Trigeminal nerve
In neuroanatomy, the trigeminal nerve, also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for sensation in the face and motor functions such as biting and chewing; it is the most complex of the cranial nerves. Its name derives from each of the two nerves having three major branches: the ophthalmic nerve, the maxillary nerve, and the mandibular nerve. The ophthalmic and maxillary nerves are purely sensory, whereas the mandibular nerve supplies motor as well as sensory functions. Adding to the complexity of this nerve is that autonomic nerve fibers as well as special sensory fibers are contained within it.
The motor division of the trigeminal nerve derives from the basal plate of the embryonic pons, and the sensory division originates in the cranial neural crest. Sensory information from the face and body is processed by parallel pathways in the central nervous system.
Structure
Origin
From the trigeminal ganglion, a single, large sensory root enters the brainstem at the level of the pons. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons slightly rostrally and medially to the sensory root.Motor fibers pass through the trigeminal ganglion without synapsing on their way to peripheral muscles, their cell bodies being located in the nucleus of the fifth nerve, deep within the pons.
Trigeminal ganglion
The three major branches of the trigeminal nerve—the ophthalmic nerve, the maxillary nerve and the mandibular nerve —converge on the trigeminal ganglion, located within Meckel's cave and containing the cell bodies of incoming sensory-nerve fibers. The trigeminal ganglion is analogous to the dorsal root ganglia of the spinal cord, which contain the cell bodies of incoming sensory fibers from the rest of the body.Sensory branches
The ophthalmic, maxillary and mandibular branches leave the skull through three separate foramina: the superior orbital fissure, the foramen rotundum and the foramen ovale, respectively. The ophthalmic nerve carries sensory information from the scalp and forehead, the upper eyelid, the conjunctiva and cornea of the eye, the nose, the nasal mucosa, the frontal sinuses and parts of the meninges. The maxillary nerve carries sensory information from the lower eyelid and cheek, the nares and upper lip, the upper teeth and gums, the nasal mucosa, the palate and roof of the pharynx, the maxillary, ethmoid and sphenoid sinuses and parts of the meninges. The mandibular nerve carries sensory information from the lower lip, the lower teeth and gums, the chin and jaw, parts of the external ear and parts of the meninges. The mandibular nerve carries touch-position and pain-temperature sensations from the mouth. Although it does not carry taste sensation, one of its branches—the lingual nerve—carries sensation from the tongue.The peripheral processes of mesencephalic nucleus of V neurons run in the motor root of the trigeminal nerve and terminate in the muscle spindles in the muscles of mastication. They are proprioceptive fibers, conveying information regarding the location of the masticatory muscles. The central processes of mesencephalic V neurons synapse in the motor nucleus V.
Dermatomes
The areas of cutaneous distribution of the three sensory branches of the trigeminal nerve have sharp borders with relatively little overlap. The injection of a local anesthetic, such as lidocaine, results in the complete loss of sensation from well-defined areas of the face and mouth. For example, teeth on one side of the jaw can be numbed by injecting the mandibular nerve. Occasionally, injury or disease processes may affect two branches of the trigeminal nerve; in these cases, the involved branches may be termed:- V1/V2 distribution – Referring to the ophthalmic and maxillary branches
- V2/V3 distribution – Referring to the maxillary and mandibular branches
- V1-V3 distribution – Referring to all three branches
Function
The sensory function of the trigeminal nerve is to provide tactile, proprioceptive, and nociceptive afference to the face and mouth. Its motor function activates the muscles of mastication, the tensor tympani, tensor veli palatini, mylohyoid and the anterior belly of the digastric.The trigeminal nerve carries general somatic afferent fibers, which innervate the skin of the face via ophthalmic, maxillary and mandibular divisions. The trigeminal nerve also carries special visceral efferent axons, which innervate the muscles of mastication via the mandibular division.
Muscles
The motor component of the mandibular division of the trigeminal nerve controls the movement of eight muscles, including the four muscles of mastication: the masseter, the temporal muscle, and the medial and lateral pterygoids. The other four muscles are the tensor veli palatini, the mylohyoid, the anterior belly of the digastric and the tensor tympani.With the exception of the tensor tympani, all these muscles are involved in biting, chewing and swallowing and all have bilateral cortical representation. A unilateral central lesion, no matter how large, is unlikely to produce an observable deficit. Injury to a peripheral nerve can cause paralysis of muscles on one side of the jaw, with the jaw deviating towards the paralyzed side when it opens. This direction of the mandible is due to the action of the functioning pterygoids on the opposite side.
Sensation
The two basic types of sensation are touch-position and pain-temperature. Touch-position input comes to attention immediately, but pain-temperature input reaches the level of consciousness after a delay; when a person steps on a pin, the awareness of stepping on something is immediate but the pain associated with it is delayed.Touch-position information is generally carried by myelinated nerve fibers, and pain-temperature information by unmyelinated fibers. The primary sensory receptors for touch-position are structurally more complex than those for pain-temperature, which are nerve endings.
Sensation in this context refers to the conscious perception of touch-position and pain-temperature information, rather than the special senses processed by different cranial nerves and sent to the cerebral cortex through different pathways. The perception of magnetic fields, electrical fields, low-frequency vibrations and infrared radiation by some nonhuman vertebrates is processed by their equivalent of the fifth cranial nerve.
Touch in this context refers to the perception of detailed, localized tactile information, such as two-point discrimination or the difference between coarse, medium or fine sandpaper. People without touch-position perception can feel the surface of their bodies and perceive touch in a broad sense, but they lack perceptual detail.
Position, in this context, refers to conscious proprioception. Proprioceptors provide information about joint position and muscle movement. Although much of this information is processed at an unconscious level, some is available at a conscious level.
Touch-position and pain-temperature sensations are processed by different pathways in the central nervous system. This hard-wired distinction is maintained up to the cerebral cortex. Within the cerebral cortex, sensations are linked with other cortical areas.
Sensory pathways
Sensory pathways from the periphery to the cortex are separate for touch-position and pain-temperature sensations. All sensory information is sent to specific nuclei in the thalamus. Thalamic nuclei, in turn, send information to specific areas in the cerebral cortex. Each pathway consists of three bundles of nerve fibers connected in series:The secondary neurons in each pathway decussate, because the spinal cord develops in segments. Decussated fibers later reach and connect these segments with the higher centers. The optic chiasm is the primary cause of decussation; nasal fibers of the optic nerve cross to keep the interneuronal connections responsible for processing information short. All sensory and motor pathways converge and diverge to the contralateral hemisphere.
Although sensory pathways are often depicted as chains of individual neurons connected in series, this is an oversimplification. Sensory information is processed and modified at each level in the chain by interneurons and input from other areas of the nervous system. For example, cells in the main trigeminal nucleus receive input from the reticular formation and cerebellar cortex. This information contributes to the final output of the cells in Main V to the thalamus.
File:Touch Pain Pathways.png|thumb|600px|center|alt=Text-and-line diagram of sensory-nerve pathways|C = Cervical segment, S = Sacral segment, VPL = Ventral posterolateral nucleus, SI = Primary somatosensory cortex, VM = Ventromedial prefrontal cortex, MD = Medial dorsal thalamic nucleus, IL = Intralaminar nucleus, VPM = Ventral posteromedial nucleus, Main V = Main trigeminal nucleus, Spinal V = Spinal trigeminal nucleus
Touch-position information from the body is carried to the thalamus by the medial lemniscus, and from the face by the trigeminal lemniscus. Pain-temperature information from the body is carried to the thalamus by the spinothalamic tract, and from the face by the anterior division of the trigeminal lemniscus.
Pathways for touch-position and pain-temperature sensations from the face and body merge in the brainstem, and touch-position and pain-temperature sensory maps of the entire body are projected onto the thalamus. From the thalamus, touch-position and pain-temperature information is projected onto the cerebral cortex.
Summary
The complex processing of pain-temperature information in the thalamus and cerebral cortex reflects a phylogenetically older, more primitive sensory system. The detailed information received from peripheral touch-position receptors is superimposed on a background of awareness, memory and emotions partially set by peripheral pain-temperature receptors.Although thresholds for touch-position perception are relatively easy to measure, those for pain-temperature perception are difficult to define and measure. "Touch" is an objective sensation, but "pain" is an individualized sensation which varies among different people and is conditioned by memory and emotion. Anatomical differences between the pathways for touch-position perception and pain-temperature sensation help explain why pain, especially chronic pain, is difficult to manage.