Mapping axon diameters and conduction velocity in the rat brain – different methods tell different stories of the structure-function relationship

Abstract

We investigated the structure-function relationship of axons projecting through the corpus callosum. We measured transcallosal conduction times using optogenetically evoked local field potentials (LFP) and estimated conduction length with tractography based on ex vivo diffusion magnetic resonance imaging (dMRI). Axon diameters were quantified using transmission electron microscopy (TEM) and dMRI. Comparing quantifications from TEM of dehydrated tissue with cryo-TEM, revealed axon diameter shrinkage in the former case. With correction for shrinkage, diameters from TEM showed better alignment with dMRI. The measured LFPs predicted axon diameters too small for quantitation by dMRI. Conversely, the large axons quantified by dMRI had conduction times too short to be quantified from LFPs. Hence, the two modalities had differing sensitivity profiles with respect to axon diameter. This emphasizes the need to combine suitable modalities to map the full spectrum of the structure-function relationship, in consideration of a given modality's domain of sensitivity.