Histology Anatomy Of Tooth

Age at eruption 7-B (years)

Upper teeth

—3rd molar (wisdom tooth)

Lower teeth

x if

Names of teeth

Central incisor-Lateral incisor ^^ Canine

Age at eruption (months)

1st molar

2nd molar

Upper teeth

Lower teeth

20-22

Chapter 25 The Digestive System 945

Figure 25.5 The Dentition and Ages at Which the Teeth Erupt. (a) Permanent teeth; (b) deciduous (baby) teeth. Which teeth are absent from a 3-year-old child?

Deciduous Dentition

Figure 25.6 Permanent and Deciduous Teeth in a Child's

Skull. This dissection shows erupted deciduous teeth and, below them and marked with asterisks, the permanent teeth waiting to erupt.

Figure 25.6 Permanent and Deciduous Teeth in a Child's

Skull. This dissection shows erupted deciduous teeth and, below them and marked with asterisks, the permanent teeth waiting to erupt.

Erupting Tooth Histology
Figure 25.7 Medial Section of a Canine Tooth and Its Alveolus. Shows typical anatomy of a tooth and periodontal tissues.

(JIN-jih-vuh), covers the alveolar bone. Regions of a tooth are defined by their relationship to the gingiva: the crown is the portion above the gum, the root is the portion below the gum, embedded in alveolar bone, and the neck is the point where the crown, root, and gum meet. The space between the tooth and gum is the gingival sulcus. The hygiene of this sulcus is especially important to dental health (see insight 25.1).

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946 Part Four Regulation and Maintenance

Most of a tooth consists of hard yellowish tissue called dentin, covered with enamel in the crown and neck and cementum in the root. Dentin and cementum are living connective tissues with cells or cell processes embedded in a calcified matrix. Cells of the cementum (cementocytes) are scattered more or less randomly and occupy tiny cavities similar to the lacunae of bone. Cells of the dentin (odontoblasts) line the pulp cavity and have slender processes that travel through tiny parallel tunnels in the dentin. Enamel is not a tissue but a noncellular secretion produced before the tooth erupts. Damaged dentin and cementum can regenerate, but damaged enamel cannot—it must be artificially repaired.

Internally, a tooth has a dilated pulp cavity in the crown and a narrow root canal in the root. These spaces are occupied by pulp—a mass of loose connective tissue, blood and lymphatic vessels, and nerves. These nerves and vessels enter the tooth through a pore, the apical foramen, at the inferior end of each root canal.

Insight 25.1 Clinical Application

Tooth and Gum Disease

Food leaves a sticky residue on the teeth called plaque, composed mainly of bacteria and sugars. If plaque is not thoroughly removed by brushing and flossing, bacteria accumulate, metabolize the sugars, and release lactic acid and other acids. These acids dissolve the minerals of enamel and dentin, and the bacteria enzymatically digest the collagen and other organic components. The eroded "cavities" of the tooth are known as dental caries.9 If not repaired, caries may fully penetrate the dentin and spread to the pulp cavity. This requires either extraction of the tooth or root canal therapy, in which the pulp is removed and replaced with inert material.

When plaque calcifies on the tooth surface, it is called calculus (tartar). Calculus in the gingival sulcus wedges the tooth and gum apart and allows bacterial invasion of the sulcus. This leads to gingivitis, or gum inflammation. Nearly everyone has gingivitis at some time. In some cases, bacteria spread from the sulcus into the alveolar bone and begin to dissolve it, producing periodontal disease. About 86% of people over age 70 have periodontal disease and many suffer tooth loss as a result. This accounts for 80% to 90% of adult tooth loss.

9caries = rottenness

Mastication

Mastication (chewing) breaks food into pieces small enough to be swallowed and exposes more surface to the action of digestive enzymes. It is the first step in mechanical digestion. Mastication requires little thought because food stimulates receptors that trigger an involuntary chewing reflex. The tongue, buccinator, and orbicularis oris muscles manipulate food and push it between the teeth. The masseter and temporalis muscles produce the up-and-down crushing action of the teeth, and the lateral and medial pterygoid muscles and masseters produce side-to-side grinding action.

Saliva and the Salivary Glands

Saliva moistens the mouth, digests a little starch and fat, cleanses the teeth, inhibits bacterial growth, dissolves molecules so they can stimulate the taste buds, and moistens food and binds particles together to aid in swallowing. It is a hypotonic solution of 97.0% to 99.5% water and the following solutes:

• salivary amylase, an enzyme that begins starch digestion in the mouth;

• lingual lipase, an enzyme that is activated by stomach acid and digests fat after the food is swallowed;

• mucus, which binds and lubricates the food mass and aids in swallowing;

• lysozyme, an enzyme that kills bacteria;

• immunoglobulin A (IgA), an antibody that inhibits bacterial growth; and

• electrolytes, including sodium, potassium, chloride, phosphate, and bicarbonate ions.

Saliva has a pH of 6.8 to 7.0. There are striking differences in pH from one region of the digestive tract to another, with a powerful influence on the activity and deactivation of digestive enzymes. For example, salivary amylase works well at a neutral pH and is deactivated by the low pH of the stomach, whereas lingual lipase does not act in the mouth at all but is activated by the acidity of the stomach. Thus saliva begins to digest starch before the food is swallowed and fat after it is swallowed.

The Salivary Glands

There are two kinds of salivary glands, intrinsic and extrinsic. The intrinsic salivary glands are an indefinite number of small glands dispersed amid the other oral tissues. They include lingual glands in the tongue, labial glands on the inside of the lips, and buccal glands on the inside of the cheeks. They secrete relatively small amounts of saliva at a fairly constant rate whether we are eating or not. This saliva contains lingual lipase and lysozyme and serves to moisten the mouth and inhibit bacterial growth.

The extrinsic salivary glands are three pairs of larger, more discrete organs located outside of the oral mucosa; they communicate with the oral cavity by way of ducts (fig. 25.8):

1. The parotid10 gland is located just beneath the skin anterior to the earlobe. Its duct passes superficially

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Chapter 25 The Digestive System 947

Irritated Sublingual Gland And Gums
Figure 25.8 The Extrinsic Salivary Glands. Part of the mandible has been removed to expose the sublingual gland medial to it.

over the masseter, pierces the buccinator, and opens into the mouth opposite the second upper molar tooth. Mumps is the inflammation and swelling of the parotid gland caused by a virus.

2. The submandibular gland is located halfway along the body of the mandible, medial to its margin, just deep to the mylohyoid muscle. Its duct empties into the mouth at a papilla on the side of the lingual frenulum, near the lower central incisors.

3. The sublingual gland is located in the floor of the mouth. It has multiple ducts that empty into the mouth posterior to the papilla of the submandibular duct.

These are all compound tubuloacinar glands with a treelike arrangement of branching ducts ending in acini (see chapter 5). Some acini have only mucous cells, some have only serous cells, and some have a mixture of both (fig. 25.9). Mucous cells secrete salivary mucus, and serous cells secrete a thinner fluid rich in amylase and electrolytes.

Salivation

The extrinsic salivary glands secrete about 1.0 to 1.5 L of saliva per day. Cells of the acini filter water and electrolytes from the blood capillaries and add amylase, mucin, and lysozyme to it. The ducts slightly modify its electrolyte composition.

Chapter 25 The Digestive System 947

Salivary Acinar Cell
(a)

Figure 25.9 Microscopic Anatomy of the Salivary Glands.

(a) Duct and acini of a generalized salivary gland with a mixture of mucous and serous cells. Serous cells often form crescent-shaped caps called serous demilunes over the ends of mucous acini. (b) Histology of the sublingual salivary gland.

Figure 25.9 Microscopic Anatomy of the Salivary Glands.

(a) Duct and acini of a generalized salivary gland with a mixture of mucous and serous cells. Serous cells often form crescent-shaped caps called serous demilunes over the ends of mucous acini. (b) Histology of the sublingual salivary gland.

Food stimulates tactile, pressure, and taste receptors in the mouth, which transmit signals to a group of saliva-tory nuclei in the medulla oblongata and pons. These nuclei also receive input from higher brain centers, so even the odor, sight, or thought of food stimulates salivation. Irritation of the stomach and esophagus by spicy foods, stomach acid, or toxins also stimulates salivation, perhaps serving to dilute and rinse away the irritants.

The salivatory nuclei send signals to the glands by way of autonomic fibers in the facial and glossopharyngeal nerves. In response to such stimuli as the aroma or taste of food, the parasympathetic nervous system stimulates the glands to produce abundant, thin saliva rich in enzymes. Sympathetic stimulation, by contrast, causes the glands to

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948 Part Four Regulation and Maintenance produce less abundant, thicker saliva with more mucus. This is why the mouth may feel sticky or dry under conditions of stress. Dehydration also reduces salivation because it reduces capillary filtration.

Salivary amylase begins to digest starch as the food is chewed, while the mucus in the saliva binds food particles into a soft, slippery, easily swallowed mass called a bolus. Without mucus, one must drink a much larger volume of fluid to swallow food.

The Pharynx

The pharynx, described in chapter 22, has a deep layer of longitudinally oriented skeletal muscle and a superficial layer of circular skeletal muscle. The circular muscle is divided into superior, middle, and inferior pharyngeal constrictors, which force food downward during swallowing. When food is not being swallowed, the inferior constrictor remains contracted to exclude air from the esophagus.

The Esophagus

The esophagus is a straight muscular tube 25 to 30 cm long (see figs. 25.1 and 25.2). It begins at the level of the cricoid cartilage, inferior to the larynx and dorsal to the trachea. After passing downward through the mediastinum, the esophagus penetrates the diaphragm at an opening called the esophageal hiatus, continues another 3 to 4 cm, and meets the stomach at an opening called the cardiac orifice (named for its proximity to the heart).

The inferior end of the esophagus is more constricted than the rest, forming the lower esophageal sphincter. This is not an anatomical feature—the muscularis externa is no thicker here than it is higher up—but is a physiological constriction that helps close the cardiac orifice. Gastroe-sophageal reflux, the backflow of stomach contents into the esophagus, is normally prevented partly by the tonus of this sphincter but more importantly by constriction of the diaphragm around the lower esophagus. "Heartburn" has nothing to do with the heart, but is the burning sensation produced by acid reflux into the esophagus.

The wall of the esophagus is organized into the tissue layers described earlier, with some regional specializations. The mucosa has a nonkeratinized stratified squamous epithelium. The submucosa contains esophageal glands that secrete lubricating mucus into the lumen. When the esophagus is empty, the mucosa and submucosa are deeply folded into longitudinal ridges, giving the lumen a starlike shape in cross section.

The muscularis externa is composed of skeletal muscle in the upper one-third of the esophagus, a mixture of skeletal and smooth muscle in the middle one-third, and only smooth muscle in the lower one-third. This transition corresponds to a shift from voluntary to involuntary phases of swallowing as a food bolus passes down the esophagus.

Most of the esophagus is in the mediastinum. Here, it is covered with a connective tissue adventitia which merges into the adventitias of the trachea and thoracic aorta. The short segment below the diaphragm is covered by a serosa.

Swallowing

Swallowing, or deglutition (DEE-glu-TISH-un), is a complex action involving over 22 muscles in the mouth, pharynx, and esophagus, coordinated by the swallowing center, a nucleus in the medulla oblongata and pons. This center communicates with muscles of the pharynx and esophagus by way of the trigeminal, facial, glossopharyn-geal, and hypoglossal nerves (cranial nerves V, VII, IX, and XII).

Swallowing occurs in stages called the buccal and pharyngeal-esophageal phases (fig. 25.10). In the buccal phase, the tongue collects food, presses it against the palate to form a bolus, and pushes it back into the oropharynx. Here the bolus stimulates tactile receptors and activates the pharyngeal-esophageal phase. In that phase, three actions block food and drink from reentering the mouth or entering the nasal cavity or larynx: (1) the root of the tongue blocks the oral cavity, (2) the soft palate rises and blocks the nasopharynx, and (3) the infrahyoid muscles pull the larynx up, the epiglottis covers its opening, and the vestibular folds are adducted to close the airway. The food bolus is driven downward by constriction of the upper, then the middle, and finally the lower pharyngeal constrictors. As the bolus slides off the epiglottis into the esophagus, it stretches the esophagus and triggers peristalsis, a wave of muscular contraction that pushes the bolus ahead of it (fig. 25.10d).

Peristalsis is moderated partly by a short reflex through the myenteric nerve plexus. The bolus stimulates stretch receptors that feed into the nerve plexus, which transmits signals to the muscularis externa behind and ahead of the bolus. The circular muscle behind the bolus constricts and pushes it downward. Ahead of the bolus, the circular muscle relaxes while the longitudinal muscle contracts. The latter action pulls the wall of the esophagus slightly upward, which makes the esophagus a little shorter and wider and able to receive the descending food.

When we are standing or sitting upright, most food and liquid drop through the esophagus by gravity faster than the peristaltic wave can catch up to it. Peristalsis, however, propels more solid food pieces and ensures that you can swallow regardless of the body's position—even standing on your head! Liquid normally reaches the stomach in 1 to 2 seconds and a food bolus in 4 to 8 seconds. As a bolus reaches the lower end of the esophagus, the lower esophageal sphincter relaxes to let it pass into the stomach.

For a review of the anatomy up to this point, see table 25.1.

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Chapter 25 The Digestive System 949

Food Boluses

Bolus of food

Pharynx

Tongue

Epiglottis Glottis

Trachea

Bolus of food

Pharynx

Tongue

Epiglottis Glottis

Trachea

Soft palate Uvula

Epiglottis

Esophagus

Soft palate Uvula

Epiglottis

Esophagus

Esophagus Constriction

Constriction-

Peristaltic wave

Bolus

Relaxation Shortening-

Constriction-

Peristaltic wave

Relaxation Shortening-

Block Glottis

Stomach

Cardiac orifice

Figure 25.10 Swallowing. (a) The tongue compresses food against the palate to form a bolus. (b) The bolus passes into the esophagus while the tongue blocks the oral cavity, the soft palate blocks the nasal cavity, and the epiglottis blocks the larynx. (c) The superior end of the esophagus constricts as the bolus passes downward. (d) A peristaltic wave moves the bolus down the esophagus. The esophagus constricts behind the bolus while it dilates and shortens in front of the bolus. (e) The lower esophageal sphincter relaxes to admit the bolus to the stomach. (f) X ray of esophagus showing a peristaltic constriction above a bolus of ingested material.

What actions prevent the pharynx from forcing food back into the mouth or nose?

Stomach

Cardiac orifice

Upper esophagus

Peristaltic constriction

Bolus of ingested matter passing down esophagus

Figure 25.10 Swallowing. (a) The tongue compresses food against the palate to form a bolus. (b) The bolus passes into the esophagus while the tongue blocks the oral cavity, the soft palate blocks the nasal cavity, and the epiglottis blocks the larynx. (c) The superior end of the esophagus constricts as the bolus passes downward. (d) A peristaltic wave moves the bolus down the esophagus. The esophagus constricts behind the bolus while it dilates and shortens in front of the bolus. (e) The lower esophageal sphincter relaxes to admit the bolus to the stomach. (f) X ray of esophagus showing a peristaltic constriction above a bolus of ingested material.

What actions prevent the pharynx from forcing food back into the mouth or nose?

Before You Go On

Answer the following questions to test your understanding of the preceding section:

5. List as many functions of the tongue as you can.

6. Imagine a line from the mandibular bone to the root canal of a tooth. Name the tissues, in order, through which this line would pass.

7. What is the difference in function and location between intrinsic and extrinsic salivary glands? Name the extrinsic salivary glands and describe their locations.

8. Describe the muscularis externa of the esophagus and its action in peristalsis.

9. Describe the mechanisms that prevent food from entering the nasal cavity and larynx during swallowing.

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