Journal of Pain & Relief
Open Access

Sheer induction; Theoretical approximation; Specificism; Pattern theories; Specialised receptors; General physiology

Introduction

Conduction involves both, peripheral induction from infraspinal Peripheral Nervous System afferents, and ‘central transduction’ at medullar and upper levels along the Central Nervous System. Pain induction has been a main issue of experimentation in early electrophysiology throughout the 19th Century, focusing first examination around infra-spinal afferents. Running the 20th Century, new methodologies started to understand the role of voltage-irritative signatures, both in the medulla and the upper Central Nervous System, as evidences of pain transduction patterns [1]. As a result, theorists began an era of pain modelling beyond sheer induction. Approaching the 21st century, reflex theories were transformed into more complex strategies, while differentiated labelling’s characterising the phenomenon of pain sprang among interdisciplinary research. The theorisation of pain as a qualitative sensing trait has been a major tenet for electro-physiological studies, which raised modern theoretical approximations towards a scientific characterisation of pain [2]. Moved by the 19th-century fad of experimental specificism, a prior perspective built the framework, arguing for specialised receptors within the organisms as explaining the physiological proxy-agents for pain, in the conviction that pain was a natural kind of perception as perceivable as colours or scents [3]. Further theorisers claimed that no perceptual meaning would be developed without an integration of any received stimulation, whether it be caused by a neural firing pattern, its summation, its partial inhibition, or a central evaluation. Explorations have not remained unproblematic, and discussions on the nature of these ideas, as Allan Basbaum reintroduced, are still on debate. With a comparative aim, the work covers a substantial repertoire of the main theoretical achievements in the western experimental inquiry on the topic in four points [4]. Departing from the implications of the initial tenets proposed by the Müllerian turn, which configured the general orchestration for a proper field of pain electrophysiology throughout the 19th Century, it overviews the incipient theories in favour of specificism and intensivity, advancing to early 20th-century integrativism, affectivity, summation and pattern theories, and the advancements of the second half of the 20th Century, which came with the exploration of transduction, mediation and modulation.

Methodology

A present recension about the complex scene of pain research in the 21st Century finishes the fourth point. Some concluding implications are sketched, exploring some of the problems to which this historical thread has landed in the present [5]. These include the lack of strongly framed inter-field explanatory strategies; the problems produced by maintaining in currency hard readings of specificity for exposing the ultimate responsible actors in the biochemical scenario of fibres performance; or the slow accommodation of fundamental intuitions into new scientific horizons. These horizons now present, in the majority of cases, a contemporary attempt at interpreting the big picture of phenomena implied in pain sensing, examining experiences, feelings and beliefs about pain beyond peripheral, spinal or brain-localist approaches of the past as shown in (Figure 1). By the end of the 19th century, three cardinal perspectives on pain were formulated in reaction to C Bell’s 1811 concept of sensory receptors, the Intensive Theory, the Specificity Theory, and the Affective Theories [6]. Bell presented, and was later confirmed by F Magendie in 1822, was that dorsal roots, non-motor pathways, ascending spinal-cordto- brain, were responsible for sensory discrimination as for engaging in particular sensations as shown in (Figure 2). These sensations, JP Müller hypothesised in the 1830’s, were characterised as particular tones of energy exhibited by specialised receptors. In Müllers mind, as proposed in his Law of Specific Nerve Energies, sensations must be appreciated as conduits of nerve qualities to consciousness, and therefore it shall be the quality of nerves excited by specific receptors what finally comes out as pain [7].

Figure 1: Phenomena implied in pain sensing about pain beyond peripheral.

Figure 2: Ascending spinal-cord-to-brain for for engaging in particular sensations.

Discussion

The notion, as argued in the previous chapter, by translating to modernity an old tradition that characterised nerves with intentional terms, actually oriented the whole understanding of algoception in general physiology, as a proper reception of pain, thread or similar suggestions, being the justification of said concepts debated until present days. Along the 19th Century, an early but slight form of algoception was introduced through the specificist interpretation of pain induction by M Schiff, who experimented with somato topical excitation. He concluded that pain perception was required of a different and specific reception away from that of touch, deriving findings to different dorsal pathways, antero laterally for hapticity and posterior for pain and temperature [8]. Beside such pathway specialisation, it was noticeable that peripheral specificity was rising through Müllerian concepts, M Blix found that differentiated skin points evoked distinct cool, warm, haptic sensations; in parallel, H Donald-son confirmed in 1885 Blix’s acknowledgement, and along with the discovery, the same years A Goldscheider started to intuit that those sensations were implicitly caused by pattern summations of different haptic skin points, which when provoked until excess begin to fire as sensed pain [9]. Conclusions from both, Blix and Goldscheider, moved away from specificity, nevertheless their findings were later used to support a new fad in electrophysiology, a fad that upholded that pain was something specific receptors were amenable to. It was not until M von Frey’s, and later his student Strughold’s research on mechanoception, that the Specificity Theory was compelled to a formulation. Pain was hence viewed as a captive process induced by particular receptors, and their determinants specific free ending fibres scattered through a mosaic of distinct spots, altered by a stimulus which excites pathways independent of pressure or temperature until reaching a CNS kernel or centre of pain [10]. The Specificity Theory was consistent with the findings of hundreds of the so-called Schmerzpunkte per skin square cm, specificists stated that the intensity of energy flows ascends from skin-to-brain pathways out of these minute areas of pain spots which recognise specific stimuli, making the body highly specialised in cultivating sensation modalities for pain too, independent from others. The Specificity Theory took advantage in the 19th Century and was reshaped several times with different arguments as neuropsychiatric research in emotions was developing new ideas from the 18th Century to modernity [11]. The Affective Theories were some of those reformulations, of which two positions were salient: Marshall supported a Pleasure-Pain Theory, which adopted a polarised perspective integrating emotional states in the specificity arguments; in parallel, Strong proposed a psychological identification of pain, associating physical states of the original noxious sensation with psychic reactions. The later characterisation was reintroduced by Hardy, Wolff & Godell as the 4th Theory of Pain, in a very suitable position for modern studies, suggesting that pain compromises together perception and reaction [12]. However, not much back in time, in 1874, a different idea was proposed by Werb in regard to the results of his experiments on skin pain induction: an orientation towards intensivity. The main concept held that summation of unspecific stimuli forms pain elicitation. Pain appeared to be manifested conditioned by a progressive sensory input, of any class, which had the intensity of a harmful stimulus when overexposed. Today we know that the discovery was partly true, and that overloading general skin regions with summative inputs can produce a salience in the CNS of a subject as to accumulate inter-neuronal activity until excess, channelled to major brain nuclei and, thus, provoking an evaluation of the signal as painful. When focused on the PNS, easily seen in injured, clinical patients, it is known as the irritative-cumulative effect [13]. Maintaining such signalling leads to lower neuronal thresholds and voltaic overreaction, and therefore to shape a condition known as allodynia. One decade later, Erb’s concept was re-debuted with the name of Intensive Theory by Goldscheider, assuming Naunyn’s experiments of 1859 with degraded nerves in syphilitic subjects, where repetitive below-threshold inputs were transformed into acute pain as the subject was rapidly prodded with a sub-acute instrument. Their conclusion followed that it was the summation of inputs affecting receptors and not the quality of such what was generating pain [14]. Today it is difficult to know whether their exploration accounts as a general explanation of pain, or if it would rather be a better historical approximation to contextualise it as an explanation of the neuropathic processes. Nevertheless, one possible reason why intensity did not earn its deserved attention until the 1940’s, could be that the theory managed to avoid in its own way some of the ideas of its own century, which came athwart homogeneity of reception: with Pacini locating vibration and pressure-related below-skin receptors in 1831-1835, Meissner and Wagner with photosensitive cells in 1852, Blix spotting cold-warm receptors in 1882, and Ruffini’s organs in 1893.

Conclusion

With such flow of acknowledgements, specificity instead of intensity was suited to be a new trend: pain was speculated to fit its own recognition in human body cells too, and it was expected that pain appeared as another genuine kind, as desired by specificity.

Acknowledgement

None

Conflict of Interest

None

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