Inhibitory basket cell synaptic input to layer IV simple cells in cat striate visual cortex (area 17): A quantitative analysis of connectivity

Abstract

In the absence of a direct and specific marker for basket cells, the aim of this paper was to use available data to estimate the density of basket cell synaptic input to smooth and spiny neurons within layer IV of cat striate visual cortex (area 17). A linear quantitative analysis of layer IV basket cell connectivity data suggests that on average basket cells (1) comprise 25–35% of all GABAergic neurons in layer IV (3552–4736 cells mm⁻³), (2) account for 30–41% of all putative inhibitory dendritic synapses of layer IV spiny stellate cells (145–195 synapses cell⁻¹) and a similar proportion of layer IV basket cells (25–37%, 71–107 synapses cell⁻¹), and (3) provide each layer IV spiny cell with 13–45 axons and each layer IV basket cell with 6–29 axons. These estimates suggest that basket cells may be less common and provide a smaller proportion of the dendritic synaptic input to layer IV spiny and smooth neurons than previously thought. In addition, the analysis indicates that a layer IV spiny stellate cell may receive on average as many synapses and axons from layer IV basket cells as from lateral geniculate relay cells. Based on this potential numerical similarity, a geniculate-basket synaptic pairing in a spine-shaft microcircuit is hypothesized. This microcircuit could implement a type of local (dendritic) push–pull interaction underlying subfield antagonism.