Synapses on axon collaterals of pyramidal cells are spaced at random intervals: a Golgi study in the mouse cerebral cortex
Bernhard Hellwig A1, Almut Schüz A1, Ad Aertsen A2
A1 Max-Planck-Institut für biologische Kybernetik, Spemannstrasse 38, D-72076 Tübingen, Germany DE
A2 Institut für Neuroinformatik, Ruhr-Universität, Postfach Box 102148, D-44801 Bochum, Germany DE
Abstract:
In this study we investigated the arrangement of synapses on local axon
collaterals of Golgi-stained pyramidal neurons in the mouse cerebral
cortex. As synaptic markers we considered axonal swellings visible at
high magnification under the light microscope. Such axonal swellings
coincide with synaptic boutons, as has been demonstrated in a number of
combined light and electron microscopic studies. These studies also
indicated that, in most cases, one bouton corresponds precisely to one
synapse. Golgi-impregnated axonal trees of 20 neocortical pyramidal
neurons were drawn with a camera lucida. Axonal swellings were marked
on the drawings. Most swellings were ‘en passant’; occasionally, they
were situated at the tip of short, spine-like processes. On axon
collaterals, the average interval between swellings was 4.5 µm. On the
axonal main stem, the swellings were always less densely packed than on
the collaterals. Statistical analysis of the spatial distribution of
the swellings did not reveal any special patterns. Instead, the
arrangement of swellings on individual collaterals follows a Poisson
distribution. Moreover, the same holds to a large extent for the entire
collection of pyramidal cell collaterals. This suggests that a single
Poisson process, characterized by only one rate parameter (number of
synapses per unit length), describes most of the spatial distribution
of synapses along pyramidal cell collaterals. These findings do not
speak in favour of a pronounced target specificity of pyramidal neurons
at the synaptic level. Instead, our results support a probabilistic
model of cortical connectivity.
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