The posterior column-medial lemniscal system originates in several distinctive specialized receptors that give rise to heavily myelinated axons whose cell bodies are also contained in the dorsal root ganglia. Their central projections enter the dorsal roots and then the dorsal horn of the spinal cord, and without synapsing or decussating, ascend as the posterior columns. An analogous the anterior trigeminothalamic tract subserves somatic sensation in the face.
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The problems of pain localized to the head and to the back (spine) represent special problems in neurology, as mentioned, because of the relationship of the dura, spinal nerve roots, and blood vessels, each of which can generate pain signals as a result of disease in the proximate structures of the brain, spinal cord, and vertebral column. The syndromes of headache and back pain occupy a large part of neurological and general practice.
The ambiguity with which the term pain is used is responsible for some of our difficulty in understanding it. One aspect, the easier to comprehend, is the transmission of impulses along certain pathways in response to potentially tissue-damaging stimuli, that is, nociception. Far more abstruse is its quality as a mental state intimately linked to emotion, the quality of anguish or suffering that defies definition and quantification, or “a passion of the soul,” in the words of Aristotle. This duality (nociception and suffering) is of practical importance, for certain drugs or surgical procedures, such as cingulotomy, may reduce the patient’s reaction to painful stimuli, leaving awareness of sensation largely intact. Further complexities are that the symptom of pain may persist despite interruption of neural pathways that abolish all sensation (i.e., denervation dysesthesia or anesthesia dolorosa), or that pain may continue to be perceived from the absent part of an amputated limb (“phantom pain”). Finally, pain can be evoked by almost any sensory modality, such as touch, pressure, heat, or cold, if it is intense enough.
It is apparent that few physicians are capable of handling difficult and unusual pain problems and it is sometimes to the neurologist that other physicians may turn for help with these matters. Although much has been learned about the anatomy of pain pathways, their physiologic mechanisms, and which structures to ablate in order to produce analgesia, the effective management of pain by medical and surgical means remains a considerable clinical challenge. The practice of pain medicine challenges every thoughtful physician, for it demands a high degree of skill in medicine, neurology, and psychiatry. More problematic are patients who seek treatment for pain that appears to have little or no structural basis; further inquiry may disclose that fear of serious disease, worry, or depression has aggravated some relatively minor ache or that the complaint of pain has become the means of seeking attention, drugs, or monetary compensation. There is also the “difficult” pain patient, in whom no amount of investigation brings to light either medical or psychiatric illness. Further complicating almost all aspects of pain medicine is concern about narcotic dependence, tolerance, and addiction, which may be informed by societal and political forces. Finally, the physician must be prepared to manage patients who require relief from intractable pain caused by established and incurable disease.
For more than a century, views on the nature of pain sensation have been dominated by two major theories. One, the specificity theory, was from the beginning associated with the name of von Frey. He asserted that the skin consisted of a mosaic of discrete sensory spots and that each spot, when stimulated, gave rise to one sensation—either pain, pressure, warmth, or cold; in his view, each of these sensations had a distinctive end organ in the skin and each stimulus-specific end organ was connected by its own discrete pathway to the brain. A second theory was that of Goldscheider, who abandoned his own earlier discovery of pain spots to argue that they simply represented pressure spots, a sufficiently intense stimulation of which could produce pain. According to the latter theory, there were no distinctive pain receptors, and the sensation of pain was the result of the summation of impulses excited by pressure or thermal stimuli applied to the skin. Originally called the intensivity theory, it later became known as the pattern or summation theory.