Figure 1: Schematic Representation of the Pain Theories
Note: From Ju Chen, 2011. Recreated under permission granted by the Creative Commons Attribution 4.0 International License of the Neuroscience Bulletin .
Specificity theory of pain The specificity theory refers to the presence of dedicated pathways for each somatosensory modality. The fundamental tenet of the specificity theory is that each modality has a specific receptor and associated sensory fiber (primary afferent) that is sensitive to one specific stimulus (Trachsel et al., 2022). For instance, the model proposes that non-noxious mechanical stimuli are encoded by low-threshold mechanoreceptors, which are associated with dedicated primary afferents that project to “mechanoreceptive” second-order neurons in the spinal cord or brainstem (depending on the source of the input). These second- order neurons project to “higher” mechanoreceptive areas in the brain. Similarly, noxious stimuli would activate a nociceptor, which would project higher “pain” centers through a pain fiber. These ideas have been emerging over several millennia but were experimentally tested and formally postulated as a theory in the 19th century by physiologists in Western Europe. René Descartes was one of the first Western philosophers to describe a detailed somatosensory pathway in humans. Descartes’ manuscript, Treatise of Man , describes pain as a perception that exists in the brain and makes the distinction between the neural phenomenon of sensory transduction (today known as nociception ) and the perceptual experience of pain. An essential part of Descartes’ theory that resonates properly with modern pain theories is his description of nerves, which he perceived as hollow tubules that convey both sensory and motor information. This understanding of neural function was influenced by earlier research submissions of the third
century BCE—Herophilus demonstrated the existence of sensory and motor nerves, and Erasistratus demonstrated that the brain influenced motor activity. Over a millennium later, Galen demonstrated that sectioning the spinal cord caused sensory and motor deficits. Following different confirmatory findings to check this demonstration, anatomical studies by Vesalius published in 1543 reiterated and finally confirmed Galen’s findings. Galen’s findings, in addition to supporting the specificity theory of pain, also described the conditions necessary for perception. These conditions include: 1. An organ must be able to receive the stimulus. 2. There must be a connection from the organ to the brain. 3. A processing center that converts the sensation to a conscious perception must exist. Descartes contributed to Galen’s model by postulating that a gate existed between the brain and the tubular structures (the connections), which was opened by a sensory cue. A sensory cue would “tug” on the tube, which would then open a gate between the tube and the brain. The opening of this gate would then allow “animal spirits” (a crude description of nerve transmissions) to flow through these tubes and within the muscles to move them. This explanation describes the pathway for the prompt movement of the heat of a hot flame from the foot, how it activates a fibril (or fiber) within the nerve tubule that traverses up the leg, and how it moves first to the spinal cord and finally, to the brain. The corresponding motor response, as described, includes turning the head and the eyes to see the flame and raising the hands, and folding the body away from the
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