Editing Trigeminal nerve (section)

== Function ==
The sensory function of the trigeminal nerve is to provide tactile, [[proprioceptive]], and [[Nociception|nociceptive]] afference to the face and mouth. Its motor function activates the [[muscles of mastication]], the [[tensor tympani]], [[tensor veli palatini]], [[mylohyoid muscle|mylohyoid]] and the [[Digastric muscle#Anterior belly|anterior belly of the digastric]].

The trigeminal nerve carries [[general somatic afferent fibers]] (GSA), which innervate the skin of the face via ophthalmic (V1), maxillary (V2) and mandibular (V3) divisions. The trigeminal nerve also carries [[special visceral efferent]] (SVE) [[axon]]s, which innervate the muscles of mastication via the mandibular (V3) division.

===Muscles===
The motor component of the mandibular division (V3) of the trigeminal nerve controls the movement of eight muscles, including the four [[muscles of mastication]]: the [[Masseter muscle|masseter]], the [[temporal muscle]], and the [[Medial pterygoid muscle|medial]] and [[Lateral pterygoid muscle|lateral pterygoids]]. The other four muscles are the [[tensor veli palatini]], the [[Mylohyoid muscle|mylohyoid]], the anterior belly of the [[digastric muscle|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 [[Cortex (anatomy)|cortical]] representation.  A unilateral central lesion (for example, a [[stroke]]), 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===
{{Main|Somatosensory system}}
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 (fast-conducting) nerve fibers, and pain-temperature information by unmyelinated (slow-conducting) fibers. The primary sensory receptors for touch-position ([[Meissner’s corpuscles|Meissner's corpuscles]], [[Merkel's receptors]], [[Pacinian corpuscles]], [[Ruffini’s corpuscles|Ruffini's corpuscles]], [[hair receptors]], [[muscle spindle organs]] and [[Golgi tendon organs]]) 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]] (smell, sight, taste, hearing and balance) 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]] (the difference between touching one point and two closely spaced points) 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 (muscle spindle and Golgi tendon organs) provide information about joint position and muscle movement. Although much of this information is processed at an unconscious level (primarily by the [[cerebellum]] and the [[Vestibular system|vestibular]] nuclei), 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:

[[File:Sensory Pathways III.png|center|alt=Flow chart from sensory receptors to the cerebral cortex]]
The secondary neurons in each pathway [[decussate]] (cross the spinal cord or brainstem), 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 (so each cerebral hemisphere receives contralateral—opposite—vision) to keep the [[interneuron]]al connections responsible for processing information short. All sensory and motor pathways converge and diverge to the contralateral hemisphere.<ref>Excerpt from [https://archive.org/details/cunninghamstextb00cunn ''Cunningham's Textbook of Anatomy'']</ref>

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 (Main V in the diagram below) 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]] (both the anterior and posterior trigeminothalamic tracts). 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]] (also called the [[anterior trigeminothalamic tract]]).

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 (as opposed to the relatively simple, straightforward processing of touch-position information) reflects a [[Phylogenetics|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.