Human Anatomy and Physiology - I. V. Gayvoronsky 2011

Anatomy and Physiology of the Digestive System
Oral Cavity

Structure. The Digestive System begins with the Oral Cavity, cavitas oris. It consists of two parts: the oral vestibule and the oral cavity proper.

The oral vestibule, vestibulum oris, is a slit-like space situated between the Lips and Cheeks externally, and the Teeth and Gums internally. The excretory duct of the parotid gland opens into the oral vestibule. Its orifice is located on the buccal mucosa at the level of the second upper molar.

Food enters the oral cavity through the oral fissure, which is bounded by the upper and lower lips. The thickness of the lips and cheeks contains facial Muscles. Their outer surface is covered by Skin, and their inner surface by mucous membrane. The latter is lined with stratified squamous non-keratinized epithelium and contains numerous small Salivary Glands.

The mucous membrane transitions from the inner surface of the lips and cheeks onto the gums. Along the midline, it forms the frenula of the upper and lower lips (Fig. 7.3). The gums, gingivae, are the mucous membrane covering the alveolar processes of the jaws.

The oral cavity proper, cavitas oris propria, has a superior wall and a floor. Through the fauces, it communicates with the Pharynx.

The superior wall is formed by the Hard and Soft palate, which separates the oral cavity from the Nasal cavity. The hard palate is a bony palate (formed by the processes of the Maxilla and Palatine bone) covered with mucous membrane. The soft palate is a continuation of the hard palate. Its foundation consists of skeletal (striated) muscles. The anterior part of the soft palate lies in an almost horizontal plane, while the posterior part—the palatine velum—hangs downward and ends in the uvula. During swallowing, the soft palate elevates, preventing the food bolus from passing from the oropharynx into the nasopharynx and nasal cavity.

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Fig. 7.3. Oral cavity:

1 — lower lip; 2 — gingiva of the Mandible; 3 — mandibular teeth; 4 — palatine tonsil; 5 — cheek (dissected); 6 — palatopharyngeal arch; 7 — palatoglossal arch; 8 — soft palate; 9 — hard palate; 10 — maxillary teeth; 11 — gingiva of the maxilla; 12 — frenulum of the upper lip; 13 — upper lip; 14 — palatine raphe; 15 — uvula; 16 — fauces; 17 — Tongue; 18 — frenulum of the lower lip

Two pairs of arches extend laterally and inferiorly from the soft palate: the palatoglossal (anterior) and palatopharyngeal (posterior) arches. These arches are folds of mucous membrane enclosing Muscles of the same name. Between them on each side is a recess containing the palatine tonsil, tonsilla palatina. The palate is lined with stratified squamous non-keratinized epithelium on the oral side, and with ciliated epithelium on the nasal side.

The floor of the oral cavity is formed by the Neck Muscles located above the Hyoid bone. They are lined from the inside by mucous membrane.

The teeth and tongue are located within the oral cavity. The ducts of the salivary glands also open into it. Food remains in this section for an average of 10–20 seconds.

Teeth. Teeth, dentes, are located in the alveolar sockets of the lower and upper jaws. Based on the time of appearance, deciduous and permanent teeth are distinguished. In a child, deciduous teeth begin to erupt at 6–7 months of age. By the end of the 1st year of life, their number reaches 8 (upper and lower incisors). At 2 years of age, a child has 20 deciduous teeth. From 3 to 7 years of age, this number remains virtually unchanged. Between 6–7 years, the gradual replacement of deciduous teeth with permanent ones begins. This process is completed by 13–15 years of age. From 17 to 25 years, the so-called wisdom teeth (third molars) appear. An adult has 32 permanent teeth.

Each tooth consists of a crown, a neck, and a ROOT (Fig. 7.4). The tooth crown rises above the gum. The neck is the constricted region located at the junction between the crown and the root. The tooth root is situated within the alveolar socket of the jaw. It is connected to the socket by Connective Tissue known as the periodontium.

The crown is externally covered by enamel, which is the hardest tissue in the body. Near the apex of the crown, its thickness can reach 3.5 mm. Enamel consists of 96–97% inorganic salts containing elements such as calcium, phosphorus, fluorine, and carbonates. Cementum externally covers the neck and root of the tooth. Its composition includes 70% inorganic and 30% organic substances. The main substance of the tooth, dentin, forms both the crown and the root. It is similar in Structure and Chemical composition to bone. Enamel, dentin, and cementum are hard Tissues. In the center of the tooth—within the dentin—There is a pulp cavity containing the dental pulp. The pulp consists of loose connective tissue, Blood Vessels, and nerves that nourish and innervate the tooth. Vessels and nerves enter the tooth through the apical foramen at the root apex.

Fig. 7.4. Tooth structure:

1 — crown; 2 — neck; 3 — root; 4 — enamel; 5 — dentin; 6 — gingiva; 7 — pulp; 8 — cementum

Fig. 7.5. Permanent human teeth:

I, II — incisors; III — canines; IV, V — premolars; VI, VII, VIII — molars; 1 — crown; 2 — neck; 3 — root

Based on their Location and function, an adult has four types of teeth: incisors, canines, premolars, and molars (Fig. 7.5). There are a total of 8 incisors, 4 in each jaw. They are located anteriorly and feature a flattened crown with a cutting upper edge. Canines are positioned posterior to the incisors, one on each side. The crown of the canines is conical and pointed at the tip. Incisors and canines have a single root and function to grasp food. Premolars are located behind the canines, totaling 8. They have a single root. The crown of a premolar is flattened on the masticatory surface and forms two conical cusps. Molars possess multiple roots and an extensive, tuberous masticatory surface. Mandibular molars have two roots, while maxillary molars have three. Premolars and molars perform the actual function of mastication (chewing). The pressure that can be generated between their masticatory surfaces reaches 100 kg/cm².

The numerical notation of the number of teeth is called the dental formula. For an adult, it is as follows:

This means that on one half of each human jaw, there are sequentially 2 incisors, 1 canine, 2 premolars, and 3 molars.

The formula for deciduous teeth is somewhat different:

A child has a total of 5 teeth in each half of the jaw, bringing the overall count to 20, as premolars are absent and there are 2 molars on each side.

Teeth function to grasp and grind food, as well as to contribute to clear and articulate speech.

The Tongue. With the jaws closed, the tongue, lingua (Greek: glossus), completely fills the oral cavity. It is a mucomuscular organ attached to the floor of the Mouth. The structure of the tongue comprises the apex, body, and root, which fuses with the hyoid bone. On the upper surface, or dorsum of the tongue, a longitudinal sulcus runs along the midline. The unpaired lingual tonsil, tonsilla lingualis, is located at the root of the tongue.

The tongue is covered by a mucous membrane, the upper surface of which bears lingual papillae that give it a rough and velvety texture. These papillae contain numerous taste, Temperature, and tactile receptors. There are five types of papillae: filiform, conical, foliate, fungiform, and vallate. Filiform and conical papillae are responsible for general sensitivity, whereas fungiform, vallate, and foliate papillae mediate the sense of taste.

Fig. 7.6. Muscles of the tongue:

1 — root of the tongue; 2 — body of the tongue; 3 — apex of the tongue; 4 — mandible; 5 — genioglossus Muscle; 6 — inferior longitudinal muscle of the tongue; 7 — mylohyoid muscle; 8 — hyoid bone; 9 — hyoglossus muscle; 10 — styloglossus muscle

Information from the tongue's receptors travels via sensory nerve fibers to the Brainstem. This reflexively activates the salivary glands, Stomach, and Pancreas, and enhances intestinal motility. It should be noted that the SENSE OF SMELL plays a major role in the perception of food flavor; therefore, during a severe cold, taste sensations lose their intensity.

The muscular tissue of the tongue consists of striated fibers. One distinguishes between skeletal (extrinsic) and intrinsic muscles of the tongue (Fig. 7.6). Skeletal muscles move the organ within the oral cavity, while intrinsic muscles alter its shape. Movements of the tongue are voluntary and consciously controlled. The tongue muscles mix the ingested food and participate in the act of swallowing by moving the food bolus through the fauces into the pharynx.

The extrinsic (skeletal) muscles of the tongue include:

✵ the genioglossus muscle (pulls the tongue downward and forward);

✵ the hyoglossus muscle (moves the tongue downward and backward);

✵ the styloglossus muscle (unilateral contraction moves the tongue to the respective side; bilateral contraction pulls the organ backward and upward).

The intrinsic muscles include:

✵ the superior and inferior longitudinal muscles (shorten the tongue and bend it in various directions);

✵ the transverse muscle (narrows the tongue, with upper fibers curling it into a tube);

✵ the vertical muscle (flattens and broadens the tongue).

These muscles are arranged in three mutually perpendicular directions, interweaving with one another and with the fascicles of the extrinsic muscles.

Thus, the tongue Functions to perceive the taste of food, mix it, form the food bolus, and propel it into the pharynx. In addition, it facilitates clear and articulate speech by participating in The production of most sounds.

Salivary Glands. Salivary glands are classified by size into Major and minor. The ducts of three pairs of major salivary glands open into the oral cavity (Fig. 7.7): the parotid, sublingual, and submandibular glands. In addition, numerous minor salivary glands are embedded within the oral mucosa: palatine, labial, lingual, buccal, and molar glands. The major salivary glands produce saliva only during Digestion, whereas the minor glands function continuously at rest, maintaining the moisture of the oral mucosa.

Based on the composition of their secretion, salivary glands are categorized as serous (proteinaceous), mucous, and mixed. The Cytology/practical/97.html">Parotid salivary gland secretes a protein-rich fluid. The palatine and lingual glands are classified as mucous. Mixed salivary glands include the sublingual, submandibular, labial, and buccal glands.

Fig. 7.7. Salivary glands:

1 — mylohyoid muscle; 2 — sublingual salivary gland; 3 — mandible; 4 — duct of sublingual salivary gland; 5 — duct of Submandibular salivary gland; 6 — sublingual caruncle; 7 — tongue; 8 — buccinator muscle; 9 — masseter muscle; 10 — parotid duct; 11 — parotid gland; 12 — sternocleidomastoid muscle; 13 — submandibular salivary gland

The major salivary glands have a fundamentally similar structure and consist of actual glandular tissue—whose Structural and functional unit is the lobule—and excretory ducts.

The parotid gland, glandula parotidea, weighs 20–30 g. It is located inferior to the external acoustic meatus, fills the retromandibular fossa, and partially overlaps the masseter muscle and the ramus of the mandible. Its duct opens into the oral vestibule on the buccal mucosa opposite the second upper molar.

The submandibular gland, glandula submandibularis, is smaller than the parotid gland. It lies medial and slightly inferior to the body of the mandible. The excretory duct of the gland opens beneath the tongue on the sublingual caruncle.

The sublingual gland, glandula sublingualis, has a narrow, elongated shape and is located directly beneath the mucous membrane of the floor of the oral cavity. The duct of the sublingual salivary gland opens at the same site as the excretory duct of the submandibular gland.

Salivary glands produce saliva. Its daily volume can reach 1.5–2.0 L. The COMPOSITION OF THE secreted fluid depends on the type of gland, but on average, saliva entering the oral cavity consists of 99% Water and 1% dry matter. One-third of the dry matter is composed of inorganic ions such as Na+, K+, Ca²+, Cl^-, HCO₃, etc.

Saliva contains a variety of organic substances, the majority of which are Proteins or Protein Complexes. Mucin (0.3% of total saliva) is a mucilaginous protein substance that helps coat the food bolus, facilitating its formation and passage into the pharynx. Lysozyme provides the bactericidal property of saliva, i.e., The ability to destroy Bacteria introduced into the oral cavity with food. Saliva also contains digestive Enzymes, the primary ones being amylase and maltase. Both enzymes belong to the class of carbohydrate-splitting enzymes. Amylase breaks down starch and Glycogen. Maltase splits maltose into two glucose molecules. It should be noted that The breakdown of CARBOHYDRATES in the oral cavity is far from complete (reaching only oligomers), with the primary action of digestive enzymes taking place in the Small Intestine. Both enzymes are active in a slightly alkaline medium (the pH of saliva secreted during meals is about 8).

Thus, saliva performs several vital functions to ensure normal digestion: it moistens and dilutes food; facilitates The formation of the food bolus; performs a protective (detoxifying) function; and its constituent enzymes provide the initial breakdown of dietary carbohydrates. Furthermore, the taste of food is perceived by the tongue receptors only when the food is moistened. The lack of salivation due to disease causes a person to lose their sense of taste.

Salivary gland secretion is regulated primarily by The Nervous System. Specifically, parasympathetic nerve stimulation leads to enhanced salivation, producing a large volume of watery saliva. Sympathetic nerve stimulation results in a sparse secretion of concentrated saliva. A decrease in saliva production is termed "hyposalivation," and an increase is termed "hypersalivation."

Thus, A number of processes take place within the oral cavity:

1) ingestion of food;

2) mechanical Processing of food (grinding);

3) moistening of food with saliva;

4) tasting of food;

5) bactericidal Treatment of food (salivary lysozyme);

Fig. 7.8. Fistular method for obtaining saliva

6) partial digestion of carbohydrates (due to enzymes present in saliva);

7) Formation of the food bolus;

8) swallowing;

9) passage of air in case of nasal breathing insufficiency;

10) phonation (vocal timbre largely depends on THE POSITION OF the tongue, lips, cheeks, and soft palate).

The Role of I. P. Pavlov in studying the mechanisms of salivation. Salivary mechanisms were first investigated by the great Russian physiologist I. P. Pavlov using the fistular method. A fistula is a surgical connection between a gland duct and the external environment (Fig. 7.8). The Essence of the experiment was as follows: the parotid duct was brought out onto the outer surface of the dog's cheek, allowing saliva to be collected in a test tube rather than entering the oral cavity. In the absence of food, secretion was virtually absent. When food entered the Oral Cavity and stimulated its receptors, salivation began. This phenomenon was termed the unconditioned salivary reflex. The appearance of saliva prior to feeding in response to the smell or sight of food, or other stimuli mediated by Sensory Organs other than oral receptors (such as Vision or Olfaction), was termed the conditioned salivary reflex. It should be noted that the composition and volume of saliva vary depending on the type of food ingested. For example, if loose dry substances or strongly acidic substances (such as lemon) enter the oral cavity, a large volume of saliva is secreted, accompanied by a decrease in the proportion of organic substances and an increase in the watery component. Conversely, when the diet is predominantly carbohydrate-rich, the amylase content in the salivary secretion increases.

It was established that the salivation center is located in the Medulla Oblongata. Information reaches the salivary glands via nerves consisting of parasympathetic and sympathetic fibers. I. P. Pavlov identified two Main phases of salivary secretion:

1) the cephalic phase — the secretion of "appetite saliva" occurs upon the sight, smell, or thought of food; the Qualitative and quantitative composition of saliva during this phase does not depend on the type and quantity of food;

2) oral phase: saliva is secreted during the chewing of food; the qualitative and quantitative composition of saliva during this phase directly depends on the type and amount of food.