Fig. 9

Putative mechanism of CD signaling in the visuomotor pathway.

Hypothesized organization of inhibitory CD signaling in the tectum. Diverse cell types in the tectum (red cells) receive motor-related inhibitory inputs during swimming. The CD signal is initiated at unknown sites in premotor circuitry controlling tail and eye movements (yellow circle) and is relayed to the tectum, possibly via the cerebellum, or other relay nodes. Because post-swim Ca²⁺ signals cluster in the most superficial layer containing the stratum marginale (SM) where axons from TL projection neurons form a narrow input layer, TL is a likely pathway whereby CD signals reach the tectum. This is supported by transient post-swim activity observed in TL neurons (Fig. 7). Similarly, afferents from the cerebellum, or from other relay nodes, which preferentially terminate in deep neuropil layers containing the SAC, could form another input channel for CD signals. Because projection neurons from both the TL and the cerebellum are mainly glutamatergic, this model posits that the afferent CD signal is sign-converted by local inhibitory interneurons (green). As CD likely enters the tectum via superficial TL fibers, this interneuron type is expected to receive excitatory input in the SM/SO and distribute inhibitory signals across several neuropil layers. This could explain how neurons with only deep dendritic branches (Fig. 8) receive phasic inhibitory inputs despite a lack of dendrites in the more superficial layers (red neuron on the right).