Journal of Neurophysiology, Vol 54, Issue 5 1167-1177, Copyright © 1985 by APS
Somatotopic organization in cat spinal cord segments with fused dorsal horns: caudal and thoracic levels
L. A. Ritz, J. L. Culberson and P. B. Brown
We have explored the somatotopic organization of the two cat spinal cord
regions where the dorsal horns are fused (i.e., continuous across the
midline): the caudal and thoracic segments. We have mapped the
low-threshold component of dorsal horn cell receptive fields (RFs) in these
segments and have charted the locations of dorsal root low-threshold
mechanoreceptive dermatomes. We also have determined the projections of
caudal and thoracic dorsal roots to laminae III and IV by using
degeneration techniques. The dorsal skin of the tail or thorax is
represented laterally, and ventral skin is represented at the midline, in
the fused dorsal horns. Many caudal and thoracic dorsal horn units had RFs
that crossed the dorsal or ventral midline of the skin; these units were
encountered near the edges or the midline, respectively, of the fused
dorsal horns. The tail is fully represented within dorsal root dermatomes
S3 to Ca5. Roots more caudal than Ca5 represent progressively smaller skin
areas of the distal tail. Adjacent dermatomes overlapped 15-65%. Thoracic
dermatomes had a nearly vertical orientation; adjacent dermatomes
overlapped by 30-75%. Dorsal roots in caudal and thoracic regions have
crossed projections to the medial and lateral (but not middle) portions of
the contralateral dorsal horn. These crossed projections are a possible
anatomical substrate for RFs that cross the ventral or dorsal midline. The
dorsal root projection patterns are consistent with those that would be
predicted from the dorsal root dermatomes and dorsal horn cell somatotopy,
assuming that the presynaptic terminals' somatotopy is in register with
that of dorsal horn cells (the presynaptic somatotopy hypothesis; see Ref.
12).
