Cellular complexity in subcortical white matter: a distributed control circuit?

The subcortical white matter (SWM) has been traditionally considered as a site for passive–neutral–information transfer through cerebral cortex association and projection fibers. Yet, the presence of subcortical neuronal and glial “interstitial” cells expressing immunolabelled neurotransmitters/neuromodulators and synaptic vesicular proteins, and recent immunohistochemical and electrophysiological observations on the rat visual cortex as well as interactive regulation of myelinating processes support the possibility that SWM nests subcortical, regionally variable, distributed neuronal–glial circuits, that could influence information transfer. Their hypothetical involvement in regulating the timing and signal transfer probability at the SWM axonal components ought to be considered and experimentally analysed. Thus, the “interstitial” neuronal cells—associated with local glial cells—traditionally considered to be vestigial and functionally inert under normal conditions, they may well turn to be critical in regulating information transfer at the SWM.