Dental Neuroanatomy 8:00-10:00 AM, Monday, April 19, 2010 David A. Morton, Ph.D. NASAL AND ORAL CAVITIES Objectives: 1. Review and understand the basic anatomy of the following structures including sensory and motor innervation including central and peripheral pathways: a. Nasal cavity b. Paranasal sinuses c. Palate d. Tongue e. Salivery glands f. Teeth g. Pharynx 1 OVERVIEW OF THE NASAL CAVITY • Divided into two lateral compartments separated by nasal septum • Lined with mucous membrane; vascular supply from maxillary, facial, and ophthalmic aa. • Innervated by branches of the CN I, CN V-1, and CN V-2 • Bordered by the following structures: • Roof. Cribriform foramina, which transmits CN I for smell • Nasal septum. Perpendicular plate of the ethmoid bone, vomer bone, and septal cartilage • Lateral wall. Superior and middle nasal conchae (ethmoid bone) and the inferior nasal concha, in addition to the maxillary, sphenoid, and palatine bones. The lateral wall contains the following openings, enabling communication with the nasal cavity: • Sphenoethmoidal recess. Openings from the sphenoid sinus. • Superior meatus. Openings from the posterior ethmoidal air cells. • Middle meatus. Openings for the frontal sinus via the nasofrontal duct, middle ethmoidal air cells, anterior ethmoidal air cells and maxillary sinus • Inferior meatus. Opening for the nasolacrimal duct, which drains tears from the eye into the nasal cavity • Sphenopalatine foramen. Opening posterior to the middle nasal concha; receives the nasopalatine nerve and the sphenopalatine artery from the pterygopalatine fossa into the nasal cavity 2 Smell – Cranial Nerve I CN I originates in the mucosa lining the superior nasal concha and the superior septum, where the nerve provides special sensation for smell; course from nasal cavity into the anterior cranial fossa via the cribriform foramina; neurons enter the olfactory bulb, where they synapse with interneurons that course along the olfactory tract, transporting information to the brain. • Olfactory bulb and tract. Neurons in the olfactory bulb, called mitral cells, are secondary sensory neurons of the olfactory system. Their axons leave the olfactory bulb and enter the olfactory tract. The olfactory tract is not a peripheral nerve. It is part of the central nervous system. 3 PARANASAL SINUSES Hollow cavities within the ethmoid, frontal, maxillary, and sphenoid bones. They help decrease the weight of the skull, resonate sound produced through speech, and produce mucus. The paranasal cavities communicate with the nasal cavity, where mucus is drained. Branches of CN V provide general sensory innervation. • Ethmoidal Sinus. Consists of numerous small air cells within the bone, as opposed to one or two large sinuses. Innervated by CN V-1. • Frontal Sinus. The frontal sinus is located in the frontal bone and opens into the anterior part of the middle meatus via the frontonasal duct. Innervated by CN V-1. • Maxillary Sinus. Largest of the paranasal sinuses; lateral to the nasal cavity and inferior to the orbit. Opens into the hiatus semilunaris in the middle meatus. Innervated by CN V-2. • Sphenoid Sinus. Contained within the body of the sphenoid bone and is inferior to the sella turcica. Opens into the sphenoethmoidal recess of the nasal cavity. Innervated by CN V-1 and CN V-2. 4 PALATE Sensory innervation of the palate: • Hard palate. Consists of the maxillary and palatine bones. The incisive canal and greater palatine foramina transmit branches of CN V-2 for sensation. • Soft palate. Possess the uvula; ensures that food moves down into the esophagus when swallowing, rather than up into the nose by acting like a flap-valve. CN V-2 provides sensation via the lesser palatine nerves. 5 Muscles of the soft palate • Tensor veli palatini muscle. Innervated by CN V-3; tenses the soft palate • Levator veli palatini muscle. Originates along the cartilage of the auditory tube; inserts into the soft palate. Elevates the soft palate; innervated by CN X. To test the function of CN X, a patient is asked to open his mouth wide to determine if the palate deviates to one side or the other during a yawning motion. A lesion of CN X would cause paralysis of the ipsilateral levator veli palatini muscle, resulting in the uvula being pulled superiorly to the opposite side of the lesion. 6 TONGUE Skeletal muscle covered with taste buds. The tongue is supported in the oral cavity by muscular connections to the hyoid bone, mandible, styloid process, palate, and pharynx. Muscles of the Tongue The tongue is divided into a left and a right half by a septum of connective tissue. As a result, all tongue muscles are bilaterally paired. • Genioglossus muscle. Attaches between the internal surface of the mandible and the tongue. Contraction of the genioglossus muscle causes the tongue to protrude out of the oral cavity (“sticking out your tongue”). • Motor innervation of tongue. CN XII (hypoglossal nerve), innervated all tongue muscles receive their somatic motor innervation via CN XII (except the palatoglossus, which is innervated by CN X). To clinically test CN XII, ask the patient to stick her tongue out. If CN XII is functioning normally, the tongue should extend out evenly in the midline. If CN XII is lesioned, the tongue will deviate toward the same side of the face as the lesion. An easy way to remember this is that a lesion to CN XII will cause a “patient licks his wounds.” 7 Sensory Innervation of the Tongue The following nerves innervate the tongue: • CN V-3 (lingual nerve). The anterior two-thirds of the tongue receives sensation from the lingual branch of CN V-3. The submandibular duct ascends from the submandibular gland to open adjacent to the lingual frenulum. The lingual nerve courses inferior to the submandibular duct. o Chorda tympani nerve (CN VII). Enters the infratemporal fossa via the petrotympanic fissure, where it joins with the lingual nerve. The chorda tympani nerve provides special sensory taste from the anterior two-thirds of the tongue, as well as visceral motor parasympathetic innervation to the submandibular and sublingual salivary glands. o Submandibular ganglion. Is suspended from the lingual nerve, where preganglionic and postganglionic neurons from the chorda tympani nerve (CN VII) synapse en route to provide visceral motor innervation to the submandibular and sublingual glands. • CN IX (glossopharyngeal nerve). The posterior third of the tongue receives both its general sensory and special sensory (taste) innervation from CN IX. 8 Taste Pathways 9 SALIVARY GLANDS Innervation of the Salivary Glands The parasympathetic nervous system is responsible for providing visceral motor innervation to the salivary glands. This is accomplished via the following cranial nerves: • Parotid gland. CN IX, via the otic ganglion. • Submandibular gland. CN VII, via the submandibular ganglion. • Sublingual gland. CN VII, via the submandibular ganglion. 10 TEETH AND GINGIVAE The teeth cut, grind, and mix food during mastication. Branches of CN V-2 and the maxillary artery and veins supply the maxillary teeth and gingivae via branches of CN V-3, and the inferior alveolar artery and veins supply the mandibular teeth and gingivae. Innervation of the Teeth The innervation of the teeth and gingivae are as follows: • Maxillary teeth. CN V-2 exits the brain and enters the pterygopalatine fossa via the foramen rotundum. CN V-2 courses into the infraorbital canal, where branches provide general sensory innervation to the maxillary teeth in a plexus of nerves formed by the anterior, middle, and posterior superior alveolar nerves. o Maxillary gingivae. The posterior, middle, and anterior superior alveolar nerves innervate the buccal surface of the gingivae, whereas the greater palatine and nasopalatine nerves innervate the lingual surface. • Mandibular teeth. CN V-3 exits the brain and enters the infratemporal fossa via the foramen ovale. A branch of CN V-3, the inferior alveolar nerve, enters the mandibular foramen and provides general sensory innervation to the teeth on that side of the mandible. The inferior alveolar nerve exits the mandibular canal as the mental nerve by traversing the mental foramen and provides general sensory innervation to the bottom lip. o Auriculotemporal nerve. Splits around the middle meningeal artery to provide general sensory innervation to the temporal region of the face and scalp. • Otic ganglion. Preganglionic parasympathetic neurons from the glossopharyngeal nerve (CN IX) exit the middle ear as the lesser petrosal nerve and enter the infratemporal fossa, through the foramen ovale, and synapse in the otic ganglion. Postganglionic parasympathetic neurons “hitch-hike” along with the auriculotemporal nerve, providing visceral motor innervation to the parotid gland. o Mandibular gingivae. The buccal and mental nerves innervate the buccal surface of the gingivae, whereas the lingual nerve innervates the lingual surface. An inferior alveolar nerve block is administered to patients who require dental work in any of the teeth on the ipsilateral side of the mandible. Injection of local anesthetics (e.g., lidocaine) into the oral mucosa at the lingula of the mandible will anesthetize the inferior alveolar nerve and potentially the proximal lingual nerve, resulting in numbness of the tongue and oral mucosa. Care must be given when extracting the third molar tooth because the lingual nerve is closely associated with it. A plexus formed by the anterior superior, middle superior, and posterior superior alveolar nerves supplies the maxillary teeth. Unlike the mandibular teeth, where anesthesia can be administered in one location to effectively block all of the teeth on that side, the maxillary teeth must be considered separately. The palate must also be considered; therefore, a greater palatine as well as a nasopalatine nerve block is often administered during dental work. • Maxillary molars. Injection is given at the region of the second molar. • Maxillary premolars. Injection is given at the region of the second premolar. • Maxillary canine and incisors. Injection can be given above the roots of the anterior teeth or via an infraorbital nerve block.