Point
3
Intro
I am sorry to say that the spinal tract and spinal nucleus V are not exclusively associated with C.N. V. It is also associated with C.N.s VII (facial), IX (glossopharyngeal) and X (vagus). While this makes the story a little confusing, it also makes a lot of sense! The pain and temperature fibers associated with C.N. VII innervate the skin of the external ear, the wall of the external auditory meatus and the outer surface of the tympanic membrane (if it's OK with you, I'll lump these together as "EAR"). These fibers are the peripheral processes of cells that lie in the GENICULATE ganglion (located in the facial canal). The central processes of these neurons enter the brain with C.N. VII (at pontine levels, caudal to the trigeminal), travel in spinal tract V and end in spinal nucleus V. The pain and temperature information is then conveyed rostrally in the TTT (trigeminothalamic tract) to reach the VPM, from which it is relayed to somatosensory cortex (areas 3,1 and 2). Thus the pain and temperature fibers of C.N. VII don't have their "own" central cell group, but instead use that of the trigeminal.
Like C.N. VII, C.N. IX is involved in the pain and temperature innervation of the "EAR". In addition, C.N. IX conveys pain and temperature from the posterior one-third of the tongue, the auditory tube and the upper part of the pharynx (touching of the pharyngeal wall results in a gag reflex which we will cover in Point 9; Nucleus Ambiguus). The cell bodies of these pain and temperature axons associated with C.N. IX lie in the relatively small SUPERIOR GANGLION IX (located just outside of the jugular foramen; you will hear about the larger inferior ganglion later). The central processes of cells in the superior ganglion enter the brain with C.N. IX (at the lateral medulla), then enter our friend spinal tract V and synapse in the caudal spinal nucleus V. You know the route from here.
FINALLY (whew!) C.N. X innervates the
"EAR" (along with C.N.s VII and IX). In addition,
pain and temperature from the lower pharynx, the larynx and
the upper esophagus are conveyed by C.N. X. The cell bodies
of these pain and temperature axons lie in the
SUPERIOR GANGLION X (located just outside the
jugular foramen). The central processes of cells in the
superior ganglion X pass into the brain at the lateral
medulla, enter our old friend spinal tract V and synapse in
the caudal spinal nucleus V. I know you can take it from
here up to cortex via the thalamus!
As mentioned above, pain fibers end only in the caudal spinal nucleus V. You should also be aware of the fact that not much is known about the other parts of the spinal trigeminal nucleus (nucleus oralis and interpolaris). The important fact is that the great majority of the pain and temperature fibers pass to the most caudal portion of spinal nucleus V. This means that lesions anywhere along the rostrocaudal extent of the spinal tract V, as well as in the caudal spinal itself, will result in deficits in pain and temperature from the ipsilateral face etc. Finally, you have probably heard of tic douloureux or trigeminal neuralgia, which is a disorder characterized by brief attacks of severe pain within the distribution of one or more divisions of the trigeminal nerve. Sometimes, section of the spinal nucleus and tract V in the caudal medulla relieves these symptoms.