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Abstract

One of the main features of the cerebral cortex is its vast internal connectivity. Understanding
this connectivity will most likely play a major part in understanding cortical function. Theoretical
studies based on quantitative neuroanatomical data are one important approach to reveal
fundamental processing principles in this complex structure. In order to provide such data,
we are studying cortico-cortical connections in the mouse cortex by way of the anterograde
tracer BDA (biotinylated dextran amine). One of the aims of this study is to gain knowledge
about the strength of connections between distant places in the cortex. The number of synapses
one region makes with another is closely related to the total length of axonal ramications the
projecting neurons make in that terminal region. This implies that the density of these axonal
ramications reects the inuence from the injection site onto this region. Therefore, the
length and density of labeled axons in a terminal region is a measure of the connectivity from
the site of injection to this region. A method was developed for estimating axonal length density
(length per volume) of stained axons in regions of termination using stereological priciples
(i.e. deriving higher dimension features based on measurements made in a low dimension). The
method consists mainly of counting intersections between labeled axons and specially designed
test lines, providing a simple quantication of tracing results.