Scientists may have finally found an explanation for what has been causing neurological symptoms in long COVID patients.

A research team at Tokyo's Keio University conducted a study on how SARS-CoV-2 infects the microglia and not the neurons in the brain using 2D and 3D cultures. The former refers to the resident immune cells of the brain, while the latter is the fundamental unit of the brain and nervous system.

While the novel coronavirus is capable of entering the brain, it has been a mystery how the virus could not invade nerve cells but still cause a buildup of inflammatory substances in the brain, according to The Mainichi.

To understand why this is happening, the research team led by neurology professor Hideyuki Okano used induced pluripotent stem (iPS) cells to come up with neurons and microglia. They then infected these cells with "pseudoviruses" that imitate the original SARS-CoV-2 and the newer strains.

During the experiment, the team learned that the virus only infected the microglia, which are responsible for cleaning unwanted substances out of the brain. Since the microglia got infected heavily, they behaved normally and died.

Without the microglia, the inflammatory substances in the brain started to build up. This subsequently led to neural tissue damage, which causes neurological symptoms associated with long COVID.

According to NHS inform, the most common long-term effects of COVID-19 on the brain include poor concentration, confusion, slowed thinking, fuzzy thoughts, forgetfulness, lost words and mental fatigue. All of these are now categorized under the medical term "brain fog," which has been linked to COVID-19.

The researchers concluded that the results of their study suggest long COVID patients develop neurological symptoms due to a significant decline in microglia and the lack of immune functionality within the brain.

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"Anti-inflammatory drugs and drugs that suppress the function of substances released by microglia could be a treatment for (COVID-19's) aftereffects," research team member Yoshitaka Kase said, per The Mainichi.

The findings of the new study are published in the journal Experimental Neurology.

The news comes a few weeks after a study published in JAMA Internal Medicine found that women over 40, obese people, smokers and those who got hospitalized due to COVID-19 are more likely to develop long COVID.

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A man wears a face mask as he check his phone in Times Square on March 22, 2020 in New York City. - Coronavirus deaths soared across the United States and Europe on despite heightened restrictions as hospitals scrambled to find ventilators. KENA BETANCUR/AFP via Getty Images