FIGURE 4. Spontaneous barrages of synaptic activity propagate down the axon.

a, Simultaneous extracellular multiple unit (MU) and somatic whole-cell recording in layer 5 during the generation of recurrent network activity of the Up state, interspersed with relatively quiet periods of the Down state. b, Simultaneous recording of an Up state in the soma and cut end of the main axon (266 m from the soma). The bottom traces are an overlay of axonal (purple) and somatic (blue) recordings of synaptic activity, along with a version of the axonal recording that has been expanded to match the peak amplitude of the somatic recording (orange). c, Morphology of the recorded cell. White arrowheads indicate putative presynaptic boutons along the axon collaterals in this focal plane. Inset shows an example of two putative en passant boutons. Note the large number of boutons that seem to be electronically close to the soma. d, Even during quiescent periods, spontaneous synaptic events (arrows point to examples) are recorded in both the soma and the cut end of the main axon of the cell in c. e, Comparison of the amplitude (area) of Up states at the soma and the cut end of the axon in different layer 5 neurons reveals an orderly decrease in the amplitude of spontaneous synaptic activity with distance from the soma. The calculated length constant of the axon is 417 m.