(a) A BDA-labeled axon with four synaptic boutons (boxes). One bouton and its postsynaptic structure labeled in red and blue, respectively: In cross section (top right) and as a surface reconstruction (bottom center). (b) Dendrites from an inhibitory interneuron, an HVC(RA) neuron, and an HVC(X) neuron (left to right) in LM. Spine locations are indicated by grey spheres. (c,d) Spine densities for each of these neuron classes from LM (c) and EM (d) reconstructions. Insets show examples with spines indicated by arrowheads. (e) SBEM-based reconstructions of two HVC(RA) somata with their proximal axons. Blue, green, and red spheres mark the location of synapses with inhibitory interneurons, HVC(RA) neurons, and HVC(X) neurons, respectively. Scale bar is 0.25 µm in a.