretory but also centripetal taste fibers. On their way, the fibers of the chorda tympani enter the sublingual and submandibular nodes (gngl. sublingualis and gngl. submaxillaris), from which the terminal branches go to the corresponding glands. For the parotid gland, the secretory and vasodilator fibers originate from the IX pair (n. glosso-pharyngeus) and through the Jacobson's nerve enter the gngl. oticum, from where through the thin branch (n. auriculo-temporalis) of the trigeminal nerve they reach the gland. The mentioned nerve nodes are obviously switching points (Umschal-tungsneurone) of impulses going from preganglionic to postganglionic nerve fibers, since after smearing the submandibular and sublingual nodes with nicotine, irritation of the chorda tympani proves ineffective. Another secretory and at the same time vasoconstrictor nerve for all salivary glands is the sympathetic, the fibers of which from the superior cervical node are directed to the a. carotis and, following its branches, reach the salivary glands. The reactions caused by irritation of the cranial and sympathetic nerves are not the same. While irritation of the cranial nerves is accompanied by abundant secretion of liquid and poor in organic substances saliva, upon irritation of the sympathetic nerve this secretion is insignificant, the saliva is thick and significantly saturated with organic substances. If the chorda tympani is cut in a dog, the submandibular gland begins to secrete saliva 24 hours after the operation. This secretion, meager at first, gradually increases, reaching a maximum by the 6-8th day and then, gradually subsiding, ceases on the 6-8th week. This phenomenon, first described by C. Bernard under the name 'paralytic secretion', has not yet found a completely satisfactory explanation. According to Langley, the reason lies in the increased excitability of the peripheral nerve elements of the gland after the preganglionic fibers are cut. In this case, the irritant is the carbon dioxide of the blood, which under normal conditions turns out to be an insufficiently active agent. The central innervation center of S. must be recognized as the medulla oblongata, where the nuclei of nn. glosso-pharyngei and facialis are located. This is proved both by the preservation of the reflex after the medulla oblongata is separated from the cerebrum, and by the positive effect when it is artificially irritated. The relationship of the cerebral cortex to the act of S. was established long ago from experiments of artificial irritation of some of its areas, as well as on the basis of the well-known fact of saliva secretion upon irritation of the already mentioned sense organs - irritations associated with the representation of food. When comparing S. as a reaction from the oral cavity with that from irritation of sense organs, it turned out that both processes can be brought under a common reflex scheme with the only difference that in the first case the reflex is carried out without the participation of the cerebral cortex, whereas to obtain a reaction from the receptive surfaces of sense organs this participation is necessary. The reflex from the oral cavity is innate, while the second is acquired through life experience; as constant as the first is, the second requires strictly defined conditions. In view of these differences, the reflex from the oral cavity should be classified according to the terminology of I. P. Pavlov as unconditional, while the reaction to irritation of sense organs as conditional reflexes. For observations of S. in humans, cases of salivary fistulas were previously used. At present, the Lashley capsule (see Lashley capsule) has been introduced into practice, which allows easy observation of the secretion of the parotid gland in any person.