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The Nervous System - Advanced Version / The Cranial Nerves
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Cranial nerves can be thought of as modified spinal nerves, since the “general” functional fiber types found in spinal nerves also are found in cranial nerves but are supplemented by “special” afferent or efferent fibers. fibers conveying olfaction (in cranial nerve I) and taste (in cranial nerves VII, IX, and X) are classified as special visceral afferent, while the designation of special somatic afferent is applied to fibers conveying vision (cranial nerve II) and equilibrium and hearing (cranial nerve VIII). Skeletal muscles that arise from the branchial (pharyngeal) arches are innervated by fibers of cranial nerves V, VII, IX, and X; these are classified as special visceral efferent fibers.
The 12 pairs of cranial nerves are commonly identified either by name or by Roman or Arabic numeral.
The trigeminal nerve is the largest of the cranial nerves. It has both motor and sensory components, the sensory fibers being general somatic afferent and the motor fibers being special visceral efferent. Most of the cell bodies of sensory fibers are located in the trigeminal ganglion, which is attached to the pons by the trigeminal root. These convey pain and thermal sensations from the face, oral and nasal cavities, and parts of the dura and nasal sinuses, sensations of deep pressure, and information from sensory endings in muscles. Trigeminal motor fibers, projecting from nuclei in the pons, serve the muscles of mastication.
Lesions of the trigeminal nerve result in sensory losses over the face or in the oral cavity. Damage to motor fibers results in paralysis of the masticatory muscles; as a result, the jaw may hang open or deviate toward the uninjured side when opened. Trigeminal neuralgia, or tic douloureux, is an intense idiopathic pain originating mainly from areas supplied by sensory fibers of the maxillary and mandibular branches of this nerve.
The trigeminal ganglion gives rise to three large nerves, the ophthalmic, maxillary, and mandibular.
The ophthalmic nerve passes through the wall of the cavernous sinus and enters the orbit via the superior orbital fissure. Branches in the orbit are (1) the lacrimal nerve, serving the lacrimal gland, part of the upper eyelid, and the conjunctiva; (2) the nasociliary nerve, serving the mucosal lining of part of the nasal cavity, the tentorium cerebelli and some of the dura of the anterior cranial fossa, and skin on the dorsum and tip of the nose; and (3) the frontal nerve, serving the skin on the upper eyelid and the forehead and scalp above the eyes up to the vertex of the head.
The maxillary nerve courses through the cavernous sinus below the ophthalmic nerve and passes through the foramen rotundum into the orbital cavity. Branches of the maxillary nerve are (1) the meningeal branches, which serve the dura of the middle cranial fossa; (2) the alveolar nerves, serving the upper teeth and gingiva and the lining of the maxillary sinus; (3) the nasal and palatine nerves, which serve portions of the nasal cavity and the mucosa of the hard and soft palate; and (4) the infraorbital, zygomaticotemporal, and zygomaticofacial nerves, serving the upper lip, the lateral surfaces of the nose, the lower eyelid and conjunctiva, and skin on the cheek and the side of the head behind the eye.
The mandibular nerve exits the cranial cavity via the foramen ovale and serves (1) the meninges of middle and parts of the anterior cranial fossae (meningeal branches); (2) the temporomandibular joint, skin over part of the ear, and skin over the sides of the head above the ears (auriculotemporal nerve); (3) oral mucosa, the anterior two-thirds of the tongue, gingiva adjacent to the tongue, and the floor of the mouth (lingual nerve); and (4) the mandibular teeth (inferior alveolar nerve). Skin over the lateral and anterior surfaces of the mandible and the lower lip is served by cutaneous branches of the mandibular nerve.
Trigeminal motor fibers exit the cranial cavity via the foramen ovale along with the mandibular nerve. They serve the muscles of mastication (temporalis, masseter, medial and lateral pterygoid), three muscles involved in aspects of swallowing (anterior portions of the digastric muscle, the mylohyoid muscle, and the tensor veli palatini), and a muscle that has a damping effect on loud noises by stabilizing the tympanic membrane (tensor tympani). Take this preliminary to see if your condition could respond to treatment.
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