General characteristics and topography of the glands
Although food stays in the mouth for a very short time, this is enough to start digestion. The main work is done by large paired salivary glands. About 30% of the total secretion volume is produced by the parotid glands, and together with the submandibular glands, they provide up to 95% of the secreted saliva. The remaining 5% come from the sublingual and minor mucosal glands.
Each gland specializes in its own type of secretion: the parotid glands secrete a predominantly proteinaceous fluid, while the sublingual and palatine glands secrete a mucous fluid.
Composition and enzymatic activity of saliva
Saliva consists of 99% water. The solid residue (0.5–1.5%) includes electrolytes, trace elements, nitrogenous substances, and proteins.
Among organic substances, the most important role is played by:
- Mucin: a mucous substance that forms the food bolus.
- Parotin: a regulatory peptide that influences protein synthesis, blood calcium levels, and the permeability of histohematic barriers.
More than 50 enzymes have been found in the secretion. The main digestive enzymes break down carbohydrates: $\alpha$-amylase (ptyalin) hydrolyzes complex starch into the disaccharide maltose, and maltase continues the process by breaking down maltose into glucose molecules.
Neurophysiology: efferent innervation
The activity of the salivary glands is strictly controlled by the autonomic nervous system:
- Parasympathetic nervous system
- The centers are located in the superior and inferior salivatory nuclei of the medulla oblongata.
- Impulses travel along the fibers of the chorda tympani and the auriculotemporal nerve.
- The neurotransmitter acetylcholine excites M1 muscarinic receptors.
- Effect: secretion of a large volume of watery (fluid) saliva with a high concentration of electrolytes.
- Sympathetic nervous system
- The centers are located in the lateral horns of the II–IV thoracic segments of the spinal cord.
- Impulses pass through the superior cervical ganglion.
- The neurotransmitter norepinephrine stimulates $\beta_2$-adrenergic receptors.
- Effect: secretion of a small amount of thick, viscous saliva rich in enzymes and mucin.
Reflex regulation and adaptation
The afferent (sensory) link is represented by cranial nerve fibers: trigeminal, facial, glossopharyngeal, and vagus. Signals from oral receptors are transmitted to the CNS, triggering a response.
According to I.I. Laptev, the composition of saliva adapts flexibly to the type of food. This is ensured by a strict sequence of perception: mechanoreceptors are irritated first, then thermoreceptors, and lastly chemoreceptors.
The salivatory process is easily inhibited under the influence of pain, negative emotions, mental stress, during sleep, or in case of dehydration.
Briefly: answers to topical questions
Which specific inorganic ions provide the buffer capacity and electrolyte composition of saliva?
The electrolyte composition of saliva is ensured by the secretion of a complex of specific inorganic cations and anions. Watery saliva with a high concentration of electrolytes is formed upon stimulation of the parasympathetic nervous system.
Components of the electrolyte composition:
- Cations — sodium, potassium, calcium, magnesium.
- Anions — chlorides, bicarbonates, phosphates, sulfates, iodides, bromides, fluorides.
- Trace elements — iron, copper, nickel, and others.
Information regarding which specific ions are responsible for the buffer capacity of saliva is absent in the provided sources.
To memorize the effects of the autonomic system: "Parasympathetic — Plenty (abundant watery saliva), Sympathetic — Scanty (little viscous saliva)".
Class and exam questions
Why might the composition of saliva change in renal failure?
The salivary glands possess an excretory function. In renal impairment, the concentration of urea, ammonia, uric acid, and creatinine in saliva increases compensatorily.
What causes excessive salivation in poisonings and asphyxia?
In asphyxia, a sharp rise in blood carbon dioxide levels directly stimulates the salivatory center. In poisoning (e.g., with heavy metal salts), hypersalivation plays a protective role — the glands attempt to eliminate toxins from the body.
Why is dry food difficult to swallow?
A sufficient volume of saliva is necessary to stimulate receptors on the posterior third of the tongue, the hard and soft palate. Without the formation of a moistened food bolus, the reflex act of swallowing is not triggered.
What else to study on this topic
- Theories of salivation (concepts of C. Ludwig, L. Heidenhain, V.P. Babkin)
- Detailed anatomy of conduction pathways (chorda tympani, otic ganglion, glossopharyngeal nerve)
- Humoral regulation of salivary gland activity by pituitary and adrenal hormones
- Pathophysiology of clinical abnormalities: hyposylia (sialopenia) and sialorrhea (ptyalism)
- The role of kallikrein and the kinin system in regulating the blood supply of the glands
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