We identified to our surprise that diffusion plays a major role in distributing small proteins within the brain. See the video below showing the movement of a photoconverted secreted protein in the brain ventricle (credits to our genetic and imaging wizard @inyoungj88.bsky.social)

and here neuronal activity in the larval zebrafish brain

There was a problem with my videos. Hope now they are working. Here is CSF movement

We next sought to identify the effects of cilia paralysis on brain physiology using two-photon calcium imaging. Our results revealed that motile cilia are necessary to maintain normal brain excitability in response to light stimuli.

It is well known that cilia contribute to CSF flow in the brain ventricles of all vertebrates (see here a video of CSF movement in zebrafish embryos from Olstad, Ringers et al, CB, 2019). Yet the function of cilia-mediated flow remains highly debated.