11.1. Internal Structure

Under the cover of enamel, the bulk of the crown is composed of dentin, which surrounds and protects the pulp chamber of the tooth. The root lacks enamel and is instead covered by a thin layer of a hard tissue called cementum. Dentin composes most of the root forming root canals that contain pulp tissue. A tooth can have a single or multiple roots. The tooth is supported at the root by an attachment apparatus called periodontium. This attachment apparatus connects the tooth to the alveolus of the jaw bone. The entire tooth is enclosed within the jaw bone before the tooth erupts, but after eruption the crown is entirely visible in an anatomically normal and healthy tooth.

Enamel

This is the hardest and most highly mineralized substance of the body. It is one of the four major tissues which make up the tooth, along with dentin, cementum, and dental pulp. It is normally visible and it is supported by underlying dentin. The composition of enamel is mostly mineral (96%), with water and organic material comprising the rest. The normal color of enamel varies from light yellow to grayish white. Enamel is semitranslucent, therefore the color of dentin and any restorative dental material deep to the enamel strongly affects the appearance of a tooth. Enamel varies in thickness over the surface of the tooth and is often thickest at the cusp, up to 2.5mm, and thinnest at its border at the neck of the tooth.

Enamel's primary mineral is hydroxyapatite, which is a form of calcium phosphate. The large amount of minerals in enamel accounts not only for its strength but also for its brittleness. Dentin is less mineralized and less brittle than enamel. Unlike dentin and bone, enamel does not contain collagen fibers. The mains proteins involved in the development of enamel are ameloblastins, amelogenins, enamelins and tuftelins, which help to form a supporting framework. Ameloblasts secrete the enamel layer during tooth development but the mature tissue is acellular (no cells).

Dentin

Dentin is the substance between enamel or cementum and the pulp chamber. It is secreted by the odontoblasts of the dental pulp. This porous, yellow-hued material is made up of 70% inorganic materials, 20% organic materials, and 10% water by weight. Because it is softer than enamel, it decays more rapidly if exposed, but it is normally completely covered in enamel or cementum.

Dentin is a mineralized connective tissue with an organic matrix of collagenous proteins. Dentin has microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp cavity to the exterior cementum or enamel border. The diameter of these tubules ranges from 2.5 μm near the pulp, to 1.2 μm in the midportion, and 900 nm near the dentino-enamel junction. Although they may have tiny side-branches, the tubules do not intersect with each other. Their length is dictated by the radius of the tooth. Although the dentin is acellular, the dentinal tubules allow odontoblasts in the pulp cavity to maintain the mineral composition of the dentin through long cytoplasmatic extensions.

Cementum

Cementum is a specialized bone-like substance covering the root of a tooth. It is approximately 45% inorganic material (mainly hydroxyapatite), 33% organic material (mainly collagen) and 22% water. Cementum is excreted by cementoblasts within the root of the tooth and is thickest at the root apex. Its coloration is yellowish and it is softer than dentin and enamel. The main role of cementum is to serve as a medium by which the periodontal ligaments can attach to the tooth for stability. The cementum is acellular (lacks cells) at the cementoenamel junction and cervical portions of the root. The more permeable form of cementum, cellular cementum, covers about one third of the root near the apex.

Dental Pulp

The dental pulp is the central part of the tooth filled with soft connective tissue. This tissue contains blood vessels and nerves that enter the tooth from a hole at the apex of the root (apical foramen). Along the border between the dentin and the pulp are odontoblasts, which form and maintain the dentin. Other cells in the pulp include fibroblasts, preodontoblasts, macrophages and T lymphocytes.

From the central pulp chamber, a pulp horn extends into each root and forms a radicular canal that ends at the apical foramen. Through this foramen, the pulp receives a dental artery and emits a vein. It also receives a dental nerve with sensory but no motor fibers. Arteries and neural plexuses in the jaw bones originate the tiny vessels and nerves that supply each tooth.

Vascularization

Maxillary

The branches of the external carotid artery include the maxillary artery. This artery forms many branches that irrigate muscles of the head. Most anteriorly, the maxillary artery forms the infraorbital artery. The anterior superior alveolar artery branches off from the infraorbital artery and irrigates the upper incisors and canines. The posterior superior alveolar artery (posterior dental artery) originates directly from the maxillary artery and divides into numerous branches to supply the molar and premolar teeth.

Mandibular

Mandibular teeth are irrigated by another branch of the maxillary artery, the inferior alveolar artery (inferior dental artery). It descends with the inferior alveolar nerve into the mandibular foramen on the medial surface of the ramus of the mandible. It runs along the mandibular canal inside the bone, accompanied by the nerve. At the first premolar tooth it divides into two branches, incisor and mental. The incisor branch is continued forward beneath the incisor teeth as far as the middle line, where it anastomoses (fuses) with the artery coming from the opposite side.

The inferior alveolar artery and its incisor branch during their course through the substance of the bone originate a series of branches which correspond in number to the roots of the teeth: these enter the minute apertures at the extremities of the roots and supply the pulp of the teeth.

Innervation

Our teeth receive sensory innervation from cranial nerve V, the trigeminal nerve. It has three major branches: ophthalmic, maxillary and mandibular that innervate the face. The maxillary nerve leaves the cranium through the foramen rotundum whereas the mandibular nerve extends through the foramen ovale.

Maxillary

The maxillary branch of the trigeminal nerve receives the sensory input from our teeth. Branches of the maxillary nerve called posterior superior alveolar nerve, middle superior alveolar nerve, and anterior superior alveolar nerve form the superior dental plexus which supplies the upper jaw. These branches are somewhat variable in structure: the middle superior alveolar nerve is sometimes missing, whereas the posterior superior alveolar nerve is most frequently split into a pair of nerves.

Mandibular

The mandibular teeth are innervated by the mandibular branch of the trigeminal nerve. The mandibular nerve produces several branches, including the inferior alveolar nerve, which enters the mandibular foramen.

While in the mandibular canal within the mandible, it supplies the lower teeth (molars and second premolar) with sensory branches that form the inferior dental plexus. Anteriorly, the inferior alveolar nerve originates the mental nerve which exits the mandible via the mental foramen and supplies sensory branches to the chin and lower lip. The inferior alveolar nerve continues anteriorly as the mandibular incisive nerve to innervate the mandibular canines and incisors.

Sensory reception

Pain

The most abundant sensory receptors in a tooth are nociceptors, a class of receptors specialized in conveying pain stimuli. The dental nociceptors are classified as A-fibers or C-fibers. A sharp pain is typically originated at A-fibers reacting to cold or mechanical stimuli, such as cold drinks or toothbrushing. C-fibers are usually involved in long-lasting dull pain produced by inflammation of the pulp. A-fibers tend to be thick and myelinated, which are neuronal traits associated with fast stimulus conduction. C-fibers are usually thin and non-myelinated.

Dental nociceptors are distributed in the entire pulp. They are found near the odontoblasts and they can extend into the dentinal tubules. These nociceptors are found at highest density and they extend furthest into the dentine at the occlusal portion of the dentin-pulp border.

Mechanical stimuli

Mechanoreception inside the teeth seems to be based on responses of odontoblasts in the pulp. When the enamel cover of the tooth is removed or damaged, the peripheral end of the dentinal tubules may be opened, and stimuli in the surrounding medium may produce fluid displacement in the tubules. Stretch-activated potassium channels on the cellular membrane of the odontoblasts are activated, leading the cell to release chemicals that activate adjacent nociceptors.

Outside the tooth, mechanoreceptors are present on the periodontal ligaments. These are collaginous fibers that anchor the root of the tooth to the surrounding alveolar bone. A type of stretch receptor called Ruffini receptor is found on the periodontal ligaments. It has finger-like projections which extend into the surrounding tissue to detect the deformation of collagen fibers. Its role is to respond to accidental movement of the tooth in its socket, such as when you bite into a hard object in your food, and activate a nociceptive reflex that causes immediate relaxation of the muscles of mastication.

Temperature

In addition to acting as mechanoreceptors, the odontoblasts of the pulp can also respond to changes in temperature. Like the thermoreceptors in our skin, odontoblasts have transient receptor potential (TRP) ion channels on their membranes. Many types of TRPs have been described, and each one opens at a certain temperature range. This allows odontoblasts to respond to cold or hot stimulation of the tooth (especially if the enamel cover is removed) and convey the stimulus to the surrounding sensory neurons (nociceptors).

Reflexes

The stimuli transduced by the sensory receptors of the teeth, together with stimuli originating in stretch receptors within the muscles of mastication, are transmitted to the CNS by the trigeminal nerve. They are processed in the spinal trigeminal nucleus at the medulla oblongata. The reticular formation then transmits them to the thalamus, from where they are conveyed to the primary sensory cortex, allowing us to become aware of the sensation in the tooth.

Our neural system also uses these stimuli to direct immediate reflex responses that do not require our awareness. One class of reflexes involves the use of feedback from the muscle spindles (stretch detectors) in the muscles of mastication to detect the position of the mandible and adjust the contraction of each muscle for stable and effective chewing. This myotatic response goes beyond simply stimulating the muscle when it is stretched. The response is dependent on chewing task, phase of movement, and site of stimulation.

Another type of reflex is triggered by Ruffini receptors in the periodontal ligaments, which signal traumatic displacement of a tooth. When activated, this reflex causes immediate relaxation of the main muscles of mastication. This is thought to be a protective response designed to prevent tooth damage by further biting into a hard object.

Summary

Human teeth have a live dental pulp surrounded by dentin and covered by enamel at the crown and by cementum at the root. They irrigated by the maxillary artery and innervated by the trigeminal nerve, with sensory receptors for pain, vibration and temperature.

Key terms

Enamel, dentin, cementum, dental pulp, hydroxyapatite, cementoblast, periodontal ligament, odontoblast.

Figure credits

Figure 1 by KDS4444 - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=52624520

Figure 2 by Mikael Häggström - File:Gray510.png, Public Domain, https://commons.wikimedia.org/w/index.php?curid=9879909

Figure 3a by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 778, Public Domain, https://commons.wikimedia.org/w/index.php?curid=526568

Figure 3b by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 779, Public Domain, https://commons.wikimedia.org/w/index.php?curid=541624

Figure 4 by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 781, Public Domain, https://commons.wikimedia.org/w/index.php?curid=541626