The peripheral nervous system serves as a medium between external stimuli and the central nervous system. More importantly, it performs the transduction of various thermal, mechanical, electromagnetic, and chemical stimuli into a modality that can be interpreted by the central nervous system, i. e. electrical impulses (Guyton & Hall, 2006, p. 573). One such stimulus crucial to survival is pain. One might ask how the brain differentiates between different stimuli when all it receives are electrical impulses.

The labeled line principle states that the modality of sensation transmitted through electrical impulses by any nerve fiber is determined by the area of the brain to which the impulse is being transmitted (Guyton & Hall, 2006, p. 572). Any signal pathway begins with a receptor. For pain, the receptor is essentially a free nerve ending; ubiquitous in the upper layers of skin, it is also widespread internally, such as in the walls of the gut, periosteum of bones, and surfaces of joints, etc (Snell, 2006, p. 86). The free nerve ending can be excited by three types of stimuli, namely chemical, thermal, and mechanical.

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These free nerve endings have a certain threshold, any stimulus greater than this threshold will elicit a response, i. e. an action potential. An average person feels pain when the skin is heated above 45 degrees Centigrade (Guyton & Hall, 2006, p. 599). Depending on what type of nerve fibers have been stimulated, pain is divided into two categories. Mechanical and chemical stimuli have the ability to elicit fast pain, while all types of stimuli can elicit slow pain (Guyton & Hall, 2006, p. 599).

Fast pain is conducted through delta A type fibers to the central nervous system (Snell, 2006, p.142), and can be felt by the individual within about a tenth of a second after the application of the stimulus (Guyton & Hall, 2006, p. 598). On the other hand, slow pain is conducted by C type fibers (Snell, 2006, p. 142), and is discernible to the individual only after a second, and has a propensity to increase in intensity after the initial stimulus (Guyton & Hall, 2006, p. 598). Before reaching the central nervous system, however, these afferent fibers pass through dorsal sensory ganglia of the spinal nerve roots, and sensory ganglia of the trigeminal, facial, glossopharyngeal and vagal cranial nerves (Snell, 2006, p. 80).

These ganglia are aggregates of cell bodies of neurons to which these fibers belong; these cell bodies are bypassed by the pain signals (Snell, 2006, p. 80). After these ganglia, the fibers enter the spinal cord through the tip of the posterior gray column and divide into ascending and descending branches (Snell, 2006, p. 142). The signals carried by these fibers then encounter the first synapse with a neuron of the central nervous system, i. e. a second-order neuron (Guyton & Hall, 2006, p. 600).

A synapse is the junction between two neurons which enables the transfer of action potentials from one to the other by chemical or electrical means (Snell, 2006, p. 48). Once inside the spinal cord, the pain signals have two major pathways to the brain, namely the neospinothalamic and the paleospinothalamic tracts (Guyton & Hall, 2006, p. 600). The neospinothalamic tract receives signals from the delta A type fibers and, hence, serves as a pathway for fast pain signals (Guyton & Hall, 2006, p. 600).

The second-order neurons give off long fibers that decussate to the opposite side and then move upwards towards the brain (Guyton & Hall, 2006, p. 600). Most of these fibers end up directly in the ventrobasal complex of the thalamus, while a few may terminate in the reticular areas of the brain (Guyton & Hall, 2006, p. 600). Conversely, the paleospinothalamic tract receives signals mainly from the slow pain conducting C type fibers; although it receives signals from some A type fibers as well, it is essentially a pathway for slow pain (Guyton & Hall, 2006, p.601).

The fibers of the paleospinothalamic tract take a convoluted path through the spinal cord before decussating and moving up to the brain (Guyton & Hall, 2006, p. 601). In the brain, these fibers are distributed widely; only a fraction goes straight to the thalamus (Guyton & Hall, 2006, p. 601). The rest are spread over the reticular nuclei, the tectal area of the mesencephalon, and the periaqueductal gray region (Guyton & Hall, 2006, p. 601).

It is interesting to note that signals from slow-pain pathways are also directed towards the reticular areas of the brainstem. These areas have a strong stimulating effect on the activity of the brain, a probable explanation for the difficulties faced by persons suffering from chronic pain in regard to sleep (Guyton & Hall, 2006, p. 601). References Guyton, A. C. , & Hall J. E. (2006). Textbook of Medical Physiology (11th Ed. ). Philadelphia: Saunders. Snell, R. S. (2006). Clinical Neuroanatomy (6th Ed. ) Philadelphia: Lippincott Williams and Wilkins.