Figure 6: Canonical SC circuits in mammals.

(A) Interlaminar connection. (A1) From the sSC to the dSC. When an electrical stimulus is delivered to narrow field vertical (NF) cells in the sSC (red lightning bolt), excitation is transmitted to brainstem-projecting neurons in the dSC (open circles) through excitatory synapses with AMPA and NMDA receptors. In parallel, GABAergic feedforward inhibition (black circle cells) curtails the excitation as shown in the inset (see the upper inset trace showing intracellular electrophysiological recordings, “w/o disinhibition”) and prevents successive spiking responses upon stimulation of the NF cells. However, if the feedforward GABAergic inhibition is removed, long lasting excitatory responses are induced in the dSC neurons ¹³⁸ (lower inset trace, “w disinhibition”) through bidirectional reverberating excitatory circuits existing in the dSC (see B2) ¹⁴⁵. (A2) Interlaminar connection from the dSC to sSC. The brainstem projection neurons in the dSC (open circle) send collaterals to the GABAergic dSC neurons (black circle), which send axons to the sSC and inhibit WF cells that project to the LP. (Adapted from Phongphanphanee et al. 2014¹⁵¹). (B) Intralaminar connections of the sSC and dSC. (B1) In the sSC, when an electrical stimulus is delivered to the NF cells (red lightening bolt), they excite nearby horizontal cells (H, black circles) which are GABAergic and send horizontal neurites to inhibit remote NF cells. Here, the extent of horizontal excitatory connections (red trace in the upper inset) is narrower than the inhibitory connections (blue trace in the upper inset) and the net effect (green trace in the lower inset) becomes ”Mexican-hat”-like center excitation and surround inhibition in NF cells . Here, “distance” in the inset indicates the medial-lateral distance of the NF cells relative to the stimulated cell. (B2) In the dSC, when an electrical stimulus is delivered to the brainstem-projecting neurons (open circle) the lateral excitation (red trace in the upper inset) is wider than the lateral inhibition in other brainstem-projecting neurons (blue trace in the upper inset) mediated by local GABAergic neurons (black circles), and the net effect (green trace in the lower inset) is an “Excitatory hill”. (Adapted from Phongphanphanee et al. 2014 ¹⁵¹).