Journal of Neuroscience, Vol 15, 3475-3489, Copyright © 1995 by Society for Neuroscience
Modular organization of the pontine nuclei: dendritic fields of identified pontine projection neurons in the rat respect the borders of cortical afferent fields
C Schwarz and P Thier
Sektion fur Visuelle Sensomotorik, Neurologische Universitatsklinik Tubingen, Germany.
Cortical afferents transferring information destined for the cerebellum
terminate in the pontine nuclei (PN) in a divergent and patchy fashion. We
investigated whether the form of dendritic fields of pontine projection
neurons which are postsynaptic to the cortical afferents are related to
this patchy pattern. To this end we used a triple combination of (1)
retrograde labeling (injection of Fluorogold into the brachium pontis), (2)
anterograde labeling [injection of Dil into cortical areas A17 and
Sml(forelimb)], and (3) subsequent intracellular fills of identified
projection neurons (Lucifer yellow) in slightly fixed slices of pontine
brainstem. In 64 projection neurons whose somata were located within 160
microns of the border defined by cortical afferent fields, most of the
dendritic trees were found to respect the border. Strikingly, proximal
dendrites which were oriented toward the border often bent in order to
avoid the boundary. This observation was supported by a quantitative
analysis. It revealed that overlap areas of dendritic fields with the
neighboring compartment were significantly smaller than those of
hypothetical, radially organized dendritic fields of the same size,
indicating that the dendritic fields are indeed confined to single
compartments. In a second series of experiments, double injections of the
anterograde tracers Dil and DiAsp into adjacent sites within one cortical
area (A17 or Sml) were made in order to test if the topology of the
cortical map is preserved within individual pontine compartments. This,
however, does not seem to be the case, since the terminal fields displayed
a complex pattern of overlap and nonoverlap rather than a consistent shift
of terminal fields expected in the case of preserved topology. The results
of the present study are consistent with the view that pontine modules
independently process information from different parts of individual
cortical areas. We suggest that this characteristic property of the
corticopontine projection system might be the morphological basis of the
well established fact that somatotopically continuous sensory maps in the
cortex are transformed into maps at the level of the cerebellar cortex,
showing a fractured somatotopy.
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