(A) A representative inhibitory axon fragment traced from a myelinated seed profile (arrow). Blue indicates the extent of myelin along the fragment; thin red axon segments are unmyelinated. The spheres indicate the location of symmetric synapses, and are color-coded red for inhibitory postsynaptic targets (synapse 1) and green for excitatory postsynaptic targets (synapses 2–5). These five symmetric synapses were sufficient to categorize the axon as inhibitory. The letters above the fragment correspond to the approximate locations of later panels. (B) The myelinated profile used as a tracing seed for the axon fragment in A. Myelin is false-colored blue; the axon is false colored in red. (C–N) EM micrographs through synapses 1–5. The presynaptic axon is false-colored in dark red; the postsynaptic inhibitory dendrite in light red; the postsynaptic excitatory targets in green; and other excitatory boutons converging onto the same targets as the myelinated inhibitory axon, in yellow. (C) Synapse 1 contacts an inhibitory dendrite, which is also shown in (D–E). In E, the dendrite is densely coated with excitatory axonal boutons making asymmetric synapses (blue triangles), a hallmark of inhibitory dendrites. (F) Cross-section through synapse 2. (G) Larger field of view of the section in F, showing that the postsynaptic target is a neuronal cell body. (H) A nearby section through the same neuron as in G, reveals it to be a pyramidal cell with a prominent apical dendrite. (I) The myelinated axon approaching an apical dendrite. (J) Synapse 3 is formed immediately after the axon unmyelinates and contacts the same apical dendrite shown in I. (K) The same bouton participating in synapse 3 forms synapse 4 with a spine arising from the same apical dendrite shown in I and J. (L) Synapse 5 targets the shaft of a dendrite, identified as excitatory by the presence of spines on near-by sections (e.g. M–N). (M) Synapse 5 can still be seen, and a spine neck arises from the postsynaptic dendrite. (N) The spine neck in M forms a small spine head, receiving an asymmetric synapse (blue triangle) from an excitatory bouton from a different axon. Scale bar in A, ~1 μm; B, 1 μm (also applies to C–F); G, 4 μm; H, 9 μm; I, 1 μm (also applies to J–L); M, 1 μm (also applies to N).