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Science 16 June 2006:
Vol. 312. no. 5780, pp. 1622 - 1627
DOI: 10.1126/science.1124593
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Research Articles

Cortex Is Driven by Weak but Synchronously Active Thalamocortical Synapses

Randy M. Bruno* and Bert Sakmann

Sensory stimuli reach the brain via the thalamocortical projection, a group of axons thought to be among the most powerful in the neocortex. Surprisingly, these axons account for only ~15% of synapses onto cortical neurons. The thalamocortical pathway might thus achieve its effectiveness via high-efficacy thalamocortical synapses or via amplification within cortical layer 4. In rat somatosensory cortex, we measured in vivo the excitatory postsynaptic potential evoked by a single synaptic connection and found that thalamocortical synapses have low efficacy. Convergent inputs, however, are both numerous and synchronous, and intracortical amplification is not required. Our results suggest a mechanism of cortical activation by which thalamic input alone can drive cortex.

Department of Cell Physiology, Max Planck Institute for Medical Research, Jahnstrasse 29, 69120 Heidelberg, Germany.

* To whom correspondence should be addressed. E-mail: bruno{at}mpimf-heidelberg.mpg.de

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