flame for protection. Descartes conceived that there are many of these fibrils and that their movements elicit multiple sensations. The modern concept of a dedicated pain pathway as proposed by the specificity theory was developed by Charles Bell in his landmark essay, ’Idea of a New Anatomy of the Brain.’’ In this submission, Bell provided an alternative perspective on the organization of the nervous system. First, he suggested that the brain is not a “common sensorium,” as suggested by Descartes, which was the accepted model of the brain at the time. Instead, he provided anatomical evidence that the brain was a heterogeneous structure, a theory first postulated by Willis in the 17th century. Bell also suggested that nerves were bundles of heterogeneous neurons that have specialized functions and that their bundling was only for ease of distribution. This idea agrees with later submission proposing different sensory neurons for different types of stimuli, motor neurons, and so-called “vital” neurons that are wired to the mind rather than the brain. Another highlight of Bell’s submission is the idea that the perception of the stimulus, including vision and nociception, is different than the perceptual experience of sight and pain. An extract of Bell’s submission on this theory properly captures this aspect of the course. Bell wrote: while each organ of sense is provided with a capacity of receiving certain changes, yet each is utterly incapable of receiving impression destined for another organ of sensation. It is also very remarkable that an impression made on two different nerves of sense, though with the same instrument will produce two distinct sensations, and the ideas resulting will only have relation to the organ affected. This extract rightly captures the fundamental tenet of the specificity theory, which postulates that there is a dedicated fiber that leads to a dedicated pain pathway to the sensory modality region of the brain. Widely regarded as the father of experimental physiology, François Magendie also made notable contributions to neurophysiology. One of François’ research highlights includes the reiteration of Bell’s findings regarding the existence of both motor and sensory nerves and that these have separate paths to and from the spinal cord (the ventral and dorsal roots, respectively (Holmes & Friese, 2020). This differentiation of spinal nerves was later known as the Bell-Magendie Law —a fundamental aspect of the organization of the nervous system. Furthermore, the discovery of specific, cutaneous touch receptors, such as Pacinian corpuscles, Meissner’s corpuscles, Merkel’s discs, and Ruffini’s end-organs in the latter part of the 19th century provided further evidence that specific sensory qualia were encoded by dedicated nerve fibers. However, there remained a debate about the nature of pain as part of the five senses, as an end-organ specific to pain stimuli (nociceptor) had not yet been discovered. Additional evidence supporting the specificity theory of pain was supported by Schiff and Woroschiloff’s findings of a pain pathway in the spinal cord in a series of experiments that lasted until 1859. These findings were built on the observations that sensory fibers decussate in the spinal cord. Schiff and Woroschiloff established the presence of two pathways through observations of the effect of incisions at different levels of the spinal cord: the anterolateral pathway for pain and temperature and the posterior bundles for tactile. In 1896, Max von Frey also carried out experiments that advanced the specificity theory. von Frey indicated that there were four somatosensory modalities: cold, heat, pain, and touch, and that all of the other skin senses were derivatives of these four modalities. To test this idea, he developed his now well-known “von Frey hairs” (termed an aesthesiometer), which consisted of a hair—usually from a human, but sometimes he used a horsehair or a hog bristle— attached to a wooden stick. By measuring the hair’s diameter,
length, and precise maximal weight that it could support without breaking off the stick (maximal tension), it was possible to measure the force applied to an extremely specific spot. Today, von Frey hairs are made of fine nylon filaments of varying thicknesses (and hence stiffness) to deliver different forces and pressures upon bending. With the use of these hairs, he could carefully determine the pressure required to elicit a sensation at each of the skin spots identified by Blix and Goldscheider. Later on, in 1967, myelinated primary afferent fibers that respond only to mechanical noxious stimuli were discovered. Soon thereafter, nociceptive, unmyelinated afferent fibers were discovered by Bessou and Pear. These discoveries revolutionized the field of pain research and helped advance and develop many theories of pain. Since Sherrington’s endorsement of the specificity theory of pain, this became the dominant theory at the time. However, its popularity waned with the postulation of the gate control theory of pain by Melzack and Wall in 1965 (Sola & Pulido, 2022). Intensity theory of pain An intensive theory of pain (now referred to as the intensity theory) has been postulated at several different times throughout history. First conceptualized in the fourth century, this theory defines pain not as a unique sensory experience but rather, as an emotion that occurs when a stimulus is stronger than usual. Centuries later, Erasmus Darwin reiterated this concept in Zoonomia . One hundred years after Darwin, Wilhelm Erb also suggested that pain occurred in any sensory system when sufficient intensity was reached rather than being a stimulus modality in its own right. Arthur Goldscheider further advanced the intensity theory, reportedly based on a different experiment performed earlier in 1859. These experiments showed that repeated tactile stimulation (below the threshold for tactile perception) produced pain in patients with syphilis who had degenerating dorsal columns (Marchand, 2021). When this stimulus was presented to patients 60–600 times/s, they rapidly developed what they described as unbearable pain. These results were produced in a series of experiments with distinct types of stimuli, including electrical stimuli. It was concluded that there must be some form of summation that occurs for the subthreshold stimuli to become unbearably painful. Goldscheider suggested a neurophysiological model to describe this summation effect: Repeated subthreshold stimulation or suprathreshold hyper-intensive stimulation could cause pain. Goldscheider further suggested that the increased sensory input would converge and summate in the gray matter of the spinal cord. This theory competed with the specificity theory of pain, which was championed by von Frey. However, the theory lost support with Sherrington’s evolutionary framework for the specificity theory and postulated the existence of sensory receptors that are specialized to respond to noxious stimuli, for which he coined the term nociceptor . Pattern theory of pain In an attempt to overhaul theories of somaesthesis (including pain), J. P. Nafe postulated a “quantitative theory of feeling” in 1929. This theory ignored findings of specialized nerve endings and many of the observations supporting the specificity and/or intensive theories of pain. The theory stated that any somaesthetic sensation occurred by a specific pattern of neural firing and that the spatial and temporal profile of firing of the peripheral nerves encoded the stimulus type and intensity championed this theory and added that cutaneous sensory nerve fibers, except those innervating hair cells, are the same. Primarily this claim was supported by earlier works that had shown that distorting a nerve fiber would cause action potentials to discharge in any nerve fiber, whether encapsulated or not. Furthermore, intense stimulation of any of these nerve fibers would cause the percept of pain.
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