Figure 4.

Action potentials are initiated in the axon of MSO principal neurons, and axon location does not change as a function of age. A, Camera lucida reconstructions of biocytin-filled MSO neurons show axon location (in red). Three axons were truncated for the figure (*). B, A cumulative frequency plot of 129 cells shows that most axons emerged directly from the soma (73%), as indicated by the points within the shaded region. In the remaining cells, 19% of the axons emerged proximally from the lateral dendrite (to the right of the shaded region), and 8% emerged from the medial dendrite within 8 μm from the soma (to the left of the shaded region). C, Dual somatic and proximal dendritic current-clamp recordings in two neurons, one with a somatically located axon (left) and one with an axon extending from the lateral dendrite ∼50 μm from the soma (right). Top panel, Camera lucida reconstructions show the location of the axons (in red and indicated by arrows). For the neuron with a dendritically located axon, the dendritic recording site and the axon were <5 μm apart. In both neurons, the medial dendrite is on the left. Middle and bottom panels, Somatic (green traces) and dendritic (lateral dendrite; purple traces) responses to suprathreshold somatic current injection (middle panels) or lateral synaptic stimulation (bottom panels). For the neuron with an axon extending from the soma, action potentials were detected first at the soma and then 35 μm out on the dendrite in an attenuated form (lag time, 20 μs for current injection, 0 μs for synaptic stimulation). However, in the neuron with the dendritically located axon, action potentials were first detected at the dendritic recording site, 50 μm from the soma, and then at the soma (lag time, 20 μs for current injection, 40 μs for synaptic stimulation).