Sparse local excitation from LR cells
Our study shows that while these LR cells are the most prevalent cell
type in the superficial layers of the RSG, they rarely synapse onto
their neighboring FS, RS and LR neurons. We have confirmed that the most
likely reason for this sparse connectivity is because the axons of LR
cells rarely ramify within L2/3, but instead travel into deeper layers
and the corpus callosum. Similar axonal trajectories for RSG neurons are
likely to exist in both mice and rats (Kurotani et al., 2013). In
additional related work, when pyramidal neurons in the superficial
layers of one RSC hemisphere were labeled, their axons very often
terminated in the contralateral RSC (Geijo-barrientos et al., 2019;
Sempere-Ferrà ndez et al., 2018). These studies did not identify whether
LR or RS neurons were the more likely source of these projections, but
combined with our results, suggest a circuit in which LR cells of one
hemisphere might serve to influence the activity of the contralateral
hemisphere. However, it is also possible that LR axons continue onto
other brain regions, as the RSC projects to several areas involved in
memory and spatial navigation processing (van Groen & Wyss, 1990). A
major challenge in the field will be to identify precise molecular
markers for LR cells to help in identifying their cell-type-specific
projections.