It's about time for thalamocortical circuits

Abstract

The timing of thalamocortical excitation and inhibition is critical to local microcircuits. Two new papers shed light on the development and performance of a somatosensory microcircuit that regulates the integration of thalamic inputs.The timing of sensory stimuli is a fundamental parameter used by the neocortex to construct representations of the external world. In the somatosensory system, the timing of cortical spikes can accurately follow the pattern of stimuli generated while touching an object 1 . This temporal precision is likely to be essential for proper tactile discrimination. To enforce precise timing, the cortex uses a simple circuit that is activated by thalamic afferents 2 . These thalamocortical afferents provide the main input to the somatosensory cortex and form excitatory, glutamatergic synapses onto a subset of cortical neurons. In this issue, two papers examine the development and function of a microcircuit activated by thalamocortical afferents. Daw et al. show that the ontogeny of this thalamocortical microcircuit involves a set of rapid, coordinated steps that transform it into a precise coincidence detector 3 . Once the circuit is mature, Cruikshank and colleagues demonstrate that it relies on divergent kinetics and strength of excitation from thalamocortical afferents for reliable performance 4 .Upon entering the cortex, individual thalamocortical afferents contact both excitatory projection neurons (glutamatergic principal cells) and local inhibitory interneurons (GABAergic cells). Thus, somatosensory information is immediately distributed to both excitatory and inhibitory cells. Surprisingly though, thalamocortical synapses onto inhibitory interneurons are much stronger than those onto excitatory principal cells 2,5,6 . By contacting both inhibitory and excitatory cells, thalamocortical afferents lay the foundation for a simple disynaptic circuit that provides powerful, local feedforward inhibition (Fig. 1).In the somatosensory cortex, thalamocortical afferents initiate feedforward inhibition by activating so-called fast-spiking interneurons, a subtype of GABAergic interneuron. Because these fast-spiking interneurons synapse onto the same excitatory principal cells that are directly contacted by thalamocortical afferents, thalamic activity results in both excitation and inhibition of cortical principal cells.Because excitation is direct, whereas inhibition is delayed by one synapse, disynaptic feedforward inhibition lags monosynaptic thalamocortical excitation by 1-2 ms. The short latency between the onset of thalamocortical excitation and the onset of feedforward inhibition presents a temporal 'window of opportunity' for principal neurons to integrate

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