All Cranial Nerves 2

            There are twelve paired cranial nerves of the head and neck that allow information to be carried to and from the brain. The cranial nerves are numbered I through XII and each has a specific name and function. The cranial nerves are an important part of the peripheral nervous system and are either afferent or efferent; however, some cranial nerves are both afferent and efferent. Afferent, or sensory, nerves allow information such as pain, temperature and taste to be carried from the periphery of the body to the brain (Fehrenbach, 2002). Efferent, or motor, nerves allow information to be carried from the brain to the body for the movement of muscles (Fehrenbach, 2002). The twelve cranial nerves transmit information between the brain or brainstem and the head and neck, and they pass through the skull by way of fissures and foramina. The twelve cranial nerves are important to the dental hygienist in many ways. The dental hygienist must have knowledge of the location of the twelve cranial nerves for administration of local anesthesia to prevent patient pain during dental treatment. Also, an understanding of the cranial nerves allows the dental hygienist to recognize diseases of the muscles of the head and neck, temporomandibular joint and salivary glands (Fehrenbach, 2002).

                The first cranial nerve is cranial nerve I and is named the olfactory nerve. The olfactory nerve is an afferent, or sensory, nerve and it transmits the sense of smell from the nasal mucosa to the brain. The olfactory nerve enters the skull through the perforations in the cribriform plate of the ethmoid bone and relays information to the olfactory bulb in the brain (Fehrenbach, 2002).

            Cranial nerve II is the second cranial nerve and is named the optic nerve. The optic nerve is a sensory nerve for visual information (Moebius syndrome, 2001). It transmits sight from the retina of the eye to the brain. The optic nerve passes through the skull by way of the optic canal of the sphenoid bone (Fehrenbach, 2002).

            The oculomotor nerve is cranial nerve III. It functions as a motor nerve for the eye muscles that move the eyeball. The oculomotor nerve also controls upper eyelid elevation and pupil reactivity ( O’Hanlon-Nichols, 1999). Cranial nerve III passes through the skull by way of the superior orbital fissure of the sphenoid bone (Fehrenbach, 2002).

            The fouth cranial nerve, cranial nerve IV or trochlear nerve, is a motor nerve for one eye muscle only. The small trochlear nerve passes through the superior orbital fissure of the sphenoid bone on its way to the orbit (Fehrenbach, 2002).

            Cranial nerve V is named the trigeminal nerve. It is the largest cranial nerve and is both a sensory nerve and a motor nerve. The trigeminal nerve has many functions. It serves as a sensory nerve for the teeth, tongue, oral cavity and skin of the face and a motor nerve for the muscles of mastication (Fehrenbach, 2002). The trigeminal nerve has two roots, a sensory root and motor root. The sensory root of the trigeminal nerve splits into three large branches: the ophthalmic, maxillary and mandibular. The ophthalmic branch serves the upper face and scalp with sensation. The maxillary and mandibular branches serve the middle and lower face with sensation. The ophthalmic branch of the trigeminal nerve passes through the superior orbital fissure of the sphenoid bone (Fehrenbach, 2002). The maxillary branch passes through the foramen rotundum of the sphenoid bone and the mandibular branch passes through the foramen ovale of the sphenoid bone (Fehrenbach, 2002). The smaller motor root emerges with the mandibular branch of the sensory root and extends to innervate muscles in the mandible and floor of the mouth (Fehrenbach, 2002). Because it innervates many tissues of the head and neck, the trigeminal nerve is the most important nerve to the dental hygienist (Fehrenbach, 2002).

            The sixth cranial nerve is the abducent nerve. The abducent nerve functions as a motor nerve for lateral movement of the eye and blinking (Moebius syndrome, 2001). Cranial nerve VI passes through the skull by way of the superior orbital fissure of the sphenoid bone (Fehrenbach, 2002).

            The facial nerve is the seventh cranial nerve. The facial nerve is both a sensory and motor nerve. It serves as a sensory nerve to a patch of skin behind the ear, sweet, sour and salty taste sensations, tear and saliva production and to the body of the tongue (O’Hanlon-Nichols, 1999). It serves as a motor nerve for the muscles of facial expression. The facial nerve exits the skull by way of the stylomastoid foramen of the temporal bone (Fehrenbach, 2002).

            Cranial nerve VIII is the vestibulocochlear nerve. The eighth cranial nerve serves as a sensory nerve and conveys signals for hearing and balance to the brain. The vestibulocochlear nerve passes through the skull by the internal acoustic meatus of the temporal bone (Fehrenbach, 2002).

            The glossopharyngeal nerve is the ninth cranial nerve. It functions as a motor nerve for the pharyngeal and stylopharyngeus muscles to help control swallowing (O’Hanlon-Nichols, 1999). It also functions as a sensory nerve for the pharynx and posterior one-third of tongue. Cranial nerve IX passes through the skull by way of the jugular foramen (Fehrenbach, 2002).

            The tenth cranial nerve or vagus nerve is both a sensory and motor nerve. It is a sensory nerve to skin around the ear and taste sensation for the epiglottis and a motor nerve for the movement of the soft palate, pharynx, larynx and for many organs in the thorax and abdomen (Fehrenbach, 2002). Cranial nerve X passes through the skull by way of the jugular foramen (Fehrenbach, 2002).

            The accessory nerve is the eleventh cranial nerve. It functions as a motor nerve for the trapezius and strenocleidomastoid muscles, which allow for shoulder movement and head rotation (O’Hanlon-Nichols, 1999). It also functions for the muscles of the soft palate and pharynx (Fehrenbach, 2002). Cranial nerve XI passes through the skull by way of the jugular foramen (Fehrenbach, 2002).

            The twelfth cranial nerve is the hypoglossal nerve. It functions as a motor nerve for tongue movement by the tongue’s intrinsic and extrinsic muscles (Fehrenbach, 2002). Cranial nerve XII exits the skull through the hypoglossal canal in the occipital bone (Fehrenbach, 2002).

            The twelve paired cranial nerves of the head and neck allow information to be transmitted to and from the brain and are either sensory nerves, motor nerves, or both. The twelve cranial nerves each have their own function and serve different parts of the head and neck. It is important for the dental hygienist to have sufficient knowledge of each individual cranial nerve of the head and neck in order to be able to provide local anesthesia to patients and recognize certain disease processes.

Resources

Fehrenbach, M.J. & Herring, S.W. (2002). Illustrated anatomy of the head and neck  

(2^nd ed.). Philadelphia: W.B. Saunders Co., 190-211.

O’Hanlon-Nichols, T. (1999). Neurologic assessment. American Journal of Nursing, 99

(6), 44-50. [electronic version].

Moebius syndrome. (2001). Bridges, 6 (4). [electronic version].