In an effort to conciliate the specificity and pattern theories, Head and colleagues, in 1905, formulated a concept of pain sensation based on observations that followed the purposeful sectioning of the cutaneous branch of the radial nerve in his own forearm. The zone of impaired sensation contained an innermost area in which superficial sensation was completely abolished. This was surrounded by a narrower (“intermediate”) zone, in which pain sensation was preserved but poorly localized; extreme degrees of temperature were recognized in the intermediate zone but perception of touch, lesser differences of temperature, and two-point discrimination were abolished. To explain these findings, Head postulated the existence of two systems of cutaneous receptors and conducting fibers: (1) an ancient protopathic system, subserving pain and extreme differences in temperature and yielding ungraded, diffuse impressions of an all-or-none type; and (2) a more recently evolved epicritic system, which mediated touch, two-point discrimination, and lesser differences in temperature, as well as localized pain. The pain and hyperesthesia that followed damage to a peripheral nerve were attributed to a loss of inhibition that was normally exerted by the epicritic upon the protopathic system. This theory was used for many years to explain the sensory alterations that occur with both peripheral and central (thalamic) lesions. It lost credibility for several reasons but mainly because Head’s original observations and deductions could not be confirmed (see Trotter and Davies; also Walshe). Nevertheless, both fast and slow forms of pain conduction were later corroborated (see below).
A later refinement of the pattern and specificity concepts of pain was made in 1965 when Melzack and Wall articulated their “gate-control” theory. They observed, in decerebrate and spinal cats, that peripheral stimulation of large myelinated fibers produced a negative dorsal root potential and that stimulation of small unmyelinated
C (pain) fibers caused a positive dorsal root potential. They postulated that these potentials, which were a reflection of presynaptic inhibition or excitation, modulated the activity of secondary transmitting neurons (T cells) in the dorsal horn and that this modulation was mediated through inhibitory (I) cells. The essence of this theory was that the large-diameter fibers excited the I cells, which, in turn, caused a presynaptic inhibition of the T cells; conversely, the small pain afferents inhibited the I cells, leaving the T cells in an excitatory state. Wall and Melzack emphasized that pain impulses from the dorsal horn must also be under the control of a descending system of fibers from the brainstem, thalamus, and limbic lobes.
At first the gate-control mechanisms seemed to offer an explanation of the pain of ruptured disc and of certain chronic neuropathies (particularly those with large fiber out-fall) and attempts were made to relieve pain by subjecting the peripheral nerves and dorsal columns (presumably their large myelinated fibers) to sustained transcutaneous electrical stimulation. Such selective stimulation would theoretically “close” the gate. In some clinical situations these procedures have indeed given relief from pain, but not necessarily as a result of stimulation of large myelinated fibers alone (see Taub and Campbell). In a number of other instances relating to pain in largeand small-fiber neuropathies, however, the clinical behavior has been quite out of keeping with what one would expect on the basis of the gate-control mechanism. As with preceding theories, weaknesses have been exposed in the physiologic observations on which the theory is based. These and other aspects of the gate-control theory of pain have been critically reviewed by Nathan.
During the last few decades there has been a significant accrual of information on cutaneous sensibility, demanding modification of earlier anatomic–physiologic and clinical concepts. Interestingly, much of this information is still best described and rationalized in the general framework of some of the older theories.