By the 1890s, physiological and medical concepts of pain, as of the body and of disease generally, had become mechanistic, localized, and empirical. The work of Rudolf Virchow in the 1850s and 1860s had demonstrated the cellular damage associated with infection and trauma. The experimental findings of physiologists in France, Germany, and England supported mechanical models of body functions. The work of Charles Bell and François Magendie, showing that the posterior roots of the spinal nerves responded to sensations whereas the anterior roots appeared to be associated with motor responses, laid the groundwork for the idea of a dedicated neural pathway of pain sensation, elaborated in 1839 by Johannes Müller's theory of "specific nerve energies." Further confirmation came from the work of Edouard Brown-Séquard on the pain pathway in the spinal cord; from the contributions of John Hughlings Jackson and others, suggesting specific locations of various function in the brain; and from the experiments of Max von Frey, who in 1896 identified "pain spots" on the skin. But the idea of specificity was by no means unquestioned.

	Portrait of Charles Bell c.1839		Charles Bell, Sir. The Anatomy and Philosophy of Expression: as Connected with the Fine Arts. 5th ed. London : Henry G. Bohn, 1865

Charles Bell

Charles Bell, the Scottish anatomist and neurosurgeon seen, described the motor role of the anterior or ventral nerve root of the spinal cord in his privately-circulated pamphlet, A New Idea of the Anatomy of the Brain, Submitted for the Observation of his Friends (1811).

The Spinal Nerve Roots

Bell's drawing of the spinal cord, ventral view, from: An Exposition of the Natural System of the Nerves of the Human Body (London: Spottiswoode, 1824)

François Magendie (1783-1855)

	Portrait of François Magendie, 1822

Magendie, the pioneer of experimental physiology in France, published a brief yet momentous paper on the functional discreteness of the spinal nerve roots ("Expériences sur les foncions des racines des nerfs rachidiens") in his own journal in 1822. He demonstrated that the posterior or dorsal roots appeared more specifically related to sensation and, without knowing it at the time, confirmed Bell's conclusions about the anterior nerve root.

	Portrait of Johannes Müller

The Law of Specific Nerve Energies

Johannes Müller (1801-1858), professor at Berlin, numbered some of the most famous names in 19th century physiology among his students, including Helmholtz, DuBois-Reymond, Schwann, Virchow, Kölliker and Remak. His formulation of the Law of Specific Nerve Energies appeared in the second edition of his Handbuch der Physiologie des Menschen für Vorlesungen (Coblenz: J. Hölscher, 1835-1840), translated here by Edwin Clarke and UCLA's Charles Donald O'Malley:

". . . (T)he same cause, such as electricity, can simultaneously affect all sensory organs, since they are all sensitive to it; and yet, every sensory nerve reacts to it differently; one nerve perceives it as light, another hears its sound, another one smells it; another tastes the electricity, and another one feels it as pain and shock. One nerve perceives a luminous picture through mechanical irritation, another one hears it as buzzing, another one senses it as pain. . . He who feels compelled to consider the consequences of these facts cannot but realize that the specific sensibility of nerves for certain impressions is not enough, since all nerves are sensitive to the same cause but react to the same cause in different ways. . . (S)ensation is not the conduction of a quality or state of external bodies to consciousness, but the conduction of a quality or state of our nerves to consciousness, excited by an external cause."

Pain Spots

Max von Frey (1852-1932) used human hairs or other bristles mounted in a handle to map cutaneous sensitivity on a piece of skin on the back of the hand. His investigation found discrete pain spots (circles) which did not coincide with pressure points (triangles; &Haare&=hair).

"Untersuchung über die Sinnesfunctionen der menschlichen Haut," Abhandlungen der mathematisch-physischen Klasse der Königichen Sächsischen Gesellschaft der Wissen- schaften, 1896

Nociception

Portrait of Sir Charles Scott Sherrington

In 1898 the British physiologist, Sir Charles Scott Sherrington (1857-1952), proposed the key concept of nociception: pain as the evolved response to a potentially harmful, "noxious" stimulus. His landmark work, The Integrative Action of the Nervous System (New York: Scribner, 1906) included examples in which various sensory stimuli (such as those which normally elicit pain or nociception vs. those evoking the scratch reflect) compete in the production of various behavioral responses using the same motor pathways, in what he called "the struggle between dissimilar arcs for mastery over their final common path."

Sherrington insisted that the essential function of the nervous system was the coordination or integration of activities of the various parts of the organism.

The function of pain, to the twentieth-century scientist, was no longer to heal, to punish, or to ennoble, but to provide a mechanical warning of actual or potential damage to cells and tissues in a specific body area. Despite Sherrington's emphasis on integration and competition, the idea of a specific pathway for pain, linking peripheral receptors to spinal neurons to brain receptors, and producing a motor response of one-to-one intensity, the &telephone exchange model,& became dominant in neurophysiology.