Change of conduction velocity by regional myelination yields constant latency irrespective of distance between thalamus and cortex

The widely spanning sensory cortex receives inputs from the disproportionately smaller nucleus of the thalamus, which results in a wide variety of travelling distance among thalamic afferents. Yet, latency from the thalamus to a cortical cell is remarkably constant across the cortex (typically, ≈2 ms). Here, we found a mechanism that produces invariability of latency among thalamocortical afferents, irrespective of the variability of travelling distances. The conduction velocity (CV) was calculated from excitatory postsynaptic currents recorded from layer IV cells in mouse thalamocortical slices by stimulating the ventrobasal nucleus of the thalamus (VB) and white matter (WM). In adults, the obtained CV for VB to WM (CVVB-WM; 3.28 ± 0.11 m/s) was ≈10 times faster than that of WM to layer IV cells (CVWM-IV; 0.33 ± 0.05 m/s). The CVVB-WM was confirmed by recording antidromic single-unit responses from VB cells by stimulating WM. Exclusion of synaptic delay from CVWM-IV did not account for the 10-fold difference of CV. By histochemical staining, it was revealed that VB to WM was heavily myelinated, whereas in the cortex staining became substantially weaker. We also found that such morphological and physiological characteristics developed in parallel and were accomplished around postnatal week 4. Considering that VB to WM is longer and more variable in length among afferents than is the intracortical region, such an enormous difference of CV makes conduction time heavily dependent on the length of intracortical region, which is relatively constant. Our finding may well provide a general strategy of connecting multiple sites irrespective of distances in the brain.