History of Pain: A Brief Overview of the 19th and 20th Centuries
In the July issue of Practical Pain Management,1 we covered the history of pain spanning the 17th and 18th centuries. Understanding the history of pain can help practitioners and researchers grasp the nature of pain, and demonstrate how the pain management specialty grew to include the current range of treatment options. This is the final installment of this series, and will review pain management practices from the 19th and 20th centuries.
Advances in the 19th Century: Pain Receptors
Scientific forces that had been building in the 18th century carried over to the 19th century, which saw an increased number of breakthroughs in the understanding of pain mechanisms and therapeutic innovations. The 19th century also saw physiology dominated by experimental research on the structure and function of primary organs, which plays a role in pain.2 Lastly, after 40 years at the patients’ bedsides or in autopsy rooms, physicians began pursuing the study of pain symptoms. Medical texts in the early part of the 19th century started to focus on the links between physical and mental aspects of pain.2
The early part of the 19th century saw the development of clinics, which increased interest in the study of pain. Pain research and thinking at this time remained within the framework of “specificity theory” advanced by Johannes Müller (1801-1858) and later Maximilian von Frey (1852-1932), which saw pain as an independent sensation with its own sensory apparatus. In France, however, the Catholic Church again opposed materialism and spiritualism and was concerned that research was reducing the mental aspects of pain to a more physiological/chemical level. At this point in time, the Church wanted to keep the concept of “the soul,” which was introduced by René Descartes in the 17th century. According to Roselyne Rey, this conflict prevented a more complete understanding of pain, and it hardened the positions of each side.2
Despite religious objections, the pace of research continued.
According to Rey, advances in microscopy benefitted the work of Johannes Müller, who lived and worked in Germany. Müller concluded that there were specific energies within the nerve fiber and that the paths of nerve fibers were rigorously ordered. This research led to his summation that there were specific fibers for pain or receptors (neuroreceptors) for painful sensations. As noted, this finding was a major step forward in understanding pain transmission and led to Müller formulating the law of specific nerve energies.
Müller used amputees in his research, since all the primitive fibers were found at the level of the stump, and the nerve trunk remained intact. In his studies, he examined the effects of a stimulus that was applied to an area restricted to the branches of the nerves found below the stimulation point. The sensations received by the brain were the same as with the stump intact.2
Müller then proposed a model where each posterior root ganglion would function like a semiconductor of the sensation. This model may represent the first step toward the concept of a “gate,” which will be discussed in more detail in the findings of the 20th century. Müller believed that the reaction of a feeling produced by a sensory nerve would occur in the sensory nerves and not the motor nerves. He stated that, “the central parts of the sensory nerves that transmit to the brain are capable of feeling independently of the nerve cords or conductors. Sensations are determined appropriate to each sense.”2
Müller proposed a theory for pain, which took into account findings from physiology, historical observations, pathology, and integrated psychological dimensions of pain. He believed that pain was not imaginary, that pain could occur without an external stimulus.2
His thinking was similar to that of Pierre Jean George Cabanis, which we discussed in Part 1, who felt that pain sensations could be generated spontaneously in the brain and provoke pains that were real.1 Müller’s conclusions about pain were important findings and can be traced back to Descartes’ models of pain transmission (The Boy and Fire). Müller’s model encompassed significant scientific advances largely due to the improvement of the microscope.
Middle of the 19th Century
In the middle of the 19th century, there was a debate over whether pain could be avoided or accepted fatalistically (ie, whether it was predetermined by fate). This debate was prompted by the role of war, which promoted further progress in the understanding of pain. Fatalism implies that we do not choose pain, but that it happens anyway—as in the case of war. As a result of lessons learned on the battlefield, many new treatment options were introduced in England, France, and the United States, including nitrous oxide, the isolation of morphine in Germany, and ether in the United States.
Toward the end of the 19th century, research started to focus on receptors that were specific for pain. The advanced microscopes allowed the identification of neural structures, which were very different from one another. Müller felt that these structures served very different functions. This research trend represented another major step toward the theory of specificity.2
Maximilian von Frey (1852-1932) elaborated on the work of Müller. However, von Frey’s work had very different implications, and led to a more restricted concept of pain.
He was trying to identify particular points on the skin, which responded specifically to one of the four cutaneous sensations: touch, heat, cold, and pain. In order to accomplish this task, he invented what he called an aesthesiometer, where the stimulus consisted of hair.
The result did not depend on pressure alone, but on location. Further, von Frey felt that pain was perceived when the stimulus went beyond a certain threshold, and this led to his conclusion that pain results from the stimulation of special organs. Von Frey’s theory was based on the mechanisms of pain that he felt were dependent on a specific neural apparatus. Today, von Frey’s theory has been generally discounted, but his work contributed to the understanding of sensory receptors and the measurement of insensitivities of stimuli and sensory thresholds.2
According to Rey, every attempt to establish the notion of specificity at the fiber or receptor level, including transmission, turned out to be insufficient in understanding pain. The methodology of reductionism led to the basic explanation that pain is a simple response to a stimulus.2