| SECTION IV | NERVES OF THE ORAL CAVITY |
OBJECTIVES
The objectives for this section are to prepare the reader to perform the following:
- List the 12 CNs and briefly describe their function.
- Describe the important branches of the trigeminal nerve and trace the route of each important branch from the brain to the structures that they innervate in the oral cavity.
- Describe the pathway to the oral cavity of the facial nerve and identify the oral structure(s) it innervates.
- Describe the pathway to the oral cavity of the glossopharyngeal nerve and identify the oral structure(s) it innervates.
- Describe the pathway to the oral cavity of the hypoglossal nerve and identify the oral structure(s) it innervates.
There are three types of nerve fibers based on their function: afferent, efferent, and secretory. Afferent [AF er ent] (or sensory) fibers convey impulses (such as feeling, touch, pain, taste) from peripheral organs (like the skin or surface of the tongue) to the central nervous system. (Hint: Afferent: where “a” (as in approach) means sending impulses toward the brain, i.e., from an organ receiving sensory input, so the brain can “feel” it; therefore, these impulses are sensory, related to the senses of feeling, touch, pain, taste, etc.)
Efferent [EF er ent] (or motor) nerve fibers convey impulses from the central nervous system to the peripheral organs, such as to muscle fibers to initiate contraction. They supply the four pairs of muscles of mastication and other muscles in the region of the mouth. (Hint: Efferent: where “e” (as in exit) means sending an impulse from the brain, which could contract a muscle and move a bone in the intended direction (or could increase force on that bone without movement); therefore, these impulses are motor.)
Secretory fibers are specialized efferent nerve fibers that, upon stimulation, can send messages to glands, such as the salivary and lacrimal (tear) glands to produce and secrete saliva or tears.
There are 12 cranial nerves (CNs) that all supply the area of the head. Table 14-2 lists the 12 CNs (indicated by Roman numerals I to XII) that are responsible for the specific functions.1,3,11,27–30
| TABLE 14-2 | The Twelve Cranial Nerves |
|
Nerve No. |
Cranial Nerve |
Function |
|
I |
Olfactory [ol FAK toe ree] |
Smell |
|
II |
Optic |
Sight |
|
III |
Oculomotor [AHK u lo MO tor] |
Orbital muscles for eye movement |
|
IV |
Trochlear [TROK le ar] |
Orbital muscles for eye movement |
|
V |
*Trigeminal [tri JEM i nal] |
Motor: movement of the jaws and muscles of mastication Sensory: sensation of feeling for the face, teeth, and periodontal ligaments and anterior two thirds of the tongue |
|
VI |
Abducens [ab DOO senz] |
Orbital muscles for eye movement |
|
VII |
*Facial |
Motor: to the muscles of facial expression |
|
|
|
Sensory: taste to anterior two thirds of tongue |
|
|
|
Secretory: to submandibular and sublingual glands |
|
VIII |
Auditory (or acoustic) |
Sense of hearing, position, and balance |
|
IX |
*Glossopharyngeal [GLOSS o feh rin JI al] |
Secretory: to parotid gland and pharyngeal movements Sensory: feeling to pharynx and posterior one third of the tongue and taste to posterior one third of the tongue |
|
X |
Vagus [VAY gus] |
Pharyngeal and laryngeal movements: digestive tract |
|
XI |
Spinal accessory |
Neck movements: sternocleidomastoid and trapezius muscles |
|
XII |
*Hypoglossal |
Motor: tongue movement (muscles) |
*The asterisked nerves in bold are most important when discussing the function of the oral cavity. A detailed discussion of these nerves will include the major branches to structures of the mouth, including teeth, periodontal ligaments and alveolar processes, gingiva (gums), the palate and floor of the mouth, and muscles of mastication, of facial expression, and of the tongue (for both muscular action and our sense of taste). Also, a pneumonic that may help you remember the cranial nerves (where the first letter of each word is the same as the first letter of each cranial nerve) is “On Old Olympus’ Towering Top, A Finn and German Viewed Some Hops.”
A. TRIGEMINAL NERVE (FIFTH CN)
When discussing the function of the oral cavity, probably the most important nerve is the trigeminal. The trigeminal nerve or fifth CN is the largest of the CNs and is the major sensory nerve of the face and scalp. It originates in the large semilunar or trigeminal ganglion, a group of nerve cell bodies located on the superior surface of the petrous portion of the temporal bone in a small depression (the semilunar fossa) medial to the foramen ovale. The trigeminal nerve divides into three major divisions (or three nerve branches). (Hint: “tri” in trigeminal refers to the nerve’s three divisions.) Division I (the ophthalmic [ahf THAL mik] nerve) and Division II (the maxillary nerve) are only afferent (sensory). Division III (the mandibular nerve) is both afferent (sensory) and efferent (motor). Its efferent fibers supply the muscles of mastication. This is the only CN with sensory (touch and pain) innervation to the skin of the face, and the divisions or branches are distributed to the face as shown in Figure 14-37.
FIGURE 14-37. General distribution to the skin of the three sensory divisions of the trigeminal nerve. Pain in these areas is felt by impulses sent through the ophthalmic (yellow), maxillary (green), and mandibular branches (red) or divisions of this nerve. These three branches are distributed to the face as indicated in this drawing. (Reproduced from Clemente CD, ed. Gray’s anatomy of the human body. 30th ed. Philadelphia, PA: Lea & Febiger, 1985:1164, with permission.)
The maxillary and mandibular divisions of the trigeminal nerve also supply afferent or sensory neurons that provide the brain with information about the position of the teeth and jaws at all times. The interpretation of postural information by the brain (sense of position) is called proprioception. Proprioceptive nerve receptors are located in muscles and ligaments, including the periodontal ligaments, and in the lateral aspects of the TMJ. The periodontal ligament around each tooth is well supplied with proprioceptive neurons from the maxillary and mandibular divisions of the trigeminal nerve. These branches send messages to the brain as to the relative position of the mandibular to maxillary teeth. This has a tremendous influence on relative jaw position, movement, and occlusion (the fitting together) of the teeth. Canines are reported to have the richest supply of proprioceptive nerve endings.
The TMJ also has proprioceptive neurons in the capsule and disc that are innervated by a branch of the mandibular division of the trigeminal nerve. To a great extent, proprioceptive information, especially from the teeth, determines the subconscious but well-coordinated function of the two complex TMJ.28,29 Otherwise, we could experience many unpleasant tooth interferences or frequent joint pain.
Each of the three divisions is divided into many branches. The branches of the maxillary nerve and the mandibular nerve are those that innervate the region of, and around, the oral cavity and will be discussed in the most depth in this section.
1. Division I (Ophthalmic Nerve) of the Trigeminal Nerve
The ophthalmic [of THAL mik] nerve exits from the skull by way of the superior orbital fissure on the superior surface of the orbit (Fig. 14-38). It has three main branches: the smallest lacrimal nerve, the largest frontal nerve, and the nasociliary nerve. The distribution of these sensory branches that supply the skin of the face is shown in Figure 14-37. The ophthalmic nerve and its branches supply general sensations (of touch, pain, pressure, and temperature) to the skin of the upper third of the face including the skin of the forehead and anterior scalp, and the skin around the eyeballs, upper eyelids, and nose, and part of the nasal mucosa and maxillary sinus. The ophthalmic nerve does not supply the oral cavity. (Hint: “Ophthalmic” is related to the eye; compare ophthalmologist, a physician who specializes in eyes.)
FIGURE 14-38. Human skull: foramen of the branches of the trigeminal nerve (CN V) (shaded red on right side). The superior orbital fissure for the ophthalmic branch, the foramen rotundum for the maxillary branch, and the foramen ovale for the mandibular branch.
2. Division II (Maxillary Nerve) of the Trigeminal Nerve
The maxillary nerve provides general sensations (of touch, pain, pressure, and temperature) to the skin of the middle third of the face and the palate (Fig. 14-37) plus provides sensory branches to the pulp of all maxillary teeth. It exits the braincase of the skull through the foramen rotundum (Fig. 14-38). After passing through the foramen rotundum, the maxillary nerve passes into the pterygopalatine space and eventually splits into four branches: the pterygopalatine, PSA, infraorbital, and zygomatic nerves.
a. First Branch of the Maxillary Nerve: Pterygopalatine Nerve
The first branch of the maxillary nerve, the pterygopalatine nerve, splits off closest to the foramen rotundum. A branch of this nerve, called the descending palatine nerve, passes through the greater palatine foramen to become the greater palatine nerve (anterior palatine nerve). The greater palatine nerve spreads anteriorly to supply the part of the hard palate and palatal gingiva that is medial to the posterior teeth (molars and premolars) (posterior red lines in Fig. 14-39). Just posterior to the greater palatine foramen, the middle and posterior (lesser) palatine nerves enter the palate through the lesser palatine foramen to spread posteriorly to supply the tonsils and mucosa of the soft palate.
FIGURE 14-39. Human skull: inferior surface including the palate, showing the foramina for branches of the trigeminal nerve that innervates the mucosa of the palate: the greater palatine foramen (for the greater palatine nerve) and the incisive foramen (for the nasopalatine nerve). The more posterior red lines indicate the diagrammatic distribution of the branches of the greater palatine nerves as they spread out along the junction of the alveolar processes with the palatine processes of the maxillae to the tissues (mucosa) of the palate located between the posterior teeth. The more anterior red lines indicate the nasopalatine nerve branches spreading out to the mucosa between the anterior teeth.
Another long branch of the pterygopalatine nerve, the nasopalatine nerve, runs along the roof of the nasal cavity, then diagonally downward and anteriorly along the nasal septum where it enters the bone of the palate to emerge onto the anterior palate through the incisive foramen. This branch innervates the soft tissue of the nasal septum and gingiva and palatal soft tissue lingual to the anterior teeth (anterior red lines in Fig 14-39). The right and left nasopalatine nerves combined with the greater palatine nerves innervate the soft tissue of the entire hard palate (shown as all red lines in Fig. 14-39).
b. Second Branch of the Maxillary Nerve: Posterior Superior Alveolar (PSA) Nerve
Just before the maxillary nerve branch enters the infraorbital fissure and canal on the floor of the orbit, it gives off its second branch, the PSA nerve. This branch descends to enter the alveolar canals on the infratemporal portion of the maxilla (Fig. 14-40). Once within the trabecular (spongy) bone of the maxilla and the maxillary sinus, its dental branches enter small openings in the tooth roots to supply the maxillary molars (except for one root, the mesiobuccal root of the maxillary first molars). It also innervates the supporting alveolar bone, periodontal ligaments, and facial gingiva next to the maxillary molars, the mucosa of part of the maxillary sinus, and cheek mucosa next to maxillary molars.
FIGURE 14-40. Maxillary division of the trigeminal nerve: branches seen as red lines (dotted red lines when within the trabecular bone) that innervate the maxillary teeth. The lateral wall of the left maxilla has been removed exposing the large maxillary sinus. One nerve branch (the PSA nerve) exits the pterygopalatine space and goes down the posterior surface of the maxilla before entering the maxilla through the alveolar canals on its way to most maxillary molar roots. Another branch, the infraorbital nerve, passes from the pterygopalatine space to the floor of the eye orbit (which also forms the roof of the maxillary sinus) where it enters the infraorbital canal (not shown). Within the infraorbital canal, two branches split off to pass downward along the walls of the maxillary sinus and into the maxilla. The MSA nerve passes through the spongy bone of the maxilla to the maxillary premolars (and the mesiobuccal root of the first molar on each side), and the ASA passes to the roots of the maxillary anterior teeth. The infraorbital branch continues through the infraorbital canal to exit the maxilla through the infraorbital foramen, which provides feeling to the skin on the side of the nose, the anterior part of the cheek, and the upper lip on that side.
c. Third Branch of the Maxillary Nerve: Infraorbital Nerve
In the pterygopalatine space, a third branch of the maxillary nerve splits off and passes through the inferior orbital fissure on the floor of the orbit and enters the infraorbital canal, where it becomes the infraorbital nerve (Fig. 14-40). While within this canal, the infraorbital nerve gives off two branches, the middle superior alveolar (MSA) and the anterior superior alveolar (ASA) nerves (shown in Fig. 14-40).
The MSA nerve passes forward along the lining of the maxillary sinus. It gives off small dental branches that enter premolars through their root openings (apical foramina) to supply the maxillary premolars (and the mesiobuccal root of the maxillary first molar), supporting alveolar bone, periodontal ligaments, and facial gingiva in the maxillary premolar region and part of the maxillary sinus. It is important to realize that the nerve supplying primary teeth is the same as that to the permanent teeth that replace them. Therefore, the nerve branch to the primary molars is the MSA, the same one that supplies their successors, the permanent premolars.
The second branch given off of the infraorbital nerve while in the infraorbital canal is the ASA nerve. Its small dental branches supply the pulp, supporting alveolar bone, periodontal ligaments, and facial gingiva of the maxillary anterior teeth and part of the maxillary sinus.
Notice that three superior alveolar branches of the maxillary nerve of CN V (PSA, MSA, and ASA) innervate all maxillary teeth. A comparison of the descriptions of these three nerves indicates a great lack of uniformity in their distribution. Sometimes, the MSA nerve is missing, and the function is taken over by the anterior and posterior alveolar nerves.
After exiting from the infraorbital foramen, the infraorbital nerve splits into its end (terminal) branches innervating the skin and mucosa of the side of the nose (nasal nerve), skin and mucosa of the lower eyelids (palpebral [PAL pe bral] nerve), and skin and mucosa of the upper lip, facial gingiva of maxillary premolars, and facial gingiva of anterior teeth (labial [LAY bee al] nerve) (Fig. 14-40).
d. Fourth Branch of the Maxillary Nerve: Zygomatic Nerve
The zygomatic nerve arises in the pterygopalatine fossa, enters the orbit via the inferior orbital fissure, and then divides into two branches: the upper zygomaticotemporal and lower zygomaticofacial nerves (Fig. 14-37). These branches supply the skin of the temporal region and lower part of the orbit.
3. Division III (Mandibular Nerve) of the Trigeminal Nerve
The mandibular nerve is a mixed nerve; that is, it contains both sensory (afferent) and motor (efferent) fibers. It is the only motor portion of the trigeminal nerve. These motor fibers of the mandibular nerve supply the eight muscles of mastication, plus the mylohyoid muscle and the anterior belly of the digastric muscles, which help to retract the mandible (Fig. 14-35). Sensory fibers provide general sensations (of touch, pain, pressure, and temperature) to the skin of the lower third of the face (as seen in Fig. 14-37) and the floor of the mouth and anterior two thirds of the tongue (not taste). Other branches enter all mandibular teeth.
The mandibular nerve exits the neurocranium through the foramen ovale (Fig. 14-38). It passes into a space just medial to the zygomatic arch and mandibular ramus and inferior to the temporal bone, called the infratemporal space. As it passes inferiorly toward the mandibular foramen in the mandible, it divides into four sensory branches: the auriculotemporal, buccal, lingual, and inferior alveolar nerves.
a. Auriculotemporal Nerve
The first branch of the mandibular division, the auriculotemporal [aw RIK u lo TEM po ral] nerve, comes off the main trunk immediately below the base of the skull, turning backward to supply pain and proprioception fibers to the TMJ and to supply the skin of the outer ear and the lateral aspect of the skull and cheek (Fig. 14-41).
FIGURE 14-41. Mandibular division of the trigeminal nerve branches (yellow). The external wall of the right mandible has been removed to expose the inferior alveolar nerve within the mandible, where it gives off the many small dental branches to each mandibular tooth. (From this view, the buccinator muscle hides the teeth.) Within the mandible near the premolar area, the inferior alveolar nerve splits into two end (terminal) branches. One branch, the mental nerve, exits through the mental foramen to innervate the skin of the chin and lip on that side, while the other branch is really a continuation of the inferior alveolar nerve anteriorly within the mandible where it is called the incisive nerve (not visible here). Also, note the other major branches of the mandibular division: the lingual nerve, which is in close proximity to the inferior alveolar nerve posteriorly but then diverges anteriorly to enter the tongue, and the buccal nerve, which innervates the cheek and tissue next to mandibular molars. Other nerve branches (not shaded) are motor branches of the mandibular nerve supplying the muscles of mastication (shaded light red). The maxillary artery is shaded red. (Motor branches can be seen entering the masseter and temporalis muscles.) (Reproduced from Clemente CD, ed. Gray’s anatomy of the human body. 30th ed. Philadelphia, PA: Lea & Febiger, 1985:1166, with permission.)
b. Buccal (Buccinator) Nerve
Another branch is the buccal (buccinator [BUCK sin a tor] or long buccal) nerve, which comes off just below the foramen ovale and passes through the infratemporal space between the two heads of the lateral pterygoid muscles and then down and forward to the buccinator muscle (Fig. 14-41) where it innervates the mucosa and skin of the cheek up to the corner of the mouth and the buccal gingiva in the area of the mandibular molars and sometimes the second premolars. The best place to anesthetize the tissue supplied by the buccinator nerve is to inject inside the cheek to deposit the anesthetic into the buccinator muscle near the mandibular molars (Fig. 14-42).
FIGURE 14-42. Location of the branches of the mandibular division of the trigeminal nerve (mandibular nerve) (in red). As the mandibular nerve passes through the infratemporal space, it gives off the buccal nerve to the cheek (lateral to the ramus). Before entering the mandibular foramen, the mandibular nerve (medial to the ramus) gives off a lingual nerve branch that passes to the tongue. The inferior alveolar nerve enters the mandibular foramen (and canal) where it and its terminal incisal branch give off branches through the spongy bone to all mandibular teeth.
c. Lingual Nerve
The next branch of the mandibular nerve, given off inferior to the foramen ovale, is the lingual nerve branch that goes to the tongue (Figs. 14-41 and 14-42). It passes downward, medial to the ramus but lateral to the medial pterygoid muscle, to the mucous membrane just lingual to the last molar. The lingual nerve provides general sensation (touch, pain, pressure, and temperature, but not taste) to the top (dorsal) and bottom (ventral) surfaces of the anterior two thirds of the tongue and adjacent tissues. The adjacent tissues include the soft tissue (mucosa) on the floor of the mouth and inner surface of the mandible and the lingual gingiva of the entire mandible.
d. Inferior Alveolar Nerve
Finally, the inferior alveolar nerve comes off the mandibular nerve on the medial side of the lateral pterygoid muscle (Fig. 14-41). This large nerve roughly parallels the direction of the lingual nerve to descend between the sphenomandibular ligament and ramus to the mandibular foramen, where it gives off the mylohyoid nerve and then enters the mandible through the mandibular foramen (represented on the medial surface of the mandible in Fig. 14-42). The mylohyoid nerve (efferent) pierces the sphenomandibular ligament and travels forward in the mylohyoid groove to supply the mylohyoid muscle.
Once the inferior alveolar nerve enters the mandible through the mandibular foramen, it is in the mandibular canal within the body of the mandible, where it gives off the many small dental branches that spread through trabecular (spongy) bone of the mandible in order to enter the apical foramen of all mandibular molars and premolars. It also innervates the periodontal ligaments and alveolar processes of these teeth. While within the mandibular canal, the inferior alveolar nerve splits near the roots of the premolars to become the mental nerve and the incisive nerve. The incisive nerve (Fig. 14-43) branch continues forward within the mandibular canal to supply the mandibular incisor and canine teeth, their periodontal ligaments, and surrounding alveolar process. The mental nerve branch of the inferior alveolar nerve exits from the body of the mandible through the mental foramen (Fig. 14-41) and supplies the facial gingiva of the mandibular incisors, canines, and premolars and the mucosa and skin of the lower lip and chin on that side up to the midline (Fig. 14-37).
FIGURE 14-43. Trigeminal nerve distribution of the branches of the maxillary and mandibular divisions. The ophthalmic branches are shaded green, the maxillary nerve and branches are shaded red, and the mandibular nerve and branches are shaded blue. Note that the buccinator (long buccal) branch (labeled No. 8) of the mandibular division passes superficial to the ramus to enter the cheek, whereas the lingual nerve (labeled No. 9) and inferior alveolar nerve (labeled No. 10) pass medial to the ramus as they go to the tongue and mandible, respectively. Also, note that the infraorbital branch of the maxillary division gives off the MSA (labeled No. 5) and ASA branches (labeled No. 6) while in the infraorbital canal on the floor of the eye orbit (roof of the maxillary sinus).
Note that if an anesthetic solution is deposited next to the opening of the mandibular foramen, it could block the passage of sensory nerve signals from all mandibular teeth on that side (by blocking the inferior alveolar and its terminal incisive branch) and also the skin of the chin and lip area (because another terminal branch, the mental nerve, has also been blocked). Further, since the lingual nerve is in close proximity to the mandibular foramen, its fibers may also be blocked, causing that side of the floor of the mouth, lingual gingiva, and anterior two thirds of the tongue to lose feeling. The only part of the mandible that would not be numb would be the tissue buccal to the molars, which requires some additional anesthetic solution in the cheek to block the buccal nerve.
Figures 14-43 and 14-44 and Table 14-3 can be used to help summarize the distribution of the mandibular and maxillary sensory nerve branches to all teeth and surrounding tissues of the mouth.
Motor (efferent) branches of the mandibular nerve supply the muscles of mastication: the masseteric nerve to the masseter muscle, as well as to the TMJ, the posterior and anterior temporal nerves to the temporalis muscle, the medial pterygoid nerve to the medial pterygoid muscle, and the lateral pterygoid nerve to the lateral pterygoid muscle.
LEARNING EXERCISE
Referring to Table 14-3, cover one column at a time, and see how many branches of the trigeminal nerve you can recall that innervate the pulps, gingiva, periodontal ligaments and alveolar processes, and hard palate in each area of the mouth. These are the nerves any dental student, dental hygiene student, or graduate of either profession should be most familiar with. You should also be able to determine the location of each nerve.
| TABLE 14-3 | Distribution of Branches of Trigeminal Nerve to the Teeth and Surrounding Structures |
aAlso supply the maxillary sinus.
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