The Journal of Neuroscience, September 15, 2002, 22(18):8170-8182
Spinal Sensorimotor Transformation: Relation between Cutaneous
Somatotopy and a Reflex Network
Anders
Levinsson,
Hans
Holmberg,
Jonas
Broman,
Mengliang
Zhang, and
Jens
Schouenborg
Section for Neurophysiology, Department of Physiological Sciences,
Lund University, S-221 84 Lund, Sweden
The projection of primary afferents onto spinal interneurons
constitutes the first step in sensorimotor transformations performed by
spinal reflex systems. Despite extensive studies on spinal somatotopy,
uncertainties remain concerning the extent and significance of
representational overlap and relation to spinal reflex circuits. To
address these issues, the cutaneous projection from the hindpaw and its
relation to the topography of lamina V neurons encoding withdrawal
reflex strength ("reflex encoders") was studied in rats. Thin and
coarse primary afferent terminations in laminas II and III-IV,
respectively, were mapped by wheat germ agglutinin-horseradish peroxidase and choleragenoid tracing. The functional weights of these
projections were characterized by mapping nociceptive and tactile field
potentials and compared with the topography of reflex encoders. Both
anatomical and physiological data indicate that thin and coarse skin
afferent input is spatially congruent in the horizontal plane. The
representation of the hindpaw in the spinal cord was found to be
intricate, with a high degree of convergence between the projections
from different skin sites. "Somatotopic disruptions" such as the
representation of central pads medial to that of the digits were
common. The weight distribution of the cutaneous convergence patterns
in laminas III-IV was similar to that of lamina V reflex encoders.
This suggests that the cutaneous convergence and features such as
somatotopic disruptions have specific relations to the sensorimotor
transformations performed by reflex interneurons in the deep dorsal
horn. Hence, the spinal somatotopic map may be better understood in
light of the topography of such reflex systems.
Key words:
somatotopic; sensorimotor; receptive field; somatosensory; spinal cord; reflex; primary afferent
Copyright © 2002 Society for Neuroscience 0270-6474/02/22188170-13$05.00/0
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