Chapter 5 - Primary Afferent Projections to the Spinal Cord

Abstract

Sensory signals are transmitted from the periphery to the dorsal horn of the spinal cord by primary afferent fibers. The central projections of these have been studied widely in the rat and cat using degeneration techniques and neuronal tracers—HRP in particular. The majority of the primary afferents enter the dorsal horn at or near their level of origin; the remaining usually ascend or descend one or two spinal segments before penetrating the dorsal horn where they synapse on second-order neurons. The projections of primary afferents to the spinal cord are to Rexed’s laminae and certain spinal cord nuclei, namely the central cervical, dorsal, lateral spinal, intermediolateral and sacral parasympathetic nuclei. In general, C fibers terminate superficially, in laminae 1–2, Aδ fibers in laminae 1, 2, 5 and 6, and Aβ fibers in laminae 3–5. There is a remarkable somatotopic organization of primary afferent projections in the spinal dorsal horn.