Pain sensations in the head and face region are conducted to the parietal lobe of the brain through the trigeminal nerve. In this case the trigeminal root ganglion (Gasserian ganglion) is the seat of the first-order neuronal system and is comparable (skin) sensations. The neurons of the trigeminal root ganglion, like those of the dorsal root ganglion, have distal, or peripheral, fibers and proximal, or central, fibers. The peripheral fibers are the incoming sensory fibers from the three divisions of the trigeminal nerve, namely the ophthalmic, maxillary and mandible nerves. The central, or proximal, fibers form the sensory root of the trigeminal nerve and enter the brain stem at the level of the level of the Pons. Soon after their entry, some of the fibers of the sensory root ascend and terminate in the sensory nucleus of the trigeminal nerve. The descending fibers form the spinal tract of the trigeminal nerve and terminate in the nucleus of the spinal tract of the trigeminal nerve and terminate in the nucleus of the spinal tract of the trigeminal nerve.
Pain fibers from the gloss pharyngeal and vogues cranial nerves which serve the tongue (taste), the pharynx, and various internal organs respectively, also join the spinal tract, and these fibers also terminate in the nucleus of the nucleus of the spinal tract of the trigeminal nerve. The second-order neuronal system lies, in this case, in the nucleus of the trigeminal nerve which is situated in the upper three cervical spinal cord segments-the medulla oblongata, the Pons and the mid-brain. This nucleus is a long one whose distal end is in close relation with the dorsal horn cells constituting the second-order neuronal system of the trigeminal nerve lies in the nucleus of the trigeminal nerve. The nucleus, though a continuous group of cells, has been called by three different names at different levels, namely, nucleus of the spinal tract, primary sensory nucleus, and mesencephalic nucleus of the trigeminal nerve. Fibers from these cells cross the mid-line and then ascend as a trigeminal luminal lemniscuses or trigeminal-thalamic tract to terminate in the thalamus. The third-order neuronal system arises from the thalamic neurons and ascends to terminate in the parietal lobe of the brain.
In addition to the two systems (spino-thalamic and trigeminal-thalamic) described above, there is a third ascending sensory pathway called the spino-reticule-thalamic system. It starts in the dorsal horn of the spinal cord and is projected through interneuron to the reticular substance of the medulla oblongata and the mid-brain and then to the thalamus. It is a slow-conducting system and is diffusely projected from the thalamus to the limbic and frontal lobes of the brain. Pain arising from deeper structures is conducted through this system and is poorly localized. This system sub serves the affective-reactive aspects of pain that is the aspects of emotions accompanying or experiencing the sensation of pain and reacting to it physically.
There are also descending sensory fibers from the brain stem structures that have an inhibitory effect on pain. The most important of these is the periaqueductal grey matter in the mid-brain. The fibers from the nuclei of the periaqueductal region, through the nucleus, rape Magnus, of the medulla, are projected to the dorsal horn cells of the spinal cord. Other structures in the descending pathway are the other reticular nuclei and the locus ceruleans. The descending inhibitory pathway uses serotonin and noradrenalin as chemical neurotransmitters in contrast to the ascending sensory system which uses acetylcholine.
