Scientists are beginning to understand how the brain flushes out waste products in order to stay healthy and fend off neurological diseases.
Two teams of scientists from Washington have uncovered signs that a slow wave of electricity that pulses through the brain during sleep pushes waste products, including the protein closely associated with Alzheimer’s disease, from deep in the brain to its surface.
A vein passing through the brain then acts as a pipe to carry waste products out through the barrier that separates the brain and the rest of the body. This deposits them into the bloodstream to be filtered out by the kidneys.
But if the disposal system stops working correctly, which can occur with aging, in cases of traumatic brain injury, and chronic stress, those waste products can accumulate in the brain.
It may also allow immune cells to penetrate the organ, leading toinflammation linked to degenerative brain disorders.
Veins passing through the brain act as pipes ferrying waste products out through the barrier between the brain and the rest of the body and depositing them into the bloodstream to be filtered out by the kidneys
The study was conducted in mice, but researchers believe their findings match up with research on potential causes of Alzheimer’s disease and other dementias.
In February, scientists led by Dr Jonathan Kipnis, a neurologist at Washington University in St Louis, answered the question of how the brain gets waste out through the rigid protective barrier without using the body’s lymphatic system, a network that drains fluids throughout the body.
The brain is not part of that system and its waste disposal pathway is known as the glymphatic system. It clears out waste generated by energy-consuming metabolic processes - all the chemical processes continuously working in the body to ensure it functions correctly - such as carbon dioxide, damaged proteins and dead cells.
Scientists uncovered exit points around the brain from which cerebrospinal fluid, which carries waste, flowed out to the thick membrane surrounding the brain and spinal cord and into the bloodstream, where the body’s lymphatic vessels took over to clear it out.
A few weeks later, that group of scientists expanded on those findings further, showing how that waste-laden liquid ends up at those exit points.
They reported that as the brain sleeps, neurons fire in synchronized, rhythmic waves that generate force to flush cerebrospinal fluid through the brain and past the blood-brain barrier.
Dr Li-Feng Jiang-Xie, a researcher at Washington University and lead author of the second study, said: ‘These neurons are miniature pumps. Synchronized neural activity powers fluid flow and removal of debris from the brain.
‘If we can build on this process, there is the possibility of delaying or even preventing neurological diseases, including Alzheimer's and Parkinson's disease, in which excess waste – such as metabolic waste and junk proteins – accumulate in the brain and lead to neurodegeneration.’
In that same issue of Nature, a team led by scientists at the Massachusetts Institute of Technology expanded upon evidence that a slow wave washes away waste.
They tested mice that had been given Alzheimer’s and found evidence that exposing them to light flickering and sound clicking at the same brain rhythm frequency prompted pulsating signals in the brain that moved CSF out of the organ.
That CSF was carrying amyloid protein, which builds up in the brains of people with Alzheimer’s disease.
Dr Jiang-Xie said: ‘We think the brain-cleaning process is similar to washing dishes.
‘You start, for example, with a large, slow, rhythmic wiping motion to clean soluble wastes splattered across the plate.
'Then you decrease the range of the motion and increase the speed of these movements to remove particularly sticky food waste on the plate.
‘Despite the varying amplitude and rhythm of your hand movements, the overarching objective remains consistent: to remove different types of waste from dishes.
'Maybe the brain adjusts its cleaning method depending on the type and amount of waste.’
The glymphatic system falls into disrepair as we age, however, accumulating waste products like amyloid proteins and tau tangles that contribute to cognitive decline and memory loss, as well as Alzheimer's disease and dementia.
Changes in sleep patterns can also upend the normal waste disposal process. CSF flows more freely between cells while we sleep, and the entire glymphatic process is optimized during sleep.
Without adequate sleep, the flow of CSF is impaired, and so is the clearance of waste products linked to neurodegenerative diseases.
