20 December 2012
The Differences and Similarities of Classical and Non-classical Pathways
The organization of the classical ascending sensory pathways for hearing, vision, touch and taste has many similarities, but differs with regard to the complexity of the pathways. The auditory and the visual systems are the most complex and those of somesthesia and taste are the least complex of the sensory systems. The nerve fibers from the sensory organs terminate on cells in nuclei that are located on the same side of the head/body. The ascending olfactory pathway reaches parts of the CNS that are different from those of the four other senses. The olfactory pathways mainly project to allocortex and have abundant connections to structures of the limbic system, mainly the medial and the central nucleus of the amygdala. The neural connections of the olfactory sensory cells that are located in the olfactory bulb have similarities with that of the retina. Like in the retina, two different paths can be identified namely, a vertical path that leads from the olfactory receptors to the cells that are the output cells of the olfactory bulb and which connect to the CNS, and a horizontal path that consists of the periglomerular cells that connect between the receptor cells, and the granular cells that connect between the output cells.
Much less is known about the non-classical pathways that the classical pathways. The best known non-classical pathways are those of the auditory system. The non-classical ascending sensory pathways for touch, hearing, vision, and taste receive input from the classical pathways and these connections branch off the classical pathways at various levels of the ascending classical pathways. The anteriorlateral tracts of the somatosensory system are the pain pathways and some of these pathways have similarities with the non-classical pathways in the auditory system. In the auditory system there is a clear branching of ascending information in this...