Primary afferent distribution pattern in the marginal zone (Lamina I) of adult monkey and cat lumbosacral spinal cord

Abstract

Using Golgi preparations from adult cat and monkey the present study describes the general arborization pattern of primary afferent collaterals to the marginal layer (lamina I) of the dorsal horn. The dorsal root origin of the golgi-impregnated afferents was verified in the cat by matching the structural properties of these fibers with horseradish peroxidase (HRP)-stained primary afferents which were filled with HRP via anterograde transport through dorsal rootlets. The marginal afferents, in both the cat and the monkey, are relatively fine-caliber fibers which emanate from the dorsolateral fasciculus of Lissauer. Superficially they form a transverse plexus of fibers which runs across the surface of the dorsal horn and is especially dense in the lateral portion of the dorsal aspect of the marginal layer where lamina I is thickest. In deeper regions of the marginal layer, marginal afferents are longitudinally oriented and have collaterals within the outer zone of lamina II. In both superficial and deep regions of the marginal layer the afferents run parallel with large numbers of marginal cell dendrites. Most marginal afferents give rise to collaterals with widely spaced, round to oval "boutons en passant" and "boutons terminaux" which are 1.5-2.0 micrometers in diameter. Structural correlation with previous physiological and anatomical studies suggest that most of these fibers are finely myelinated, A-delta, high-threshold mechanoreceptive fibers.