ABSTRACT
Sir Henry Head conception is that the somatosensory apparatus is divisible, morphologically and functionally, into protopathic and epicritic components. Head and his collaborators published a notable series of papers on altered somatic sensation after lesions at various levels of the nervous system. Epicritic sensation, as judged by the presence of spatially accurate, graded responses, must have antedated the emergence of the new system. The tiny, nonmyelinated peripheral nerve fibers were originally thought to be the only carriers for pain, but even after it became known that the small, myelinated A-fibers could mediate pain, the C-fibers were still held to be responsible for the protopathic form. Head's theory that the peripheral somatosensory pathways are divisible, morphologically and functionally, into protopathic and epicritic components has been adapted by some contemporary investigators to the central pathways. The modern version is based largely on the contrasting anatomical and physiological properties of the phylogenetically old (extralemniscal) and new (lemniscal) systems.
