Brain motion is driven by mechanical coupling with the abdomen.

The brain moves within the skull, but the drivers and consequences of this motion are not well understood. Here we visualized motion of the dorsal cortex relative to the skull in awake head-fixed mice using high-speed, multiplane two-photon microscopy. Brain motion was directed primarily rostrally and laterally, and was correlated tightly with locomotion, but not with respiration or the cardiac cycle. Specifically, brain motion was driven by abdominal muscle contractions that activate a hydraulic-like vascular connection between the nervous system and the abdominal cavity, and could similarly be induced by pressure applied to the abdomen. Model simulations suggest that brain motion may drive interstitial fluid through and out of the brain into the subarachnoid space, in the opposite direction of fluid flow seen during sleep. These results suggest that the brain is linked mechanically to the abdominal compartment, and that fluid flow in the brain could be coupled to body movements.