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1 the Trigeminal Nerve. Part I: an Over-View

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1 the Trigeminal Nerve. Part I: an Over-View The Trigeminal Nerve. Part I: An Over-View Wesley E. Shankland, D.D.S., M.S., PhD. The Journal of Craniomandibular Practice October 2000, VOL. 18, NO. 4 ABSTRACT: The trigeminal nerve is the largest and most complex of twelve cranial nerves. Its vas size and influence are greatly appreciated when one attempts to diagnose and treat patients suffering from orofacial pain and temporomandibular joint disorders. Without a thorough knowledge of the trigeminal nerve, the efficacy of diagnostic and therapeutic procedures will be very disappointing. This is the first of a four-part series of articles about the trigeminal nerve, a basic over-view of both the gross and neuroanatomical structures is presented. As practitioners from various backgrounds and specialties, we are all charged with evaluation patients suffering with orofacial, temporomandibular joint, head and neck pain. The finest service we can offer our patients is to discover an accurate diagnosis or diagnoses, as the case may be. Due to the great anatomical complexity of the head and neck regions, diagnostic procedures are often difficult at best. Through knowledge of the trigeminal nerve is a must if one expects to gain any degree of success with these types of patients. In anatomy lectures and laboratories, each of us learned the basic anatomy of this largest of the cranial nerves; however, trigeminal anatomy was soon forgotten once we passed the National Board Examination. As doctors and therapists who diagnose and treat orofacial pain and temporomandibular joint disorders, we have no choice but to be experts of the trigeminal system. This statement is not s suggestion, but rather, a commandment. This four-part series of articles will serve as in-depth study guides and sources of information concerning the trigeminal nerve. The first article is an over-view; each of the additional three articles describes and discusses the individual divisions of the trigeminal nerve. Cranial Origin of the Trigeminal Nerve The trigeminal, or fifth cranial nerve, is comprised of three major divisions: the ophthalmic, the maxillary, and the mandibular. The largest cranial nerve is the great afferent nerve of the face, of the mucous membranes of the head, the afferent nerve (The term afferent, “to bring to,” is interchanged with the term sensory in this article.) of internal cranial structures, the afferent nerve of the teeth and temporomandibular joints, and efferent (motor) nerve of the first branchial arch (Table 1). The trigeminal nerve has its origin at the lateral border of the pons through two roots, the sensory (portio major), which is much larger than the motor root (portio minor). The afferent fibers, except for those associated with pressure and stretch receptors and proprioception1, have their cell bodies of origin located in the trigeminal (Gasserian or semilunar) ganglion. They enter the pons through the lateral portion of its ventral surface. The small motor root, whose cell bodies of origin are located within the pons and , unlike the sensory portion of this nerve, not in the Gasserian ganglion, also includes proprioceptive fibers (proprioception refers to a special type of sensory perception conveyed through muscle and skeletal mechanoreceptors for reflex adjustments of muscle actions and for awareness of position of movement.) derived from the mesencephalic nucleus in the pons. This anterior root is comprised of six to eight rounded filaments of Vulpian, and emerges from the pons slightly superior to the larger posterior sensory root until it passes out of the cranium through the foramen ovale in the sphenoid bone, where it becomes closely united with the mandibular division of the trigeminal nerve. After leaving the pons in the posterior cranial fossa, the motor and sensory roots pass anteriorly under the tentorium cerebelli in a grove in the petrous portion of the temporal bone in the middle cranial fossa to reach the Gasserian ganglion. This huge ganglion sits in a depression in the temporal bone (Meckle's cave) in the floor of the middle cranial fossa near foramen lacerum and is encased between two layers of dura mater. This collection of nerve cells was first called the Gasserian ganglion by anatomist 1 Anton B. R. Hirsch in 1765 in admiration of his mentor and professor, Johann Ludwig Gasser.2 This size of the cavernous area which contains the Gasserian ganglion varies from person to person and even one side to the other in the same person3. The mass of this ganglion is basically flat, semilunar in shape, and measures approximately 1 x 2 centimeters4. The central cellular processes (i.e., the portio major) exit the concavity of the ganglion, passing under the superior petrosal sinus of the temporal bone. The peripheral fivers leave the convexity of the crescent of the ganglion as the three divisions of the trigeminal nerve. The internal carotid artery and the posterior portion of the cavernous sinus are positioned on the medical aspect of the ganglion, from which sympathetic innervation is provided to the ganglion. The greater superficial petrosal nerve is situated between the ganglion and the petrous portion of the temporal bone. This ganglion is unique in that it is the only site in which primary afferent neuron cell bodies are derived from neural crest cells during development. In addition, it is the largest collection of cell bodies outside of the central nervous system in the body (Figure 1). The cells of the trigeminal ganglion are pseudo-unipolar, this making this structure analogous to the dorsal root ganglion of the spinal cord. Theses neurons are somatotopically (Somatotopic organization: an orderly representation of the various body parts within central nervous system structures that process sensory information such that neighborhood relation in the periphery are preserved in the central nervous system. This organizational representation in the central nervous system is maintained in the spinal cord and throughout the various areas of the brain.) organized from medial to lateral. Each neuron in the ganglion has two branches or processes. The peripheral processes of these ganglion cells form, from medial to lateral, the ophthalmic, maxillary, and mandibular divisions of the trigeminal nerve. The other branches of these ganglion neurons, the central processes, form the large sensory root which courses to the lateral border of the pons, through a groove in the petrous portion of the temporal bone, entering the pons and dividing into upper and lower roots. The upper roots synapse in the principle (chief sensory, main sensory, or pontine) nucleus positioned lateral in the pons. The lower roots descend as the spinal trigeminal root and become continuous with the substantia gelatinosa of Rolando of the spinal cord. Blood supply to the Gasserian ganglion is delivered from branches of the cavernous portion of the internal carotid artery as this vascular structure passes medially and superiorly to the ganglion through the carotid canal in the temporal bone. Table 1 Components of the Trigeminal Nerve _______________________________________________________________________________ Component Function ________________________________________________________________________________ Special visceral efferent (Branchial motor) To muscles of mastication, (Autonomics) tensor tympani and tensor veli palatine muscles, zygomandibularis muscle, mylohoid muscle, and the anterior belly of the digastric muscle. General somatic afferent From the face, scalp, conjunctiva, bulbus of eye; (General sensory) mucous membranes of the paranasal sinuses, nasal, and oral cavities; the tongue, teeth, and gingivae; part of the external portion of the tympanic membrane; the meninges and dura mater of the anterior and middle cranial fossae. Afferent Portion of the Trigeminal Nerve (Portico Major) The larger afferent portion of the trigeminal nerve provides somatosensory sensations (i.e., mechanoreception, thermal sensitivity, pressure sensation, nociception, and proprioception) to the entire face (except the angle of the mandible), the temple, the external acoustic meatus, and the anterior scalp as 2 far posterior as the vertex of the skull. In addition, portions of the eye, nose, maxillary sinus, nasal pharynx, auditory tube, the temporomandibular joint, the dura mater and periosteum are also innervated by branches of the three divisions of this greatest of all the cranial nerves. Although the cell bodies of the somatosensory neurons are located in the Gasserian ganglion, the cell bodies for proprioception5 and stretch receptors in the muscles of mastication are located in the mesencephalic nucleus in the dorsal pons The number of nerve fibers in the human Gasserian ganglion has been calculated by Young6. Using an electron microscope, he counted an area and then calculated that there were 124,000 nerve fibers, with approximately half being myelinated and half unmyelinated. The myelinated fibers ranged from 0.2 to 1.3 microns in diameter and the largest unmyelinated fibers ranged up to eleven microns in diameter. Embryologically, the trigeminal nerve is the nerve of the first branchial arch. Yet, unlike the maxillary (V2) and mandibular (V3) divisions, the ophthalmic (V1) portion is not a branchial component in the human being7. Phylogenetically, the trigeminal is the principle cutaneous afferent (sensory) nerve of the head and face as a result of the fusion of the two branchial nerves of the first branchial arch in lower chordates; namely, the ophthalmic and maxillomandibular nerves8. The
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