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If someone is asked, “which muscle is the most powerful/strongest of all the muscles of the human body?” The answer of most people (especially those who don’t have medical or anatomical knowledge) will be biceps (muscle of the forearm) or some other muscle from the lower limb, like the quadriceps. To your surprise, this is one of the muscles of our mouth, the masseter, the chief chewing muscle, based on the comparison with other muscles of the same size. The power of the masseter is estimated to be about the force exerted by 200 pounds of weight on the molars! It elevates the lower jaw and helps in chewing. This muscle is innervated by the largest cranial nerve of the body, the trigeminal nerve (CN V). In the following text, we shall learn about the trigeminal nerve’s origin, course, classification, functional components, and divisions. We shall have a look at trigeminal nuclei, trigeminal lemniscus, and functions of the trigeminal nerve. Lastly, some diseases related to the trigeminal nerve and their treatment will be discussed.
As the name implies, cranial nerves are the nerves that supply sensor and motor innervation to the structures present in the head and neck region of the body. There are 12 pairs of these cranial nerves. The trigeminal nerve is the fifth cranial nerve, and it is the chief sensory nerve of the forehead and face area. The motor part of the trigeminal nerve provides motor innervation to the muscles of mastication and some other muscles. It also carries afferent information in reflexes like the jaw jerk reflex or corneal reflex. It is the largest cranial nerve based on its size. According to length, the Vagus nerve (CN X) is the longest cranial nerve. The word trigeminal implies that it is formed by the three big divisions (discussed later).
Classification and Functional Components
The trigeminal nerve is a mixed nerve and contains general somatic afferents and special visceral afferents. The first two divisions, i.e., ophthalmic and maxillary, contain only general somatic afferent fibers. The mandibular division contains both sensory afferents and visceral motor efferents.
Origin, Course, and Divisions
The trigeminal nerve emerges from the brain stem at the anterior aspect of the pons in the posterior cranial fossa by two roots; the large sensory root and a small motor root. It passes forwards over the petrous part of the temporal bone in the middle cranial fossa, where it ends in a crescent-shaped ganglion, the trigeminal ganglion. This ganglion is present in a pouch of dura mate, the outermost meninge of the brain; this pouch is known as Meckel cave. From the ganglion, the parent nerve is divided into three divisions; ophthalmic, maxillary, and mandibular.
The ophthalmic nerve arises from the trigeminal ganglion; it leaves the skull and enters the orbital cavity through the superior orbital fissure. It ends by giving different branches; supratrochlear and infratrochlear nerves, supraorbital nerve, lacrimal nerve, etc. It contains only sensory fibers and supplies cutaneous innervation to the area of the forehead, conjunctiva of the eye, cornea, upper eyelid, side of the nose, nasal cavity, mucous membrane of the paranasal sinuses, etc. The ophthalmic division is the smallest of all three divisions of the trigeminal nerve.
The maxillary division is the second division of the trigeminal nerve arising from the trigeminal ganglion. The maxillary division also contains only sensory fibers, just like the ophthalmic nerve. It leaves the skull through the foramen rotundum and provides cutaneous innervation to the lower eyelid, skin over the part of the cheek, part of the nose, and upper lip; all these areas are related/close to the maxilla (justifying its name). it also innervates the mucous membranes of the nose, maxillary sinus, and palate. Its branches include zygomaticotemporal, zygomaticofacial, and infraorbital nerve (Remember; supraorbital nerve is the branch of ophthalmic division.
The third and largest division of the trigeminal nerve arising from the trigeminal ganglion is the mandibular division. It is a mixed nerve as it contains both sensory and motor fibers. It leaves the cranial cavity via the foramen ovale. It collects sensory information from the skin over the mandible, part of the cheek, auricle, and temple, as well as from the mucous membrane of the mouth and anterior part of the tongue (only general sensations). Motor information is provided to the muscles of mastication and some other muscles, i.e., tensor vali palatini, tensor tympani, etc. The mandibular division also supplies the temporomandibular joint.
Nuclei of the Trigeminal Nerve and Trigeminal System
The nuclei of cranial nerves are the collection of cell bodies of axons forming that cranial nerve. There are four nuclei of the trigeminal nerve. These include the main sensory nucleus, mesencephalic nucleus, spinal nucleus, and the trigeminal nerve motor nucleus. All four nuclei collectively constitute the trigeminal system.
Main Sensory Nucleus
This nucleus is present in the pons. The sensations of touch and pressure are relayed here. The fibers coming here from the periphery have cell bodies in the trigeminal ganglion.
The mesencephalic nucleus is present in the mid-brain and extends inferiorly to the main sensory nucleus. This nucleus is concerned with the proprioceptive information coming from the muscles of mastication, facial muscles, and extraocular muscles. These proprioceptive fibers have their cell bodies in the mesencephalic nucleus and not in the trigeminal nucleus, bypassing the trigeminal ganglion.
It is in the form of a column extending from the main sensory nucleus present in the pons, continuing through the medulla to the upper part of the spinal cord as far as the second cervical segment of the cord. The sensations of pain and temperature coming through the fibers of all three divisions of the trigeminal nerve are relayed here.
Motor nuclei contain cell bodies of axons providing motor innervation to the muscles of mastication and others. It receives impulses from the motor cortex via corticonuclear fibers, the tectum of the midbrain, the red nucleus, and the medial longitudinal fasciculus. It also receives fibers from the mesencephalic nucleus and forms a reflex pathway for muscles of mastication.
From the main sensory nucleus, the spinal nucleus, and the mesencephalic nucleus, the axons of the second-order neurons ascend upward in the form of a neuronal band or bundle named as trigeminal or spinal lemniscus to the thalamus. Thalamus is the main sensory collection center of the brain. From the thalamus, sensory information is directed to the cerebral cortex. Axons of the third-order neurons reach the somatosensory area located in the post-central gyrus of the cerebral hemisphere, where all the general sensory information from the body is received, integrated, and used for making an appropriate and proper response by the motor cortex and also used for decision making and thinking process of the brain.
Functions of the trigeminal nerve include both sensory and motor in nature. Please note that the trigeminal nerve does not contain any special somatic or visceral afferents, so it does not involve in the transmission of special sensations such as taste, smell, or vision, although it supplies mucous membranes of the nose and mouth, anterior part of the tongue and cornea and conjunctiva of the eye.
Sensory Functions include the transmission of general sensory information like touch, pain, temperature (both warmth and cold), pressure, etc., from the skin of the forehead, conjunctiva, skin of the upper eyelid, and side of the nose via the ophthalmic nerve (CN V1). Cutaneous information from the mucous membrane of the nose, maxillary sinus, and upper lip is transmitted via the maxillary nerve (CN V2). Sensory information from the oral cavity, anterior part of the tongue, and skin over the mandible is transmitted through the mandibular nerve (CN V3).
Sensory functions also include the reception of proprioceptive information from the muscles of mastication, facial muscles, and extraocular muscles. This provides information about the position, extent of flexion/extension, the extent of a stretch of muscles, position of the joint, etc.
The motor part of the trigeminal nerve supplies the muscles of mastication. Muscles of mastication include masseter, temporalis medial pterygoid, and lateral pterygoid muscle. These muscles produce movements at the temporomandibular joint (the only moveable joint of the skull). These movements include protraction (forward motion of mandible), retraction (backward motion of mandible), elevation and depression of the mandible, and side-to-side rotatory movement of the mandible. These muscles not only help in chewing, grinding, and mixing food but also helps in talking, etc.
Jaw Jerk Reflex
The jaw jerk reflex is used to check the trigeminal nerve. When the lower jaw is stroked gently at the area just under the lower lip or at the chin when the mouth is slightly opened, the lower jaw reflexively moves upward to close the mouth. This is due to the stretch of the masseter tendon, which evokes the stretch reflex. The pathway for the jaw jerk is the trigeminal nerve. The afferent information goes to the mesencephalic nucleus, from where it is sent to the motor nucleus of the trigeminal nerve. The impulse from the motor nucleus causes the masseter to contract to result in elevation of the mandible, i.e., closure of the mouth. A similar reflex occurs during the mastication process.
The blinking of the eye characterizes the corneal reflex in response to light. When light stimulus touches the cornea or conjunctiva of the eye, it causes blinking of the eye. The afferent information is carried by the general sensory afferent fibers of the ophthalmic division of the trigeminal nerve to its sensory nucleus. From the sensory nucleus, information is passed onto the facial nerve nuclei via the medial longitudinal fasciculus. Motor or efferent signals through the facial nerve cause contraction of the orbicularis oculi muscles, which close the eyes.
Diseases Related to Trigeminal Nerve and Their Management
There are many diseases that affect the nervous system. Some of them are systemic, which affect the nervous system as a whole, and some are localized to particular nerves. We will limit our discussion to the trigeminal nerve and its branches.
Trigeminal Nerve Palsy
Injury of the trigeminal nerve may result in sensory loss from a large part of the face and mucous membranes of the nose, paranasal sinuses, and mouth. In addition, muscles of mastication are paralyzed, resulting in problems with chewing and grinding the food.
Causes of trigeminal nerve palsy are diabetic neuropathy, aneurysm or tumor pressing the nerve, stenosis of the foramen from where the branches of the nerve leave the skull, inflammation (neuritis), multiple sclerosis or vascular diseases resulting in ischemia, etc. Trauma to the head can result in crush injury of the nerve. This can occur especially to the ophthalmic nerve due to road traffic accidents or falls resulting in orbital fractures.
Trigeminal nerves can be clinically tested by a different method. For ophthalmic division, the corneal reflex is checked using cotton to stimulate the conjunctiva. This reflex is absent in lesions involving the ophthalmic division. The motor part of the trigeminal nerve is tested by asking the subject to clench his/her teeth. Failure to do so indicates trigeminal nerve palsy.
Acute severe stabbing or burning pain on the face in the area supplied by the 2nd and 3rd divisions of the trigeminal nerve. This is also known as tic douloureux. The pain is episodic and completely subsides when the acute attack is over. There is no facial muscle dysfunction. Usually, the pain is unilateral. The cause of trigeminal neuralgia is unknown yet. Pain can be triggered by talking, chewing, or even by a light touch on the face. Tic douloureux is one of the most painful conditions in which pain occurs. Treating trigeminal neuralgia involves resectioning one or more divisions of the trigeminal nerve. Pharmacological interventions include the use of NSAIDs and other analgesics.
Trigeminal Nerve Block
Nerve block means anesthetizing a nerve. A trigeminal nerve block is a procedure used in routine clinical practice. This block is performed to treat trigeminal neuralgia or other conditions in which pain suppression from the area of supply if trigeminal nerve is required. This results in local anesthesia and analgesia in that area. In extremely painful conditions, a trigeminal nerve block can be performed to relieve that pain, but this is a temporary remedy, and treatment of the underlying cause is required to dissolve the pain fully.
The trigeminal nerve is the fifth cranial nerve (CN V). It is a mixed nerve that contains general somatic afferents and special visceral efferents as its functional components.
The trigeminal nerve emerges from the mid-brain at its anterior aspect at the level of the pons. It passes forward in the posterior cranial fossa, rests on the petrous temporal bone in the middle cranial fossa, and ends at a crescent-shaped ganglion, the trigeminal ganglion, located in a pouch of dura mater. Here, it divides into three divisions; 1) Ophthalmic nerve (CN V1), which leaves the skull through the superior orbital fissure, 2) Maxillary nerve (CN V2), which leaves the skull through the foramen rotundum, 3) Mandibular division (CN V3) which leaves the cranial cavity via foramen ovale. There are four nuclei of the trigeminal nerve; the main sensory nucleus, mesencephalic nucleus, spinal nucleus, and motor nucleus.
The ophthalmic division is only sensory and supplies conjunctiva, cornea, the skin of the forehead, paranasal sinuses, and the side of the nose. The maxillary division is also sensory and supplies part of the cheek, mucous membrane of the nose, maxillary sinus, palate, and area over the upper lip. The mandibular division contains both sensory and motor fibers. Sensory fibers receive information from the skin over the mandible, part of the cheek, the mouth’s mucous membrane, and the tongue’s anterior part. Motor fibers innervate muscles of mastication, tensor vali palatini, tensor tympani, etc. Sensations carried by the trigeminal nerve include pain, touch, pressure, temperature, and proprioception.
The trigeminal nerve is also involved in some reflexes, such as the corneal reflex and jaw jerk reflex.
Diseases related to the trigeminal nerve include trigeminal nerve palsy and trigeminal neuralgia. Causes include inflammation (neuritis), pressure due to tumors, ischemic injury, traumatic injury, exposure to toxins or drugs, and demyelinating diseases like multiple sclerosis, diabetic neuropathy, etc. Lesions of the midbrain involving the spinal lemniscus may result in the same signs and symptoms. Treatment involves the solution of the underlying cause by using either medications or surgical interventions.
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