The present study is concerned with the numbers of primary afferent axons in the tract of Lissauer in the cat. The data show that approximately 27% of the axons in mid‐thoracic and lumbosacral tracts are primary afferent fibers from the segment in question and another 20% of the axons are primary afferent fibers that come from nearby segments. In addition the data show that approximately 80% of the axons in the tract are unmyelinated and that there is a slightly higher proportion of unmyelinated as opposed to myelinated primary afferents. There is also a higher proportion of primary afferents in the medial as opposed to lateral parts of the tract, but there are significant numbers of primary afferents in lateral parts of the tract. Thus it seems clear that the tract contains more primary afferent fibers than was previously believed and if these data are confirmed, the conclusions will have a bearing on considerations of the primary afferent input into the dorsal horn.
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