TRIGEMINAL NERVE (V)

The trigeminal nerve is the General Somatic Afferent (GSA) nerve of the face, oral cavity, nasal cavity and the dura of the anterior and middle cranial fossae. 

The perioral region is bilaterally innervated. 

Parts of the trigeminal complex are analogous to the Spinothalamic Tract and convey protopathic sensations, whereas other parts are analogous to the DCML and convey epicritic sensations

Divisions of the Trigeminal Nerve

Ophthalmic (S)  -  upper 1/3 face

 Maxillary  (S) -  middle 1/3 face, nasal & upper oral        cavity

Mandibular (S&M) - (S) lower 1/3 face, lower oral  cavity and temporomandibular joint

(M) mastication & MATT muscles

Three Neurons of Trigeminal Nerve

1^o Sensory - Mesencephalic Nucleus  Contains only proprioceptive neurons and is the only brainstem nucleus containing primary sensory neurons and therefore analogous to peripheral ganglia.

          Trigeminal Ganglion  Contains the primary sensory neurons except for proprioceptive neurons in the mesencephalic nucleus.  

Some proprioceptive neurons (innervating extraocular muscles, temporomandibular joint, periodontal ligaments) are located in the trigeminal ganglion.

2^o Sensory - Main (Chief) Nucleus (mediates epicritic sensations)

          It consists of Dorsomedial and Ventrolateral subdivisions.

          Spinal Trigeminal Nucleus (mediates protopathic sensations)

          This nucleus is continuous with the spinal cord dorsal horn and has the same  laminar organization (posteromarginal nucleus, substantia gelatinosa, magnocellular nucleus).  Rostrocaudal subdivisions are the Oral, Interpolar and Caudal parts.  The caudal part receives all of the facial and nasopharyngeal pain and temperature fibers.  GSA fibers of nerves VII, IX & X also synapse in the spinal nucleus.  For this reason, constant pain in one ear with no evidence of middle ear disease may suggest that the pain is referred from disease in the pharynx.

3^o Sensory - Ventral Posteromedial Nucleus (VPM) This is the main thalamic projection of the trigeminal system and mediates discriminative touch, vibratory sense, proprioception and fast, well-localized pain.

Motor Nucleus

 consists of lower motor neurons that innervate the mastication muscles. 

A related area of the RF, the Supratrigeminal nucleus, is a pattern generator for mastication.  In sleep, this nucleus activates the lateral pterygoids so the pharynx is not occluded by the tongue.  General anesthesia inactivates the supratrigeminal nucleus.

Projections of Touch Affarent Secondary Sensory Neurons

 Secondary fibers of the spinal trigeminal and Ventrolateral subdivision of the main nucleus cross the midline and ascend to form the contralateral Ventral Trigeminothalamic Tract which comes to lie on the dorsal surface of the medial lemniscus, and synapses in the VPM thalamic nucleus.

    From the Dorsomedial subdivision of the main nucleus - an ipsilateral Dorsal Trigeminothalamic Tract projects to VPM (This tract carries epicritic information from the oral cavity only.) From the spinal nucleus - fibers project not only to VPM but also to PO, the Intralaminar thalamic nuclei and the brainstem RF.

Pain Afferents:

    All primary pain fibers turn caudally in the Spinal Trigeminal Tract and synapse in the Spinal Trigeminal Nucleus (caudal part).  Pain afferents from the midline of the face synapse rostrally, and those from the side of the head caudally in this part of the nucleus.