Figure 1.

Neurolucida reconstruction and identification of synaptic contacts formed between L2/3 pyramidal and bitufted neuron. A, The somatodendritic domain of the presynaptic pyramidal neuron of a pyramid-to-bitufted cell pair is shown in red and the axon arbor in blue. The main axon descends toward the white matter while giving rise to several long horizontal collaterals in L2/3-L5. The soma and dendritic arbor of the postsynaptic bitufted interneuron innervated by the pyramidal neuron are drawn in black and the axon arbor in green. Note the two axonal domains of the bitufted interneuron: a dense projection confined to supragranular L2/3 and L1 and a vertically oriented, descending projection spanning L2/3-L6. Scale bar, 300 μm. B, Superposition of part of the pyramidal cell axonal arbor (blue) and the somatodendritic domain of the bitufted interneuron (black) at higher magnification. Blue dots indicate four putative synaptic contacts established by axon collaterals of the pyramidal cell on bitufted neuron dendrites. Note that all synaptic contacts in this cell pair were established within 100 μm from the soma of the bitufted interneuron. Scale bar, 100 μm. C, Photomicrographs of putative synaptic contacts, numbers correspond to those in B. Synaptic contacts were established on dendritic shafts (1, 2) or dendritic spines (3, 4), but not on the soma. Black arrows point to the putative synaptic contacts identified light microscopically; white arrowheads indicate the en passant axon. Scale bar, 10 μm.