Axons from LR cells do not ramify locally but head to deeper layers and towards the corpus callosum
The rarity of connections from LR neurons onto their neighboring L2/3 cells suggested that LR axons have more distant targets. In order to investigate the projections of the LR cells, we used biocytin to fill cells for morphological consideration after characterizing their physiological properties. Of the 3 LR neurons whose cell body, dendrites, and axons were sufficiently filled, all exhibited projections to the deeper layers of RSG (Figure 7A,B,D). Of the three, one axon clearly entered and traveled within the corpus callosum (Figure 7A&C). Additionally, LR neurons had very few axonal ramifications within layers 2/3, matching their extremely low likelihood to synapse onto local neurons (Figure 7A&D). Upon further examination of our four paired recordings in which LR cells did directly excite the paired FS cell, we noticed that all of these LR cells were located more superficially than their paired FS cell. Additionally, of the four pairs in which the FS cell was located more superficial to the LR cell, none exhibited connections from the LR to the FS cell. This supports the finding that LR axons travel to deeper areas and do not ramify locally. It also suggests that FS cells in L2, even more so than L3 FS cells, are likely to be completely devoid of local excitation from LR cells.