Thalamocortical synaptic connections: efficacy, modulation, inhibition and plasticity.

The thalamic input to the neocortex is communicated by glutamatergic synapses. The properties and organization of these synapses determine the primary level of cortical processing. Similar to intracortical synapses, both AMPA and NMDA receptors in young and mature animals mediate thalamocortical transmission. Kainate receptors participate in thalamocortical transmission during early development. The shape of thalamocortical synaptic potentials is similar to the shape of intracortical potentials. On the other hand, thalamocortical synapses have on average a higher release probability than intracortical synapses, and a much higher number of release sites per axon. As a result, the transmission of each thalamocortical axon is significantly more reliable and efficient than most intracortical axons. Thalamic axons specifically innervate a subset of inhibitory cells, to create a strong and secure feed-forward inhibitory pathway. Thalamocortical connections display many forms of synaptic plasticity in the first postnatal week, but not afterwards. The implications of the functional organization of thalamocortical synapses for neocortical processing are discussed.