The basic computational unit of the thalamus, the glomerulus, was described exactly 50 years ago, by the late János Szentágothai. The essence of the glomerulus is a giant excitatory terminal which establishes multiple synapses on thalamocortical neurons. This synaptic arrangement evolved to secure faithful information transfer between a well-defined sensory input and the cortex. Recent ultrastructural and physiological investigations of several thalamic nuclei in the rodent and primate brains, however, revealed a rich diversity of excitatory terminals and actions in the thalamus. The data showed that only a minority of the thalamus is dedicated to faithful relay of sensory information. Large thalamic regions are driven exclusively by the cortex, whereas in others large excitatory terminals are entirely missing and are replaced by giant inhibitory terminals of extrathalamic origin. Convergence of large excitatory terminals with different origin on single thalamocortical cells has also been found and the size and complexity of the excitatory terminals displayed great nucleus specific variations These data suggests that the computations performed by individual thalamic regions is not necessary a simple relay and display considerable nucleus specific variability. Revealing the basic computational units of different thalamic nuclei also allow us to resolve the biological basis of neurological diseases involving the thalamus.
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