Figure 2.

Morphological characterization of synaptic connections using paired recordings in combination with biocytin fillings. (A) Morphological reconstruction of a synaptically coupled cell pair between a L4 spiny stellate cell and a L2/3 pyramidal neuron. The somatodendritic and axonal compartments of the presynaptic spiny stellate cell are drawn in red and blue, respectively. The somatodendritic and axonal compartments of the postsynaptic L2/3 pyramidal neuron are drawn in black and green, respectively. The gray square represents the L4 barrel where the spiny stellate cell is located. Left inset, four putative synaptic contacts established by the axon of the L4 spiny stellate cell with the dendrites of the L2/3 pyramidal neuron are marked by blue dots. Right inset, electron micrographs of the synaptic contacts. All four synaptic contacts which were identified with the light microscope were confirmed at the electron microscopic level. The axonal boutons (b) of the L4 spiny stellate cell established synaptic contacts on dendritic shafts (d) in contacts 1–3 while on a dendritic spine in contact 4 of the L2/3 pyramidal neuron. (B) Histogram of the geometric distances from the somata of putative synaptic contacts in 13 L4 spiny neuron-L2/3 pyramidal cell pairs. Inset, distribution of number of synaptic contacts per connection. (C) 2D maps of axonal (left) and dendritic (middle) “length density” of synaptically coupled L4 spiny neurons and L2/3 pyramidal cells (n = 9), aligned with respect to the barrel center. The predicted innervation domain (right) of L2/3 dendrites by L4 axons is given by the product of the L4 axonal density and the L2/3 dendritic density. Contours (thin lines) enclosing 80% of the integrated density are superimposed. Positions of L4 spiny neuron sonata (red dots), L2/3 pyramidal cell sonata (white triangles), putative synaptic contacts (cyan dots), and outlines of barrels (thick lines) are indicated symbolically. Inset, zoom in the predicted innervation domain superimposed by putative synaptic contacts. (A,B) have been adapted from Feldmeyer et al. (2002) with permission and (C) from Lubke et al. (2003) with permission.