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The Fifth Cranial Nerve “The Trigeminal” By Prof. Dr. Muhammad Imran Qureshi The Mandibular Nerve - Vc or VIII This is the third and largest division of the trigeminal nerve. It is formed by the junction of the slender motor root of the nerve with the most lateral branch of the trigeminal ganglion, which is predominantly sensory. These two nerve bundles leave the cranial cavity through the foramen ovale and unite immediately to form the trunk of the mixed mandibular nerve that passes into the infratemporal fossa. Here, it is sandwiched between superior head of lateral pterygoid and tensor vali palati muscles, anterior to the middle meningeal artery. After a short course during which a meningeal branch to the dura mater, and the nerve to part of the medial pterygoid muscle (and the tensor tympani and tensor palati muscles) are given off, the mandibular trunk divides into a smaller anterior and a larger posterior division. The anterior division receives most of the fibres from the motor root and distributes them to the other muscles of mastication i.e. the lateral pterygoid, medial pterygoid, temporalis and masseter muscles. The sensory fibres that it receives are distributed as the buccal nerve, to the skin in the region of the angle of the month and to a corresponding area of the mucous membrane lining the cheek. The posterior division is mostly sensory and gives origin to the inferior alveolar, lingual and auriculotemporal nerves. It receives only a few motor fibres that are distributed, by the mylohyoid branch of the inferior alveolar nerve to the mylohyoid and the anterior belly of digastric muscles. The Inferior Alveolar Nerve This is the largest branch of the mandibular nerve. From its origin it passes downwards behind the lingual nerve, on the outer surface of the medial pterygoid muscle and deep to the lateral pterygoid muscle, and then between the mandibular ramus and the sphenomandibular ligament to enter the mandibular canal. Immediately before entering this canal it gives off its mylohyoid branch. The inferior alveolar nerve passes forwards in the mandibular canal, in company with the inferior alveolar artery, and distributes branches to the lower molar and premolar teeth and the adjacent parts of the gingiva. At the level of mental foramen it divides into an incisive branch, which continues forwards in the mandible to supply the canine and incisor teeth, and the larger mental branch, which passes out of the bony canal by the mental foramen and is distributed to the skin and subcutaneous tissues of the chin region and the lower lip. The Lingual Nerve This nerve passes downwards and forwards anterior to the inferior alveolar nerve and between the lateral and medial pterygoid muscles. Near its origin, the chorda tympani nerve, which is a branch of the facial nerve, joins it from behind. The lingual nerve inclines obliquely to the side of the tongue, passing over the mandibular attachment of the superior constrictor muscle of the pharynx. Here the nerve lies against the deep surface of the mandible on the medial side of the roots of the third lower molar tooth and above the deep part of the submandibular gland. The nerve then passes forwards on the upper aspect of the mylohyoid muscle and lateral to the hyoglossus muscle. The lingual nerve then passes laterally and hooks below the submandibular duct. Here it gives branches to the mucous membrane of the floor of the mouth and the gingivae. At the anterior border of the hyoglossus muscle, the lingual nerve enters the substance of the tongue where it distributes branches of the common sensation to the mucous membrane of its anterior two thirds. The fibres of the chorda tympani are of two types. The majority of the fibers are sensory, with cell bodies in the geniculate ganglion; they are distributed with the lingual nerve and sub serve taste sensibility in the anterior two thirds of a tongue, excluding circumvallate papillae. The others are preganglionic parasympathetic fibres, which relay in the submandibular ganglion. The postganglionic fibres are secretomotor to the submandibular and sublingual glands. AURICULOTEMPORAL NERVE This nerve arises from the posterior trunk of the mandibular nerve by two roots, which surround the middle meningeal artery, and is at first directed posteriorly, deep to the lateral pterygoid muscle, to the medial aspect of the neck of the mandible. From here it passes upwards and outwards, between the temporomandibular joint and the cartilage of the external auditory meatus, and in the substance of the parotid gland. It finally emerges from the gland and ascends over the zygomatic arch with the superficial temporal artery to terminate by supplying the skin of the temporal region and the lateral part of the scalp In its course the auriculotemporal nerve gives cutaneous branches to the external auditory meatus, the anterior part of the tympanic membrane, the tragus of the pinna and a variable, but usually small area of the anterior and upper part of the pinna itself. It also supplies twigs to the temporomandibular joint, and carries secretomotor fibres from the otic ganglion to the parotid gland. Clinical Considerations For The Trigeminal Nerve Damage to the ophthalmic nerve is revealed by disturbances of sensation from the skin supplied by this nerve and from the eye. It is tested by determining the responsiveness of the skin of the forehead (frontal nerve) to touch and pin prick. A second test involves the corneal reflex. When the cornea is touched, the sensation travels via VI back to the trigeminal nerve and thence to the brain. Here fibers synapse with facial neurons innervating the palpebral portion of orbicularis oculi, which is caused to contract, producing a blink. Like the pupillary light reflex, the corneal reflex is consensual, i.e., both eyelids blink when either cornea is touched. Obviously, disturbances of the corneal reflex will occur if either the sensory or motor limb is damaged. If the sensory limb is damaged, neither eyelid will blink when the affected cornea is touched. On the other hand, if touching the cornea of one eye produces a blink in the opposite eye, the examiner knows that VI is working and that the defect is in the facial nerve. Damage to the maxillary nerve leads to disturbance of sensation over its region of distribution. Usually this is only tested by assessing the responsiveness of the skin over the front of the cheek (infraorbital nerve) to touch and pain. Nasal, palatal and upper dental sensations are affected by damage to maxillary nerve, but these are not tested for routinely. Damage of the sensory fibers that run in VIII leads to disturbances in sensation in its region of supply. This is very broad, but during a routine examination the test is usually confined to the skin over the chin (mental nerve) and side of the cheek (buccal nerve). General sensation to the front of the tongue (lingual nerve) may also be tested. Obviously, a thorough neurological examination can involve tests over other regions (e.g., temple, ear). Damage to the motor fibers within VIII leads to severe disturbances in chewing. Wasting of the temporalis and masseter can be seen. There is also an obvious symptom due to paralysis of the lateral pterygoid. As we know, this muscle is the main depressor of the mandible. When both lateral pterygoids work properly, the jaw moves straight down during voluntary opening of the mouth. If only the right lateral pterygoid is working the right side of the jaw will be pulled forward during opening and the chin will deviate to the left. If only the left lateral pterygoid is working, the chin deviates to the right upon opening of the mouth. Tests for strength of the temporalis, masseter, and pterygoids are made in all routine examinations. The examiner places one hand over the left temporalis and the other hand over the right temporalis and then asks the patient to clench his or her teeth. An assessment is made about the degree to which one side may be contracting less strongly than the other. The test is repeated with the examiner's fingers placed over each masseter. If the patient has a complete molar dentition, a tongue depressor may be placed sideways in the mouth between the upper and lower teeth. The patient is asked to bite down and relax. The examiner removes the tongue depressor and assesses if the impressions made by the teeth are equally deep on both right and left sides. The lateral pterygoid, medial pterygoid, and superficial masseter, when acting together on one side, protract that side and cause the jaw to deviate toward the opposite side. The left protractors push the chin toward the right; the right protractors push the chin to the left. If the examiner places a hand on the right side of the chin and attempts to push the jaw to the left, the patient must use the left protractors to resist this. If the examiner places a hand on the left side of the chin and attempts to push the jaw to the right, the right muscles must be used to resist this. By asking the patient to resist such pushes on the jaw, an assessment of strength of the jaw protractors on one side compared with those on the other may be made. Trigeminal neuralgia (TN) Also known as Tic Douloureux, is considered to be one of the most painful afflictions known to medical practice. It is a disorder of the fifth cranial (trigeminal) nerve. The typical or “classic” form of the disorder (called TN1) causes extreme, intermittent, sudden burning facial pain in the areas of distribution of the trigeminal nerve – lips, eyes, nose, scalp, forehead, upper jaw, and lower jaw. The pain episodes last from a few seconds to as long as two minutes. These attacks can occur in quick succession, in cascades lasting as long as two hours. The “atypical” form of the disorder (called TN2), is characterized by constant aching, burning, stabbing pain of somewhat lower intensity than TN1. Both forms of pain may occur in the same person, sometimes at the same time.