Journal of Neuroscience, Vol 11, 298-309, Copyright © 1991 by Society for Neuroscience
Somatotopic organization of single primary afferent axon projections to cat spinal cord dorsal horn
PB Brown, WE Gladfelter, JC Culberson, D Covalt-Dunning, RV Sonty, LM Pubols and RJ Millecchia
Department of Physiology, West Virginia University, Morgantown 26506.
Horseradish peroxidase injection of identified low threshold cutaneous
mechanoreceptor (LTCM) primary afferent axons was used to assess the
somatotopic organization of hindlimb projections to laminae III and IV of
cat dorsal horn. Multiple injections in the same animals were used to
assess bilateral symmetry and precision. Thirty-one axons were injected,
with more than 1 axon injected in each of 8 animals (25 axons). Somatotopic
relations between their receptive field (RF) centers and the centers of
their dorsal horn projections were similar to the somatotopic relations
between dorsal horn cell RF centers and cell locations. Very few reversals
of mediolateral somatotopic gradients (proximodistal RF location as a
function of mediolateral projection center) were observed. Two afferents
with nearly identical RFs in 1 animal had nearly identical projections.
These observations held for many different combinations of receptor types.
A simple mathematical model was used to demonstrate that assembly of dorsal
horn cell RFs via passive sampling of the presynaptic neuropil by dorsal
horn cell dendrites cannot account for the sizes of dorsal horn cell LTCM
RFs. Hypothesized mechanisms for assembly of dorsal horn cell RFs must take
into account the functional selectivity of connections required to produce
RFs smaller than those predicted by the passive assembly model.
