All reviews are selective: here, we have chosen to concentrate on some of the recent points of interest in somatosensory afferent systems. The reader is warned that one of the authors is naturaIly biased in favor of his own survey on the dorsal columns (111). However a more traditional and com prehensive review on this subject is available (85). GENERAL SCHEMES FOR AFFERENT SYSTEMSEach succeeding A nnual Review tends to define the nervous system in terms of more and more specific subdivisions. The intention of this progres sion is to identify and study the ultimate and indivisible particles whose reconstitution forms the entire system. This search for elementary particles is the most respectable of scientific analytic tactics; final success nevertheless depends on selecting the correct elemental categories. The history of science shows many examples of considerable progress in analysis based on categories which eventually became non-productive. Generations of men at least as clever as we are developed an elaborate technology and, in that sense, an understanding of materials based on the categories of earth, air, fire, and water. Generations of neurophysiologists have also developed categories, each based on a model of the way the system is assumed to operate; the justification and usefulness of these categories, stated or implicit in previous work, are now being questioned.What are these categories in which the organization and function of afferent systems are described? First, there are the categories of origin, as implied in the names of the cutaneous, proprioceptive, and visceral afferent systems. Within these there are subdivisions which may depend either on morphologically recognizable end organs such as muscle spindles, Golgi tendon organs, and Pacinian corpuscles, or on physiologically recognizable axons such as nociceptors, heat detectors, and cold detectors. No one now doubts the existence of these structures and functions as described, although there remain the questions whether all the morphological types are funda mentally different from each other-especially, whether the physiological 315 1082 Annu. Rev. Physiol. 1972.34:315-336. Downloaded from www.annualreviews.org Access provided by Lancaster University -UK on 02/05/15. For personal use only.Quick links to online content Further ANNUAL REVIEWS