Cell-type identity: a key to unlocking the function of neocortical circuits

Abstract

A central tenet of neuroscience is that the precise patterns of connectivity among neurons in a given brain area underlie its function. However, assigning any aspect of perception or behavior to the wiring of local circuits has been challenging. Here, we review recent work in sensory neocortex that demonstrates the power of identifying specific cell types when investigating the functional organization of brain circuits. These studies indicate that knowing the identity of both the presynaptic and postsynaptic cell type is key when analyzing neocortical circuits. Furthermore, identifying the circuit organization of particular cell types in the neocortex allows the recording and manipulation of each cell type's activity and the direct testing of its functional role in perception and behavior.

Figure 1

Inhibitory interneurons of the neocortex show cell-type specific patterns of GABAergic and electrical connections. Low threshold spiking (LTS; orange) cells form GABAergic connections with neighboring fast spiking (FS; blue) and pyramidal neurons (gray), but only rarely synapse onto other LTS cells. FS cells, in contrast, form GABAergic connections with neighboring FS cells as well as with LTS cells and pyramids. LTS are highly interconnected via electrical connections as are FS (blue) cells, but each cell type rarely forms gap junctions with other classes of inhibitory neuron.

Figure 2

The patterns of local connections among layer 5 pyramidal neurons reflect their long-range axonal targets. The probability of identifying a synaptic connection from pyramids projecting to the contralateral cortex (red) onto pyramids projecting to the ipsilateral superior colliculus (green) is almost fourfold the probability of identifying a connection between neighboring corticocortical pyramids.