For the first time, researchers have successfully visualized the intricate network of blood vessels spanning the cortex of awake mice. Their groundbreaking observations revealed that these blood vessels rhythmically expand and contract, creating “waves” that wash across the brain's surface. This discovery, published in the journal Neuron, significantly enhances our understanding of cerebral blood flow, although the precise function of these waves remains an enigma.
The brain's surface is covered by a network of elastic and actively pumping blood vessels that transport oxygenated blood. This blood then enters the cortex and feeds into a secondary network of capillaries, which supply oxygen deeper into the brain tissue. Using sophisticated physics-based experimental methods and analyses, the researchers uncovered that, beyond the regular pulses of blood flow synchronized with heartbeats, there are slower, rhythmic waves of blood flow changes occurring approximately every ten seconds. These slow waves result in blood flow changes accounting for up to 20% of the brain's total blood supply. Interestingly, these waves appear to have only a weak correlation with fluctuations in brain activity.
The observed waves generate visible bulges in the blood vessels, which facilitate the mixing of fluids surrounding the brain cells. This finding is crucial as it suggests a mechanism by which waste products and other materials are cleared from the brain's extracellular fluid. Since these bulges move in various directions, the researchers propose that this process is more about mixing the fluid rather than propelling it in any particular direction.
This mixing action could play a pivotal role in expelling misfolded proteins and other potentially harmful components from the brain into the cerebrospinal fluid. This clearance mechanism is particularly vital in protecting against neurological disorders like Alzheimer's disease and other dementias and is known to be more active during sleep.
Additionally, these findings could have significant implications for interpreting functional magnetic resonance imaging (fMRI) scans. fMRI measures changes in blood oxygenation within brain structures as they activate. The discovery that these blood flow waves are largely independent of brain activity introduces a new layer of complexity in understanding the relationship between fMRI data and brain function. It suggests that current models of interpreting fMRI results may need to be refined to account for these independent waves of blood flow.
Overall, this research not only provides new insights into the cerebral blood flow dynamics but also opens new avenues for exploring the brain's protective mechanisms against neurological diseases and refining neuroimaging techniques.
Source: National Institutes of Health