Figure 3. Inhibitory and Excitatory Local Connectivity in Cortex.

(A) Inhibitory interneurons vary in their laminar distribution across the cortex. 5-HT_3AR⁺ neurons are concentrated in L1 and L2/3, including the VIP⁺ interneurons. PV⁺ and SST⁺ interneurons are present across all of the layers, except L1 (after Lee et al., 2010; Xu et al., 2010). Subtypes are highlighted in the table at right.

(B) In L2/3, bipolar VIP⁺ interneurons disinhibit the cortex by specifically inhibiting SST⁺ neurons. SST⁺ cells tonically inhibit PV⁺ neurons and pyramidal (Pyr) neuron apical dendrites. Tonic inhibition is released when VIP⁺ neurons become active (Pfeffer et al., 2013). Input to VIP neurons (black and magenta arrows) may vary across cortical areas. Local inhibitory circuits differ in L4, where SST⁺ interneurons also inhibit PV⁺ cells, but VIP⁺ neurons are less abundant.

(C and D) Major excitatory connections of the granular and supragranular visual cortex. The local excitatory circuit includes connections from excitatory cells in a given layer to those in nearby layers (Xu et al., 2016). Ascending (left) and descending projections (right), with thicknesses proportional to excitatory connection strength. Laminae targeted by thalamic input shown in blue and red. Strength is shown in the illustration (C) and as a connectivity matrix (D), with pre- and postsynaptic layers labeled.

(E) Local excitatory connectivity in L2/3 and L4.