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 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
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