The physical and psychological factors of nociception and pain perception in humans: A review. University of Vermont
The direct experience of pain is often defined via the conscious, as the degree to which we feel pain is based purely on psychological perception. There is, however, a physical component to pain, as pain perception relies on a stimulus and the transmission of the signal this stimulus produces. Inclusively, the transmission of signals following stimuli and the resulting sensory activity is known as nociception (FURST, 1999). Pain perception refers to the conscious processing and interpreting of these signals (BALDO, 1999). Recent advances in functional brain imaging and anatomical methods in animal studies have allowed researchers to examine the physical aspect of nociception on a neurological level, especially regarding the active components of the cerebral cortex within the nociceptive system. Here, there are two major somatosensory pathways working simultaneously. These pathways are broadly known as the lateral and medial pain systems (MELZACK, 1990). Both pathways work closely with the hypothalamus, which has exhibited a crucial role in pathway and cortex communication during nociception (MATHARU, 2007). These three components are active with pain that results from physical activation of tissues as well as pain that occurs without any peripheral physical input, such as psychogenic pain (BINZER et al., 2003). This suggests that the interaction between the somatosensory systems and the hypothalamus offers a connection between the physical aspect and psychological aspects of pain. Therefore, in order to understand the perception of pain, one must establish an understanding of these systems, but more so the interaction between them. Furthermore, by examining the abnormalities in pain perception – such as chronic psychogenic pain and phantom limb syndrome – we can develop a greater understanding of the nociceptive role of these sensory and hypothalamic pathways as well as their ability to alter our perception. This understanding will be crucial in developing analgesic therapies and treatments.
First, it is important to understand the nociceptive process. This physical pain process begins in the nociceptors. These are differentiated receptors found in free afferent nerve endings (FURST, 1999). In terms of synapse orders, these neurons are typically first-order neurons, distributed throughout the skin, vessels, muscles, joints and viscera (JULIUS et al., 2001). These are activated by a noxious stimulus bridging an electrical potential threshold (BURGESS et al., 1967; MILLAN, 1999). They are divided into three classes. Mechanoreceptors are sensitive to intense mechanical and motion stimuli, observed through the tension of tissues. Thermoceptors, on the other hand, are specifically sensitive to thermal stimuli. Then, polymodal nociceptors are inclusively sensitive to mechanical, thermal and chemical stimuli, as well as inflammation sensations (TEIXEIRA et al., 1994; BESSON, 1999). All of these receptors show an increased sensitivity to continuous stimulation (CHEN,1996; LIEBESKIND, 1976). These stimuli promote a local release of chemical mediators - hystamine, serotonin, acetylcholine – that alter the membrane permeability of the nerve and allow for the propagation of an action potential (JULIUS et al., 2001).
The first-order neurons synapse along the spinal cord via afferent sensory fibers. A-beta fibers are slower conducting, myelinated fibers that respond to tactile stimuli. A-delta fibers, on the other hand, are myelinated fibers responsible for the fast conduction of painful stimuli. C fibers are unmyelinated and responsible for a slower form of transmission. C fibers are possibly the most active and prominent of the spinal fibers (FURST, 1999; GRUBB, 1998; SHELLEY et al., 1994). Pain seems to come in two steps: first, a quick and sharp pain, which is followed by a slow and dull...
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