Nervo trigemino: differenze tra le versioni

Versione delle 14:31, 27 apr 2008

Versione delle 19:54, 29 apr 2008

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==Peripheral Anatomy==

[[ImageImmagine:Gray778_Trigeminal.png|left|440px]]

The '''trigeminal nerve''' is the largest of the [[cranial nerves]]. Its name derives from the fact that it has three major branches: the '''[[ophthalmic nerve]]''' (V₁), the '''[[maxillary nerve]]''' (V₂) and the '''[[mandibular nerve]]''' (V₃). The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both sensory and motor functions.

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===Sensory Branches of the Trigeminal Nerve===

The ophthalmic, maxillary and mandibular branches leave the skull through three separate [[foramina]]: the [[superior orbital fissure]], the [[foramen rotundum]] and the [[foramen ovale]]. The [[mnemonic]] ''<u>s</u>tanding <u>r</u>oom <u>o</u>nly'' can be used to remember that V₁ passes through the <u>s</u>uperior orbital fissure, V₂ through the foramen <u>r</u>otundum, and V₃ through the foramen <u>o</u>vale. [[ImageImmagine:Gray784.png|thumb|right|420px|Dermatome Distribution of the Trigeminal Nerve]]

The '''ophthalmic''' nerve carries sensory information from the scalp and forehead, the upper eyelid, the conjunctiva and cornea of the eye, the nose (including the tip of the nose), the nasal mucosa, the frontal sinuses and parts of the [[meninges]] (the [[dura]] and blood vessels).

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Anatomically, each pathway consists of three bundles of nerve fibers, connected together in series:

[[ImageImmagine:Sensory_Pathways_III.png|center]]

Remarkably, the ''secondary'' neurons in each pathway '''decussate''' (cross to the other side of the spinal cord or brainstem). The reason for this is unknown.

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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 by input from other areas of the nervous system. For example, cells in the main trigeminal nucleus (“Main V” in the diagram) receive input (not shown) from the reticular formation and from the cerebral cortex. This information contributes to the final output of the cells in Main V to the thalamus.

[[ImageImmagine:Touch Pain Pathways.png|center]]

Touch/position information from the body is carried to the thalamus by the '''medial lemniscus'''; touch/position information from the face is carried to the thalamus by the '''trigeminal lemniscus'''. Pain/temperature information from the body is carried to the thalamus by the '''spinothalamic tract'''; pain/temperature information from the face is carried to the thalamus by the '''trigeminothalamic tract''' (also called the '''quintothalamic tract''').

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==Trigeminal Nucleus==

[[ImageImmagine:Gray696_Trigeminal.png||thumb|right|300px|Brainstem Nuclei: Red = Motor; Blue = Sensory; Dark Blue = Trigeminal Nucleus]]

It is not widely appreciated that ''all'' sensory information from the face (all touch/position information and all pain/temperature information) is sent to the trigeminal nucleus. In classical anatomy, most sensory information from the face is carried by the fifth nerve, but sensation from certain parts of the mouth, certain parts of the ear and certain parts of the meninges is carried by “general somatic afferent” fibers in cranial nerves VII (the [[facial nerve]]), IX (the [[glossopharyngeal nerve]]) and X (the [[vagus nerve]]).

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====Somatotopic Representation====

Exactly how pain/temperature fibers from the face are distributed to the spinal trigeminal nucleus has been a subject of considerable controversy. The present understanding is that ''all'' pain/temperature information from ''all'' areas of the human body is represented (in the spinal cord and brainstem) in an ascending, caudal-to-rostral fashion. Information from the lower extremities is represented in the lumbar cord. Information from the upper extremities is represented in the thoracic cord. Information from the neck and the back of the head is represented in the cervical cord. Information from the face and mouth is represented in the spinal trigeminal nucleus. [[ImageImmagine:Onion_Skin.png|thumb|right|170px|Onion Skin Distribution of the Trigeminal Nerve]]

Within the spinal trigeminal nucleus, information is represented in an '''onion skin''' fashion. The lowest levels of the nucleus (in the upper cervical cord and lower medulla) represent peripheral areas of the face (the scalp, ears and chin). Higher levels (in the upper medulla) represent more central areas (nose, cheeks, lips). The highest levels (in the pons) represent the mouth, teeth, and pharyngeal cavity.

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===Touch/Position Sensation===

[[ImageImmagine:Sensory_Homunculus.png|thumb|right|250px|The Sensory Homunculus (Updated)]] Touch/position information from the body is sent to the '''ventral posterolateral''' ('''VPL''') nucleus of the thalamus. Touch/position information from the face is sent to the '''ventral posteromedial''' ('''VPM''') nucleus of the thalamus. From the VPL and VPM, information is projected to the '''[[primary sensory cortex]]''' ('''SI''') in the postcentral gyrus of the [[parietal lobe]].

The representation of sensory information in SI is organized '''somatotopically'''. Adjacent areas in the body are represented by adjacent areas in the cortex. When body parts are drawn proportion to the density of their innervation, however, the result is a strangely distorted “little man,” the '''sensory homunculus'''.